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i
^^^RA^"
mwi^'i
PRIMARY BATTERIES
3 '/: ; V
BY
HENRY S. CARHART, A.M.
Professor of Physics in thb University of Michigan
SIXTY-SEVEN ILLUSTRATIONS
." V ; s .
Boston
ALLYN AND BACON
1891
^-
Copyright, 1891,
By henry S. CARHART.
Typography by J. S. Gushing & Co., Boston.
Presswork by Berwick & Smith, Boston.
1
PREFACE.
With the exception of a single translation from the
French, the material on primary batteries hitherto accessi-
ble to English readers has been in detached portions, partly
in books on the general subject of electricity, and partly
in scientific journals and technical papers.
A thorough knowledge, systematically arranged, of the
principles involved in the construction, operation, and
theory of primary batteries is of undoubted service to
those beginning an extended course of study in the appli-
cations and engineering of electricity; while it is indis-
pensable to one whose occupation requires familiarity with
these most simple and useful means of producing electric
currents for practical purposes.
This little book has been written with both of these
classes of readers in mind. No attempt has been made
to compile anything like a complete list and descrip-
tion of all the combinations proposed or actually used as
primary batteries. A large proportion of them are more
curious than useful, and many have scarcely the merit of
novelty.
It is hoped that the reader will find a satisfactory
account of the theory of a voltaic cell from the point of
• • •
111
iv PREFA CE,
view of the transformation and conservation of energy.
In this connection the author desires to acknowledge his
obligation to Dr. Lodge's " Modern Views of Electricity."
The divisions of the subject are considered to be as
logical as the nature of the material permits ; each one is
fully illustrated by the most useful types of cells. Some
prominence has been given to standards of electromotive
force, since they are employed much more commonly than
formerly as secondary standards for the measurement of
both currents and electromotive forces. Their convenience
and, with proper precautions, their accuracy as well com-
mend them for general use.
It is hoped that the chapter on testing will be of interest
to the student, and useful as an outline guide for laborar
tory purposes. With scarcely an exception the tests de-
scribed have been made either by the author himself or
under his immediate supervision. They are believed to be
free from bias and to exhibit some facts not heretofore
accessible to the public.
Universitt op Michigan,
Jane 1, 1891.
CONTENTS.
•«o*-
CHAPTER I.
INTRODUCTION.
SECTION PAeS
1. Battery defined 1
2. Batteries : primary and secondary 1
3. Origin of the voltaic cell 2
4. Volta's pile 3
6. The dry pile 4
CHAPTER n.
THE SIMPLE VOLTAIC CELL.
6. Fundamental phenomena 7
7. Theory of the voltaic element 8
8. Chemical reaction in the simple voltaic cell 10
9. Inconstancy of the simple voltaic cell 11
10. Experiments on the polarization of a simple cell 12
CHAPTER IIL
POTENTIAL AND ELECTROMOTIVE FORCE.
11. Electric potential 16
12. Positive and negative work 16
13. Electromotive force , 17
14. Relation of electromotive force to difference of potential 18
15. Relation of potential differences to external and internal
resistance 20
16. Volta's contact force 21
17. Explanation of the Volta effect 22
V
VI CONTENTS,
CHAPTER IV.
CLOSED CIRCUIT BATTERIES.
SECTION PAGE
18. Distinction between open and closed circuit batteries 27
19. The Daniell battery 28
20. Chemical reactions in the Daniell cell 30
21. Chemical reactions of the cell in relation to energy 32
22. Local action and amalgamation 33
23. The effect of amalgamation 34
24. Relative protection of alloying and amalgamating 35
25. Defects of the Daniell cell 36
26. The effect of temperature changes on a Daniell battery 37
27. The gravity battery 38
28. The Gethins battery 40
29. Delany's modified gravity cell 41
30. Sir William Thomson's tray battery 42
31. Grove's battery 43
32. Bunsen's battery 46
33. The bichromate battery 47
34. Chemical reactions in the bichromate battery 49
35. The advantages of sodium bichromate over potassium bichro-
mate 50
36. Directions for setting up a bichromate battery 51
37. The Fuller bichromate cell 53
38. Chromic acid as the depolarizer 53
39. The Partz acid gravity battery 55
40. Taylor's battery 57
41. The copper oxide battery 58
42. The Edison-Lalande battery 60
43. The chloride of silver cell 62
44. Modifications of the silver chloride cell 64
CHAPTER V.
OPEN CIRCUIT BATTERIES.
45. The Leclanch6 cell 66
46. Chemical reactions in the Leclanch6 cell 67
47. The prism Leclanch^ battery 69
48. The closed Leclanch6 cell 71
" eclanch6 cells with carbon cup 73
CONTENTS. VU
SECTION PAGB
60. Leclanch6 cell with agglomerated carbon 74
61. Roberts' peroxide battery 74
62. The sulphate of mercury battery 75
53. The Fitch "chlorine" battery 76
CHAPTER VI.
BATTERIES WITHOUT A DEPOLARIZER.
54. The Smee cell 78
55. The sea salt battery 79
56. The Law battery 80
57. The diamond carbon battery 80
58. Cylinder carbon battery 82
59. The Gassner dry battery 83
CHAPTER VII.
STANDARDS OF ELECTROMOTIVE FORCE.
60. Latimer Clark's standard cell 86
61. Lord Rayleigh's form of the Clark element 87
62. A standard Clark cell with low temperature coefficient 90
63. The oxide of mercury standard cell 95
64. Sir William Thomson's standard Daniell cell 97
65. Lodge's standard Daniell cell 98
66. Fleming's standard Daniell cell 99
67. The chloride of lead standard cell 102
68. To measure the E.M.F. of a standard cell 103
CHAPTER Vin.
MISCELLANEOUS BATTERIES.
69. Grove's gas battery 106
70. Upward's chlorine battery 109
71. Powell's thermo-electrochemical battery 110
72. A battery absorbing oxygen from the air Ill
73. Minchin's seleno- aluminum cell 112
74. Shelford Bidwell's dry battery 113
75. Jablochkoff's battery 114
76. Battery with two carbon electrodes 114
vm CONTENTS.
CHAPTER IX.
BATTBBY TB8TS.
BXOTXOH PAGE
77. What a systematic test includes 115
78. Theory of the method of measuring E.M.F. and internal
resistance 116
79. To obtain data for curves of polarization, recovery, internal
resistance, and current 119
80. Test of a typical Leclanch6 cell 121
81. Test of Leclanchg cell with depolarizer enclosed in carbon
cylinder , . . . 124
82. Test of zinc-carbon cell without depolarizer 127
83. Test of a ** dry " cell 128
84. Test of a silver chloride cell , 130
85. Efficiency test of copper oxide battery 131
86. Testing battery designed for small lamps 134
87. Analysis of the temperature coefficient of a battery 136
88. To determine the thermo-electric power of zinc— zinc sulphate, 136
89. Thermo-electric power of copper— copper sulphate 141
90. Application to a Daniell cell 142
91. Temperature coefficient of a Daniell cell .^. 145
92. Thermo-electric power of mercury— mercurous sulphate 146
93. The experimental cell as a Clark cell 149
94. Electromotive forces of various combinations 151
95. Relative value of oxidants in batteries 153
96. Manganese dioxide in Leclanch6 cells 155
CHAPTER X.
OROUPINQ OF CELLS.
97. Activity and efficiency 157
98. Application of Ohm's law to a single cell 157
99. Cells in series 158
100. Grouping in parallel or multiple arc 159
101. Grouping in multiple series 160
102. Arrangement to produce the greatest current 160
103. Grouping of a battery for quickest action 161
104. Grouping together dissimilar cells 164
N
CONTENTS. ix
CHAPTER XL
THERMAL RELATIONS.
8BCTI0K p^Q,
105. General considerations 166
106. Units of force, work, activity, and heat 168
107. The heat equivalent of a current 169
108. Heat evolved in a circuit with no counter electromotive force, 170
109. Counter electromotive force in a circuit 172
110. Division of the energy in a circuit with counter electromotive
force 173
111. Counter electromotive force of electrolysis 173
112. Failure of a cell to effect decomposition 176
113. Calculation of E.M.F. from the heat of combination 176
114. Application to the Smee cell 178
115. Application to the Daniell cell 179
116. Application to the Bunsen cell 179
117. Application to the silver chloride cell 180
118. Helmholtz's formula for electromotive force .'180
PRIMARY BATTERIES.
-»Or<KO«"
CHAPTER I.
INTRODUCTION.
1. Battery Defined. — An electric battery, or cell, as a
single element is called, is a device for the conversion
of the potential energy of chemical separation into the
energy of an electric current.
Thus the metal zinc and sulphuric acid, which acts
chemically on it, represent energy of chemical separa-
tion in the potential form. If now the zinc i« placed
alone in the acid, this energy of chemical separation is
converted simply into heat, when the zinc displaces the
hydrogen of the acid with the formation of zinc sul-
phate. But if the displacement of hydrogen by zinc is
made to take place under certain less simple conditions,
.. . then a part at least of the kinetic energy developed
takes the form of the energy of an electric current.
The arrangement of parts necessary to secure these con-
ditions, which determine that the transformed energy
shall be electrical, is called a battery, or voltaic cell.
2. Batteries: Primary and Secondary. — Electric bat-
teries may be either primary or secondary. A primary
battery is usually understood to be one in which the
materials are combined in the cell in such a state as to
2 PRIMARY BATTERIES.
be immediately utilizable in producing an electric cur-
rent; while, in a secondary battery, the materials or
elements of which it is composed need to be modified by
electrolysis, due to the passage of a current of electricity
from some external source, before the cell is in condi-
tion to yield any considerable energy in the form of an
electric current. The former possesses a store of poten-
tial energy in the materials which admit of chemical
reactions ; while the latter is only a reservoir, capable
of storing energy by means of the chemical changes
produced by electrolysis.
Some batteries may combine both characters in one.
These are capable of having the chemical changes which
take place in them, during the production of a current,
reversed wholly or in part upon the passage of a reverse
current from some other source ; so that, after they have
been exhausted by performing their function as a pri-
mary battery, they may again be restored to activity by
the passage through them of a current in the opposite
direction to the one normally furnished by the cells
themselves. This reverse current must be kept flowing
for a sufficient time to effect the necessary chemical
changes. Such cells are not as efficient in their sec-
ondary capacity as storage cells which are designedly
such. The energy which they can restore after recharg-
ing must always fall far short of the energy expended
on them.
3. Origin of the Voltaic CelL — As early as 1767 Sulzer
announced to the Berlin Academy of Science the dis-
covery that a peculiar taste is perceived when two
different metals are placed together on the tongue and
brought into contact at their edges. Such a combina-
tion of two metals, as copper and silver, and the saline
INTRODUCTION. S
saliva constitutes, as we now know, a voltaic couple. But
the significance of Sulzer's observation was not appre-
ciated till more than thirty years later, when Galvani
had made his capital discovery (1786) that freshly pre-
pared frogs' legs, hung by a copper wire on an iron
balcony railing, twitched convulsively whenever the
frog touched the iron ; and Volta had demonstrated
that the effect was not due to
animal electricity, but to the
two metals ; and that electricity,
identical with that excited by
friction, could be produced by
means of the metals without the
i^ncy of animal tirauea, nerves,
or muscles. Hence arose Volta's
contact theory of electrical ex-
citation. This ascribes what is
now called the difference of po-
tential exhibited by two metals
to their mere contact, independ-
ently of the medium in which
they are immersed. The reader
is referred to a later chapter for
a discussion of this subject.
4. Volta's Pile. — In pursu-
ance of his view of the origin
of the electricity producing the
muscular contractions of the ^ -voiia'a pub.
frog, and in order to increase
the electrical action, Volta constructed a chain of ele-
ments, to which be gave the name of artificial ei«otiia
oiBUt, but which has since been known as the Voltaic
pUe. It consisted of many discs of copper and zinc, or
4 PRIMARY BATTERIES.
preferably silver and zinc, either placed in contact or
soldered together in pairs, and piled up with interposed
layers of cloth moistened with pure water, or better, with
a solution of salt. An essential condition was that the
order zinc-copper-cloth, zinc-copper-cloth, must be main-
tained from bottom to top. Pig. 1 shows one of the
original forms of a voltaic pile. The discs were kept in
position by glass rods. The bottom disc of zinc was called
the negative pole, and the top one of copper the positive
pole. A pile composed of from twenty to forty such
pairs of plates produced appreciable physiological effects
when the experimenter touched the two poles with
moistened hands, or when the positive and
negative terminal wires were held in the
mouth or touched the eyes.
Volta's pile was the immediate forerunner
of his " crown of cups," which was the first
real voltaic battery. Each element of it was
called a galvanic element. Thus the names
of both Galvani and Volta became inseparably
associated mth this earliest device to produce
a. continuous flow of electricity.
5. The Dry Pile. — Following the principle
of Volta, Behrens constructed a pile, in which
" the moistened cloth was replaced with paper,
"'' and which was called, in consequence, a dry
pile, though it is inactive unless the paper
holds more or less moisture. Zamboni, who interested
himself in it and modilied it, gave to it the name of
Zamboni's pile. It was made of so-called gold and silver
paper, the former being coated on one side wifli copper
foil, and the latter with tin. The pairs were made of
small discs of the coated paper, from ^ to 4 cm. in
INTRODUCTION. 6
diameter, placed together with their metallic sides out-
ward, and then pUed up to the number of many hun-
dreds in such a way that the copper of every pair was
turned in the same direction. The whole column was
then firmly pressed into a glass tube, varnished with shel-
lac, and finally closed with brass caps, as shown in Fig. 2.
Dry piles were made consisting of as many as 20,000
pairs of discs. These were capable of charging a thin
Leyden jar of 350 sq. cm. surface, in ten minutes, to
such an extent that the discharge melted 2.5 cm. of
platinum wire 0.05 mm. in diameter.
The dry pile has been applied to the construction of
a device for the continuous motion of a light insulated
carrier, called an electric pendulum, or perpetual motion.
Two columns, 8 and S\ Fig. 3, of about 2000 pairs each,
are placed so that the positive pole of one and the nega-
tive of the other are uppermost. The lower poles are
then connected metallically by a wire tw, and the whole
is placed on an insulating stand. The small metal ring
r is attached to a glass rod forming the upper part of
the pendulum, which is supported on a knife edge at a,
and has a device at h for adjusting the centre of gravity,
which is made to assume a position slightly above the
point of support. The pendulum, therefore, inclines
toward one side, receives a charge from the pole touched,
is repelled, and carries its charge over to the opposite
pole, by which it is neutralized, and has given to it a
charge of the opposite sign. It then reverses its motion
toward the pole first approached ; and this action is
repeated indefinitely.
Such a pendulum has been in continuous motion, it is
said, in the University at Innsbruck since 1823.^ The
1 MuUer's Lehrbuch der Physik, Vol. III. p. 249.
6 PSIMARY BATTERIES.
period of oscillation changes within limits with the
humidity of the atmosphere. -^
The energy expended by the moving system is exceed-
ingly small, and is at the expense of the internal chemical
energy of the pile, which is necessarily limited.
Tig. 8. —Electric Pendulniu,
The dry pile has been applied in a similar way to the
construction of a delicate electrometer for the detection
of minute charges of electricity on a piece of gold leaf
suspended between the poles ; or for keeping charged
the pairs of quadrants of an electrometer, similar in
principle to Sir William Thomson's.
THE SIMPLE VOLTAIC CELL.
CHAPTER 11.
THE 8IMFI.B VOLTAIC CELL.
6. Fnndameiital Fhenomena. — 1£ a strip of pure zinc
is placed in sulphuric acid, diluted with from fifteen to
twenty times its volume of water, hubbies of hydrogen
may be seen to collect on the zinc, but the chemical
action soon ceases. If now a strip of copper is placed
in the same solution with the zinc, no change is observ-
able so long as the two metals are kept out of contact ;
but as soon as they are made to
touch each other, or are con-
nected together by means of a
wire or metal strip (Fig. 4), vig-
orous chemical action is set up,
the zinc is attacked by the acid,
attd hydrogen gas is liberated in
abundance at the anrfaae of the
capper plate or strip. Thus, while
the chemical action takes place
apparently at the zinc, the gas-
eous product of the reaction appears only at the copper.
As soon as the connection between the two metals is
interrupted, the chemical action ceases, and hydrogen
is no longer disengaged.
If now the proper tests are applied, it will be found
that the energetic chemical activity, taking place while
the two metals are connected, is accompanied by the
8 PRIMARY BATTERIES.
passage of a current of electricity from the copper to
the zinc through the metallic connector, and from the
zinc to the copper through the liquid in which the plates
are immersed. The plates, the liquid, and the connect-
ing wire or other conductor constitute the electric cir-
cuit. The wire connected with the copper plate is called
the positive electrcde, and the other the negative. The
copper plate itself is called the negative plate, and
the zinc the positive plate. This is because it has
been demonstrated that zinc in contact with copper in
air, either directly or through an intervening metal,
assumes a positive charge of electricity, and the cop-
per a negative one.
Such a system of two different metals, immersed in a
liquid which acts chemically on one of them when the
circuit is closed, constitutes vjhfaFls known as a voltaic
cell or element. The positive luetal is usually zinc ; the
negative may be copper, silver, or platinum; while for
the exciting liquid water, salt water, sulphuric acid,
hydrochloric acid, or a caustic alkali may be used.
7. Theory of the Voltaic Element. — To make as siniple
a case as possible, let us suppose that the zinc OtMiCv
copper are immersed in dilute hydrochloric acid, every
molecule of which consists of one atom of hydrogen --
combined with one of chlorine (HCl).
Clausius supposed that in a liquid a continual inter-
change takes place between like atoms of different mole-
cules. Thus the hydrogen of any acid molecule of
hydrochloric acid is not permanently attached to the
chlorine of the same molecule, but is occasionally sep-
arated from it, and then combines with the free chlorine
atom of some other molecule. This interchange goes
on indifferently in all directions so long as no directive
'\}
■k
:«
,'t-
I-
(i
THE SIMPLE VOLTAIC CELL. 9
force is introduced from without. The theory of Clausius
is supported by certain facts of double decomposition
with strongly combined salts. When their solutions are
mixed, the interchange of atoms allows the formation of
weaker compounds ; and that such compounds do form
is proved by their appearing as a precipitate, if they are
sufficiently insoluble.
The chlorine and hydrogen atoms then interchange
frequently from njolecule to molecule at random ; and
while in the free state between successive pairings, each
hydrogen atom carries a charge of positive electricity,
and each chlorine atom an equal charge of negative.
If now we assume a chemical attraction between the
zinc and the chlorine atoms, or imagine with Helmholtz
that both zinc and copper have an attraction for the
negative charge of the c' Torine atoms, the zinc superior
to the copper, then it will follow that when the zinc and
copper are immersed in the liquid, an extraneous force
has been introduced among the chlorine atoms, so that
their molecular interchanges are constrained to take
place in the direction of the zinc. They unite with the
zifec, giving up their negative charge, till this action is
arrested by the repulsion between the negative charge
accumulated on the zinc and that of the free chlorine.
Only incipient chemical action can therefore take place
till electrical connection is made between the charged
zinc plate and the copper immersed in the liquid with
it. Negative electricity then flows toward the copper,
through the connecting conductor, and unites with the
positive charge of the hydrogen atoms which move
toward the copper plate to meet the negative current.
The hydrogen gas thus escapes at the copper plate ; a
procession of hydrogen atoms moves steadily in that
10 PRIMARY BATTERIES.
direction, either directly or, with greater probabiKty, by
successive molecular interchanges ; and the separated
electrical charges are reunited through the connecting
electrical conductor. When the circuit is interrupted,
the charges which quickly accumulate check the move-
ment of the disengaged atoms by repulsion of like
charges, and all chemical activity ceases.
The condition assumed when the circuit is open is
one of electrostatic equilibrium. The chlorine atoms
continue to unite with the zinc and to deliver to the
zinc plate their negative charge, till the repulsion be-
tween the negative charges of the zinc and of the
momentarily free chlorine atoms equals the chemical
attraction between the zinc and chlorine. The two
electrodes will then be oppositely charged, and will
exhibit a difference of potential dependent upon a
number of conditions to be described later.
8. Chemical Eeaction in the Simple Voltaic Cell. — If we
suppose that the arrangement of metals and acid in the
cell is as follows, —
Zn I H2SP4 I H2SO4 I Cu ,
Zinc Sulphuric Acid Sulphuric Acid Copper
then the operation which repeats itself over and over
when the two metals are electrically connected may be
represented thus, —
givmg
Zn I H2SO4 I H2SO4 I Cu,
» ^ ' V ^ >
» >
ZnSOi I H2SO4 I Hj I Cu .
Zinc Sulphate Sulphuric Acid Hydrogen Copper
The arrow represents the direction of the current
through the cell. The zinc and hydrogen are both dis-
THE SIMPLE VOLTAIC CELL. 11
placed in the direction of the current, while the so-called
"sulphion," or SO4 part of the acid, is displaced in the
other direction. All metals and hydrogen are electro-
positive, and travel in an electrolyte with the positive
current. Zinc sulphate is formed at the expense of
zinc and sulphuric acid, and hydrogen gas is set free
at the copper plate. The simple chemical action taking
place is the displacement of the hydrogen of the acid
by zinc, forming zinc sulphate in place of hydrogen
sulphate.
9. Inconstancy of the Simple Voltaic Cell. — If the cir-
cuit, consisting of zinc, dilute acid, copper, and con-
necting wire, is kept closed for some time, the electric
current will rapidly decrease in intensity, the chemical
action wiU diminish, and, if the connecting wire offers
but little electrical resistance, the action in the cell will
shortly cease altogether. This diminution of activity is
due to several causes. The chief one is the accumula-
tion of hydrogen on the copper plate, causing what is
known as the polarization of the cell.
The flow of the current is ascribed to what is called
the electromotive force (E.M.F.), and by Ohm's law the
strength of the current is the quotient of this E.M.F.
and the resistance offered by the entire circuit to the
flow of electricity. Any condition operating to decrease
the E.M.F., to increase the resistance, or to do both,
will cause the current to diminish in intensity. Now
the hydrogen on the copper plate sets up an inverse
E.M.F., so that the effective E.M.F., producing a cur-
rent, is diminished by the value of this inverse one.
Returning to the theory of the cell, it will be readily
seen that both the hydrogen collected on the copper
plate 9.nd the zinc will attract the free chlorine atoms.
12 PRIMARY BATTERIES.
Thus the chlorine atoms will be solicited to cany their
negative charge in both directions, and the effective
impulse will be the difference of the two.
The hydrogen also increases the internal resistance
which the cell offers to the passage of electricity, since
by its accumulation on the plate a smaller metallic sur-
face is actually in contact with the liquid.
Independently of the hydrogen, the E.M.F. decreases
because of the exhaustion of the acid and the increase
in density of the zinc sulphate. Furthermore, when the
zinc sulphate in solution reaches the copper plate by
diffusion, some of it is liable to be decomposed by the
freshly liberated or nascent hydrogen. The zinc is then
deposited on the copper, the hydrogen taking its place
and forming sulphuric acid. Thus —
Ha + ZnS04 = Zn + H2SO4.
When the copper has received a coating of zinc, the
two plates are electrically the same, and all action
ceases. Because of these faults the simple voltaic cell
is of little or no practical value.
10. Experiments on the Polarization of a Simple Cell. —
Place enough mercury in a quart jar to cover the bot-
tom, and hang near the top of the jar a piece of zinc.
Fill up the jar with a nearly saturated solution of salt
water, and place the exposed end of a wire, insulated
with gutta percha, in the mercury, the upper end form-
ing the positive pole of the battery. If now the circuit
is closed through some simple current indicator, such as
a common telegraph sounder, of a few ohms resistance,
the armature will at first be drawn down strongly; but
in the course of a few minutes, the time depending
upon the total resistance of the circuit, the armature
THE SIMPLE VOLTAIC CELL,
13
will be released by the magnet, and will be drawn up
by the retractile spring. Polarization has then pro-
ceeded so far that the current is insufficient to operate
the instrument.
Next take a small piece of mercuric chloride (HgCU)
no larger than the head of a pin, and drop it in on the
surface of the mercury. It will set up a spinning move-
ment along the mercurial surface, and the sounder
armature will be at once drawn down, indicating that
the current has recovered its initial value. The mer-
1.0
>
O.R
V.
—
-
-
S
I
iAMjO 20 40
Fig:. 5. — Polarization Curve of Simple Cell.
60
curie chloride furnishes chlorine for the removal of the
hydrogen, and so reduces the polarization. In a few
minutes the chloride will be exhausted, and polarization
will again set in. The introduction of a little mercuric
chloride will again restore the cell to activity.
A graphical representation of the progress of the
polarization in a simple voltaic element is shown by
the curve of Fig. 5. A plate of clean zinc and one of
14 PRIMARY BATTERIES.
clean copper were immersed in dilute sulphuric acid,
specific gravity 1.06. The plates were 5 cms. apart, and
96 sq. cms. surface on each plate were under the liquid.
The ordinates of the curve denote the total E.M.F. at
intervals of time indicated by the abscissas. The first
observations were taken at as short intervals as possible,
but after the first few minutes they were less frequent,
as the change in the E.M.F. was only slight. The ex-
ternal resistance was 20 ohms.
With a smaller external resistance the polarization
curve is still steeper during the first half-minute, and
in the same time the E.M.F. falls to a still lower level.
ELECTROMOTIVE FORCE.
15
CHAPTER III.
POTENTIAL AND ELECTROMOTIVE FORCE.
11. Electric Potential. — Electric potential is defined
in terms of work, and work done is the measiu'e of the
energy expended or transformed. It is sufficient for
purposes of current electricity
to define the difference of po-
tential between two points. It
is numerically equal to the
work done in carrying a unit
of electricity in the positive
direction from one
point to the other.
Thus in Fig.' 6 the
potential differ-
ence between the
terminals A^ B^ of
Fig. 6.-Simple Battery Circuit ^^^ battery is the
work required to transport a unit quantity of electricity
from A round through the external resistance R to the
point B. In general it is not the same as the work
done in carrying the unit of electricity from B \jo A
through the internal resistance r of the cell, from the
negative to the positive terminal.
The unit employed in this definition is the " absolute "
or centimetre-gramme-second (C.G.S.) unit of quantity,
which is ten times the practical unit, called the coulomb.
16 PRIMARY BATTERIES,
A point is said to have the practical zero of potential
when it is the same as that of the earth.
Since difference of potential is the work done on unit
quantity, the total work done when any quantity Q is
transferred from one point to the other is Q times the
potential difference between the points. This remains
true whether all the energy expended in the transfer is
converted into heat because of the ohmic or frictional
resistance JB; or whether a portion is converted into
mechanical work by means of an appropriate motor
device inserted in the external circuit ; or whether the
energy is in part stored up by means of electrolysis, as
in a secondary battery ; or in producing a magnetic field.
The work done in one second on any portion of a cir-
cuit, included between two points, is the product of the
current and the potential difference, both in C.G.S.
units. The work is expressed in ergs.
It is important to note that the portion of the circuit
between the two points considered must not include
any source of positive E.M.F. ; that is, an E.M.F.* act-
ing in the direction of the positive current flow.
12. Positive and Negative Work. — Work done upon
the current, or work done in producing a current, is to
be considered positive ; while work done by the current
is negative. Where the work has the positive sign,
energy in some other form is converted into the energy
of an electric current ; but when the work is negative,
the energy of the electric current is in general expended
in heating the circuit, in doing mechanical work, or in
effecting chemical dissociation. In the voltaic element
the energy of chemical separation is transformed into
that of the electric current. The same is true of a sec-
ondary battery during its discharge.
ELECTROMOTIVE FORCE, 17
In the dynamo-electric machine the power expended
in driving the armature is largely reproduced in the
energy of the currents traversing it.
13. Electromotive Force. — Electromotive force is the
name given to the cause of an electric flow. It is now
often called electric pressure frqm its superficial analogy
to water pressure. The origin of the E.M.F. of a vol-
taic battery is in the superior affinity of zinc for oxygen
as compared with copper. If equivalent weights of
zinc and copper are oxidized, the heat of combustion
is found to be 85,400 and 37,200 calories respectively.
That is, the oxidation of 65 gms. of zinc and 63.4 gms.
of copper, requiring equal weights of oxygen, will pro-
duce enough heat to raise the temperature of 85,400
and 37,200 gms. of water 1° C. respectively. The strain
of the oxygen atoms toward zinc is more than twice as
great as toward copper. This strain need not extend
to a greater distance in a liquid than the "molecular
range," which Quincke has calculated to be about one
ten-millionth of a millimetre, or one five-hundred-thou-
sandth of an inch. As fast as the oxygen is exhausted
from the layer of liquid in immediate proximity to the
zinc, diffusion supplies the waste. The heat of formar
tion of equivalent weights of zinc and copper with chlo-
rine is 97,200 and 51,600, calories respectively. With
chlorides, therefore, zinc is still the positive plate, and
copper the negative. If platinum is made to replace
copper, the negative strain on the oxygen or chlorine
atoms is reduced nearly or quite to zero, and the E.M.F.
of the combination is accordingly increased.
The E.M.F. of any form of battery depends, therefore,
on the materials employed, and is entirely independent
of the size and shape of the plates. The condition of
18 PRIMARY BATTERIES,
the surface of the plates and the density of the solution
or solutions also affect the value of the E.M.F. Thus
oxidation of the copper plate increases the E.M.F.,
while oxidation of the zinc plate decreases it. This
result is easily explained in accordance with the theory.
The oxygen on the copper plate serves to remove
the nascent hydrogen, thus obviating polarization. On
open circuit the hydrogen is then attracted toward the
copper oxide, and the oxygen toward the zinc. Both
operations facilitate the electric separation and transfer
of charges in opposite directions.
The view here adopted is that the effective E.M.F. of
a primary battery is at the contact of the zinc and the
exciting liquid rather than at the contact of zinc and
copper.
14. Eolation of Electromotive Force to Difference of
Potential — The two expressions are not synonymous,
neither are they always interchangeable. E.M.F. estab-
lishes difference of potential rather than the reverse.
This is evident from the fact that there may be a current
without any difference of potential between successive
points in a circuit, but not without an E.M.F. Such
would be the case if a straight bar magnet were thrust
through a perfectly uniform circle of wire along the
axis of the ring. An induced current would flow along
the ring during the motion of the magnet. Every part
of the wire would cut equally lines of force, but all
points would have precisely the same potential if meas-
ured by an electrostatic voltmeter of small capacity.
The difference of potential between two points is,
however, numerically equal to the effective E.M.F. pro- I
ducing a current from one point to the other when the I
circuit between the points contains no source of B.M.F. j
ELECTROMOTIVE FORCE. 19
In such a case the current flows from the place of higher
potential to that of lower, and the loss of potential is
proportional to the resistance passed over. Thus in
Fig. 6 (p. 15), A has a higher potential than B^ and
the current flows in the external circuit from the higher
potential to the lower. Moreover, the difference of po-
tential between A and B is equal to that part of the
total E.M.F. of the cell which will produce the given
current through the resistance R between the two
points. The loss of potential in passing over different
portions of this conductor is strictly proportional to the
resistance of the several portions.
If, however, we direct our attention to the interior of
the cell, we find that the current flows across from the
zinc to the liquid, or from lower to higher potential.
It is so impelled by the vera causa there acting to pro-
duce an electric flow. This cause, which is called an
electromotive force, may be compared to a pump which
lifts water against gravity ; while in the remainder of
the closed system of pipes, conveying the water, the
liquid flows back again by gravity. It is convenient,
therefore, to divide an electric circuit into two regions,
one containing the source or sources of E.M.F., and
the other containing none. Within the latter region
the cui-rent flows from higher to lower potential, and the
loss of potential is proportional to the resistance passed
over. Within the other region, or at some points in that
region, the current passes from lower to higher poten-
tial, and the change in potential bears no relation to the
resistance. In all cases, however, the loss or shrinkage
of potential, due to ordinary ohmic resistance, is propor-
tional to the resistance passed over. The change in
E.M.F. in passing over any resistance is the loss due to
20 PRIMARY BATTERIES,
this resistance, added to all the E.M.F.'s encountered,
taken with their proper sign.
15. Belation of Potential Differences to External and In-
ternal Resistance. — It will be useful to consider atten-
tively the distribution of potential throughout the
circuit of a simple cell containing no source of coun-
ter E.M.F.
If the circuit is open so that the external resistance
is infinite, then the potential difference between the two
electrodes is the total E.M.F. of the cell. Under these
condition^ the internal resistance of the cell is zero in
comparison with the external resistance. Hence the
total fall of potential is through the air from one ter-
minal to the other. If now the external resistance
is gradually diminished, the potential difference between
the two poles of the battery becomes less and less, the
E.M.F. of the battery remaining constant. If E is the
total E.M.F., E the fall of potential between the ter-
minals of the cell, and e the loss due to the resistance
of the battery itself, then
E^ ^ E - e, OY E = E' -k- e\
also
E : e II R:r.
If now the poles of the cell are connected by a stout
conductor of negligible resistance, then E becomes
zero, and e equals E, In other words, the total loss of
potential is then entirely internal.
If we suppose the seat of the E.M.F. at the surface
of the zinc, neglecting the negative E.M.F. at the other
plate, then the zinc and connected conductors are at
the lowest potential, a sudden rise occurs in passing
from the zinc to the liquid, and there is a gradual fall
ELECTROMOTIVE FORCE. 21
through the liquid to the negative plate. If the inter-
nal resistance is increased, the slope of potential per
unit of resistance is diminished, but the total loss
through the electrolytic conductor remains the same,
and equals the E.M.F. of the cell. It is immaterial
whether the two plates with the connecting conductor
are partly or wholly immersed in the conducting liquid.
16. Volta's Contact Force. — The muscular convulsions
which were observed when the lumbar nerves and the
crural muscles of a frog were connected with a bimetal-
lic arc of iron and copper, Galvani attributed to a sep-
aration of the two electricities at the junction of nerves
and muscles. Volta showed that no effect was obtained
with a continuous wire of a single metal ; he therefore
attributed the effect to the contact of dissimilar metals.
After the invention of his pile in 1800 another theory
arose, which assigned chemical action as the origin of
the E.M.F. In Volta's pile the water moistening the
cloth discs was said to be the exciting liquid oxidizing
the zinc. Volta assigned to it the function of a con-
ductor only. In pursuance of his theory, Volta invented
a condensing electroscope with one plate of polished
copper and the other of polished zinc. When the zinc
plate was placed on the copper and then deftly lifted
by means of an insulating handle, the gold leaves of the
electroscope diverged with negative electricity.
In recent times Sir William Thomson has illustrated
the Volta effect, as this has been called, with the appa-
ratus shown diagrammatically in Fig. 7. It consists of
two half-rings of zinc (Zn) and copper (Cu), placed on
insulating supports in the same plane, with a narrow
space between their ends. A light aluminum needle
is suspended so as to turn freely round the axis of the
22
PRIMARY BATTERIES.
Fig. 7.
Thomson's Contact Apparatus.
ring. It is adjusted to hang over one of the spaces
between the zinc and copper.
If now the needle is charged
to a high potential with positive
electricity, it will turn toward
the copper in the direction of
the arrow whenever the two
half-rings are metallically con-
nected at AB, If the needle
is negatively charged, it turns
towards the zinc. This motion
may be interpreted as meaning
that the zinc is charged posi-
tively, and the copper negatively.
It also means that there is a fall of potential in the air
from the zinc toward the copper, for the positively
charged needle moves in the direction of lower electric
potential. It has been supposed by many to demon-
strate that the seat of E.M.F. in a voltaic cell is at the
contact of the zinc and copper.
17. Explanation of the Volta Effect. — The positive and
negative charges exhibited by zinc and copper in con-
tact in air may be explained as a simple variation from
the ordinary voltaic element. They constitute an air-
battery, with the plates immersed in a dielectric or
non-conducting fluid ; while the plates of the latter are
immersed in an electrolytic conductor. But in each
case the fluid bathing the plates acts chemically on both
of them. The oxj^gen is attracted bj'- the zinc and cop-
per both, but unequally ; and the effective E.M.F. is a
differential result of the two chemical actions. Insu-
lated zinc is at a potential of about 1.8 volts lower than
the air, while insulated copper is only 0.8 volts lower.
ELECTROMOTIVE FORCE. 2S
these values being proportional to the heat of formation
of ZnO and CuO. When the two metals are brought
into contact, their potential becomes the same through-
out ; the equalization is brought about by an exchange
of electricities, the zinc receiving a positive charge, and
the copper a negative one. Their mean potential is
then about 1.3 below the average potential of the air.
But the normal difference of potential between each
metal and the air in the immediate vicinity remains the
same as before contact. Hence there is a slope of poten-
tial from the air next to the zinc to the air next to the
copper of about one volt ; and it is this slope of poten-
tial which is indicated by the movement of the needle
in the Thomson instrument.
The relation of the air voltaic battery to the liquid
voltaic battery may be illustrated in a different way.
It will be recalled that on open circuit or with infinite
external resistance, the potential difference between the
zinc and copper is equal to the total E.M.F. of the bat-
tery. The copper has then a positive charge, and the
zinc an equal negative one, the potential sloping from
the positive to the negative. But if the metals are
brought into contact, their potential is equalized, and
the extreme potential difference is then between the
liquid in contact with the zinc and that in contact with
the copper, the former being the higher. The plates
have no charge, because as fast as oxygen (or chlorine)
brings negative to the zinc, and hydrogen brings posi-
tive to the copper, both charges are conveyed away by
the conductor. This slope of potential in the fluid
bathing the plates coexists with their uncharged state
only when there is an incessant transfer of electricity
throughout the entire circuit.
64 PRIM Any BATTERtES.
If now air replaces the liquid, the plates remaining in
contact, and hence at the same potential, the internal
resistance is infinite, the total E.M.F. is the difference
of potential existing in the air surrounding the plates,
and the plates acquire a charge, since no current, is
established. But since in the interior of a battery the
current direction is from zinc toward copper, the slope
of potential is in this same direction ; therefore the zinc
is positively charged, and the copper negatively.
To sum up: There are two paths between the zinc
and copper plates, the external portion of the circuit
and the internal. The plates are charged with electric-
ity corresponding to the whole difference of potential
of the battery only when one of these resistances or the
other is infinite. When the external resistance is infi-
nite, and the embracing fluid is an electrolytic conduc-
tor, the potential slopes from the copper to the zinc,
from the positive charge on the copper to the negative
charge on the zinc.
When the internal resistance is infinite (air), the
plates being directly connected, the slope of potential is
from the layer of air in contact with the zinc to the
layer in contact with the copper through the non-elec-
trolytic medium; while the zinc assumes a positive
charge, and the copper a negative one, since in no other
way can their potentials be equalized.
With two couples of zinc and copper plates in con-
tact, one pair immersed in a conducting liquid and the
other in air, the potential in both cases slopes from the
zinc toward the copper through the medium ; but in
the former there will be a dynamic current, and in the
latter only a slight electrostatic displacement sufficient
to charge the plates. The displacement in the one is
ELECTROMOTIVE FORCE. 26
continuous, in the other momentary. . The seat of the
electromotive force in either case is at the contact of
the metals with the medium, rather than at their con-
tact with each other. This is the more apparent from
the fact that when zinc and copper in contact are placed
in an atmosphere of sulphuretted hydrogen, the zinc
acquires a negative charge, and the copper a positive
one. In this case the chemical action on the copper is
greater than on the zinc, and the electrical conditions
are reversed as compared with the same metals in air.
Similarly, iron and copper in sulphuric acid give a cur-
rent from copper to iron through the external conduc-
tor ; but in a solution of potassium sulphide the current
is from iron to copper.
It is not intended to assert that there is absolutely no
true contact force at the junction of two different metals.
There is such a contact E.M.F. or potential difference,
but it is of very small magnitude, and the evidence of its
existence is very different from that furnished by the
simple voltaic element. This evidence is furnished by
what is known as the Peltier effect. It is a reversible
heat phenomenon. The passage of a current through
a homogeneous conductor produces heat irrespective
of the' direction of the current. But when a weak cur-
rent is made to pass across a junction from copper to
iron, the junction is cooled. This is due to a true con-
tact E.M.F. which helps forward the current. Positive
work is done at the junction, and energy in the form of
heat is absorbed. When the current passes in the
opposite direction across the junction, heat is produced
additional to that depending upon ordinary ohmic
resistance. The same reversible heat production may
be observed at the junction of other metals and of dis-
26 PRIMARY BATTERIES.
similar substances. But in any case the contact E.M.F.,
which explains the reversible heat, is at most only a
few hundredths of a volt ; it is included in the result-
ant electromotive force of a voltaic element, but it is
altogether insignificant in comparison with that due
to chemical agency.
CLOSED CIRCUIT BATTERIES, 27
CHAPTER IV.
CLOSED CIRCniT BATTERIES.
18. Distinction between Open and Closed Circuit Bat-
teries. — It has been seen that the inconstancy of the
current furnished by a battery through a fixed resist-
ance is largely accounted for by polarization, due to
the liberated hydrogen. The agent introduced into the
cell to avoid polarization, either by removing the hydro-
gen as fast as it is formed or by preventing altogether
its disengagement, is called a depolarizer. The distinc-
tion between open and closed circuit batteries depends
chiefly upon the nature and action of this depolarizer.
A battery is entitled to be included in the closed cir-
cuit type only when it is capable of working on a closed
circuit of moderate resistance for a considerable period,
with but slight diminution in the intensity of the cur-
rent. It is thus clearly differentiated from those cells
that are adapted to stand on open circuit, without
wasteful local action, and to furnish current only at
intervals, and of a few seconds duration.
In a closed circuit cell the depolarizer must act with
sufficient promptness and efficiency to prevent polariza-
tion quite completely, thus removing this cause of the
decrease in the current.
In open circuit batteries the depolarizer may indeed
be entirely absent, or it may act with so much sluggish-
ness that it cannot prevent polarization taking place to
28 PRIMARY BATTERIES,
some extent during the action of the cell, but it destroys
polarization after the circuit has been again opened.
The promptness with which a cell recovers from a
depression of its E.M.F. by polarization is a good cri-
terion of the elEcacy of this class of depolarizers.
Batteries provided with such depolarizers occupy an
intermediate position between those with a prompt act-
ing one and those with none, of which the simple voltaic
element is the type.
The more eflBcient depolarizers in general are liquid ;
the less efficient or slower acting ones, with only a few
exceptions, are solid. The first class must be employed
when a continuous current is required, especially if the
current is of considerable magnitude. If but a small
current is taken from a cell through a high resistance,
then a solid depolarizer will suffice. But batteries with
no depolarizer for the removal of hydrogen, or an equiv-
alent, are adapted only to open circuit use, in which
the circuit is to be closed for only a few seconds at a
time.
19. The Daniell Battery. — The first constant battery
was invented by Professor Daniell, of Ediaburgh , in>CN4./M«
1836. To prevent the disengagement of hydrogen at
the copper plate, it is immersed in a solution of copper
sulphate (CUSO4). The nascent hydrogen then decom-
poses the CUSO4, the result being the formation of
sulphuric acid (H2SO4) and a deposit of metallic copper
on the copper plate.
One form of the cell is represented in Fig. 8. (7 is a
cleft cylinder of copper, and Z one of zinc. Between
the two is a porous cup of unglazed earthenware, so
that a continuous liquid circuit is maintained between
the zinc and the copper. The zinc is immersed either
CLOSED CIRCUIT BATTERIES. 29
in dilute sulphuric acid, or better, in a weak solution of
zinc sulphate ; while the copper is surrounded by the
solution of copper sulphate contained in the porous cup.
Crystals of copper sulphate are shown surrounding the
copper cylinder. These are held in a copper wire or
perforated basket, and are for the purpose of keeping
the solution of the copper salt saturated. The porous
Eif. 8. — DuUeU CeU.
cup serves no purpose except as a partition to separate
the hquids surrounding the two plates. Each metal
is placed in a salt of itself.
The more recent forms of this battery have a zinc
prism and the zinc sulphate in the porous cup, while
the sheet copper and the copper sulphate solution are
outside. The action in either case is the same.
80 PRIMARY BATTERIES.
20. Chemical Keactions in the DanieU CelL — With
acidulated water the chemical action may be represented
as follows : —
Zn. I H2SO4 I H2SO4 II CUSO4 I CUSO4 I Cu^
^ V ' ^ ^ '
>^ >
After the first step in the reaction this becomes —
Zn._, I ZnS04 I H,S04 || HsS04 j CuSO* | Cu,+i.
The arrow indicates the direction of the current, and
the porous partition is represented by the double verti-
cal line. The hydrogen and the metallic elements all
migrate in the direction of the current from the zinc
toward the copper plate; ZnS04 is formed at the ex-
pense of CUSO4; metallic zinc disappears, and metallic
copper is deposited on the copper plate. The hydrogen
is intercepted by the CUSO4 and never reaches the nega-
tive plate.
If the zinc is immersed in dilute zinc sulphate instead
of acidulated water, the electrolytic circuit, prior to the
first step in the chemical reaction, is as follows : —
Zn. I ZnS04 | ZnS04 || CUSO4 | CUSO4 | Cu^.
V ^ ' > ^ '
>^^ >
After the first step : —
Zn,_i I ZnS04 | ZnS04 || ZnS04 j CUSO4 | Cu,+i.
The action taking place is a very simple one. There
is, as before, a decrease of metallic zinc and an increase
CLOSED CIRCUIT BATTERIES.
31
of metallic copper, as indicated by the subscripts ; zinc
crowds copper out of the copper sulphate, so that there
is a continuous transformation of CUSO4 into ZnS04 by
this process of replacement.
The E.M.F. of a Daniell cell, as ordinarily set up, is
about 1.08 volts. The curves in Fig. 9 express the
results of a test made on a Daniell cell set up with
s._
I.O
0.5
•
1
>
Min. 20 40
Fig. 9. — Polarization Curvea of Daniell Cell.
60
saturated copper sulphate and a 5 per cent zinc sulphate
solution. The zinc was amalgamated and the copper
carefully cleaned. The external resistance was 5 ohms
and the internal 0.85.
The upper curve represents the total E.M.F. at small
intervals of time, which are laid off horizontally as
abscissas, the E.M.F.'s being laid off on the vertical lines
as ordinates. The ordinates of the lower curve denote
the values of the potential differences at the terminals
or electrodes of the cell for the same period of one hour.
32 PRIMARY BATTERIES,
This potential difference is the efPective E.M.F. pro-
ducing the current through the external resistance
of 5 ohms. It is then only necessary to divide this
terminal E.M.F. by five to obtain the current in
amperes.
These curves should be compared with the polariza-
tion or E.M.F. curve of Fig. 5. They serve to bring
out in a forcible manner the contrast between the rapid
polarization in a simple voltaic element and the prac-
tical freedom from polarization of a well-constructed,
clean Daniell cell. The contrast would have been still
greater if the voltaic element had been tested with the
same external resistance ; but it was not practicable to
make a satisfactory time test with an external resist-
ance of less than 20 ohms in that case, the polarization
being too rapid to follow it with accuracy.
21. Chemical Reactions of the Cell in Relation to Energy.
— The question has often arisen why any chemical
action should take place upon closing the circuit of a
Daniell cell, set up with zinc and copper in their respec-
tive sulphates. The answer involves an explanation of
the conversion of potential chemical energy into the
kinetic energy of dynamic electricity, or at least a
statement of the principle upon which this conversion
of energy is conditioned. It depends entirely upon
whether the heat of formation of the salt that can be
formed by the process of replacement is greater than
that of the salt or compound decomposed.
In the Daniell cell the heat of formation of equivalent
weights of ZnS04 and CUSO4 are 242,000 and 191,400
calories respectively. Hence for every 65 grms. of zinc
entering into combination as ZnS04, with the reduction
of 63.4 grms. of copper from CUSO4, the difference
CLOSED CIRCUIT BATTERIES. 33
between 242,000 and 191,400, or 60,600, calories of heat,
or the equivalent in the kinetic energy of an electric
current, must be developed. In the form in which the
materials are placed in the cell they represent, therefore,
potential energy.
Now potential energy always tends to become kinetic
whenever the conditions admit of the transformation.
The sole condition in the Daniell cell is that the circuit
shall be closed.
A continuous transformation then goes on, the kinetic
energy appearing in the form of an electric current
because of the special conditions determining the con-
version; and the process continues so long as there
is any available energy left to take part in the opera-
tion.
22. Local Action and Amalgamation. — Any chemical
action taking place in a cell on open circuit, tending to
reduce its available potential energy, or going on when
the circuit is closed and not contributing to the produc-
tion of the current, is called local action. Local action
is always prominent with commercial zinc in an acid
solution. The zinc contains foreign particles, such as
bits of iron, carbon, or other conducting bodies ; as soon
as these are exposed to the liquid, they form closed
local circuits, and the zinc is eaten away in patches, or
pits.
To prevent this wasteful action, the zinc is amal-
gamated. Alloys of mercury with other metals are
called amalgams. The process of amalgamation consists
in forming a zinc-mercury alloy on the surface of the
zinc plate or prism. This is best accomplished by first
cleaning the zinc by immersion in sufficiently diluted
sulphuric acid, and then rubbing mercuiy over the sur-
84 PRIMARY BATTERIES.
face by means of a swab made by tying a piece of cloth
round the end of a stick. All excess of mercury should
be allowed to drain off. If, however, the plates of zinc
stand out of the liquid for some time, the mercury will
largely separate, and collect in small globules on the
surface.
Another method of amalgamating zinc is to dip it in
an acid bath containing a mercury salt in solution. This
may be prepared by dissolving one part of mercury in
three parts by weight of aqua regia (one of nitric to
three of hydrochloric acid), and then adding three parts
more of hydrochloric acid.
There are other forms of local action which amalga-
mation does not prevent. Some of these will be more
specifically described in connection with the types of
batteries most unfavorably affected by them.
The zinc of a battery should always be amalgamated
when the exciting liquid is acid.
23. The Effect of Amalgamation. — The action of the
amalgam appears to be to bring to the surface pure zinc,
while foreign materials, especially iron, are left behind.
Amalgamated zinc, therefore, acts like pure zinc ; foreign
bodies, as soon as they are dislodged, fall to the bottom
of the cell ; and wasteful action, due to local currents,
is avoided. But amalgamated zinc possesses the singular
property of not being attacked when immersed in dilute
sulphuric acid. Since this is equally true of pure and
commercial zinc, the exemption of amalgamated zinc
from attack is not due to the suppression of local cur-
rents. The following facts tend to show that the pro-
tection of the zinc is to be ascribed to the adhesion of a
film of hydrogen to the amalgamated surface.
When amalgamated zinc is plunged in water, acidu-
CLOSED CIRCUIT BATTERIES. 35
lated with one-twentieth of its yolume of sulphuric acid,
it is not attacked at ordinary atmospheric pressure. But
if a vacuum is produced above the liquid, bubbles of
hydrogen are again freely evolved from the zinc surface.
Upon readmission of the air, bubbles again adhere to the
plate, and the chemical action is arrested.
If two plates of ordinary zinc, one amalgamated and
the other not, are immersed in dilute acid, the amal-
gamated zinc comports itself as the zinc, and the other
as the copper, of a simple voltaic couple. The amalga-
mated zinc is thus rather more readily attacked by tlie
acid than the unamalgamated.
With pure electrolytic zinc and neutralized sulphate
of zinc, there is no potential difference between two
plates, one of which is amalgamated and the other not.
24. Kelative Protection of Alloying and Amalgamating,
— The investigations of Reynier show that the protec-
tion secured by mercury is much greater than is gener-
ally supposed. In certain liquids the local waste of
amalgamated zinc is 50, 100, or even 10,000 times less
than that of ordinary zinc.
A further question is the relative value of alloying
with mercury as compared with amalgamating on the
surface. Reynier concludes^ that zinc alloyed with
mercury is, in general, better than zinc amalgamated,
especially in experiments of long duration.
The first superficial layer of amalgamated zinc is rich
in mercury ; but, as the deeper layers are attacked, the
proportion of mercury diminishes, and so also the protec-
tion obtained. The opposite takes place with the alloy,
which is visibly enriched in mercury as its weight
1 Pile Electrique, p. 21.
36 PRIMA RV BATTERIES.
diminishes. It is evident that on closed circuit the
superiority of the alloy shows itself after a much shorter
time. The alloys are more brittle than amalgamated
zinc, and they become more so by use, — a fact confirm-
ing the preceding observation.
The utility of amalgamating the zinc in batteries of
the Daniell type has often been contested. Experiment
demonstrates that the mercury reduces the loss by one-
half yi a solution of 15 per cent sulphate of copper.
In the alloys referred to the mercury constituted 4 per
cent of the entire mass.
In a chromic mixture, amalgamated zinc soon loses its
brightness, and takes on a dark tint, while the alloy be-
comes brighter and brighter up to complete exhaustion.
The employment of zinc alloys contributes to the
economy of batteries, and increases their constancy.
25. Defects of the Daniell Cell. — The Daniell cell has
several rather serious defects. A prominent one is that
the copper is sometimes deposited upon the porous cup
instead of the copper plate. This deposit grows in the
pores, fills them up, and finally cracks the cup and
renders it useless.
Again, the diffusion of the copper salt through the
porous cup, when the battery is not in action, brings it
in contact with the zinc ; a spontaneous displacement of
copper by zinc then takes place, equivalent to local
action. The copper separates in a finely divided state,
and is usually oxidized and deposited on the zinc as
black cupric oxide (CuO) ; hydrogen is at the same
time given off. If the zinc becomes thoroughly black-
ened in this way, it should be cleaned.
Because of this local action, the Daniell battery should
be taken down when not in use.
CLOSED CIRCUIT BATTERIES, 37
This reduction of copper and its subsequent oxidation
may be illustrated by placing a piece of zinc in a dilute
solution of copper sulphate. Immerse a large test-tube
filled with the solution so that its open end shall be
over the zinc. As it stands, gas will collect in the tube,
displacing the liquid, and the solution will at length
lose all its blue color. The black oxide of copper will
be found in the vessel, the solution will contain zinc
sulphate, and the collected gas will be found, upon test-
ing, to be hydrogen. With dense solutions spongy
copper will also be found mixed with, the oxide.
Another objection to the Daniell cell for some pur-
poses is its rather large internal resistance, considering
its low E.M.P. Only a moderate current, about an
ampere, can be taken from a Daniell cell as a maximum.
The internal resistance will depend upon the thickness
and quality of the porous cup, the size of the plates, and
the distance between them. The density of the solu-
tions affects the resistance in a minor degree.
26. The Effect of Temperature Changes on a Daniell
Battery. — Professor Daniell himself found that his
battery yielded a largely increased current when its
temperature was raised to 100° C. He attributed this
result to increased chemical activity. It is now known
that the E.M.F. of this cell changes but slightly with
rise of temperature, the decrease per degree Centigrade
being less than 0.015 per cent.
The most important effect of a rise of temperature of
the Daniell cell is the decrease in its internal resistance.
It is well known that the resistance of electrolytes
diminishes with increase of temperature, and that this
inverse relation between resistance and temperature
distinguishes electrolytic from metallic conductors, the
38 PRIMARY BATTERIES.
temperature coefficient of all metallic conductors being
positive, with one exception, — an alloy of f erro-man-
ganese and copper.'
Mr. W. H. Preece found' that when a Daniell cell
was heated from 0° C. to 100° C, ita resistance decreased
abruptly at firet, and afterwards more gradually, falling
from 2.12 to 0.66 ohms. This large decrease of resist-
ance accounts for the augmented activity observed by
Daniell, the external resistance in circuit having doubt-
less been small.
27. The Gravity Battery. — The gravity battery is a
simple modification of the Daniell, designed to avoid
the use of a porous cup. It takes its name from the
fact that in it the zinc and copper sulphates are sepa-
rated by their difference in den-
sity. One form of this battery is
shown in Fig. 10. The zinc is
suspended, by means of a stout
copper wire, from a brass tripod
resting on the top of the jar. Thin
sheets of copper, riveted together
and to the conducting wire, are
I placed in the bottom of the cell
' and surrounded with crystals of
., . ., copper sulphate, known commer-
cially as " blue stone or " blue
vitriol." The zinc easting is hung in a weak solution
of zinc sulphate from two and & half to three inches
above the copper plates.
The saturated copper salt has a density greater than
the dilut* zinc salt. It therefore remains in the bottom
' American Journal of Science, Vol. XXXIX. p. 471.
" Proceedings Koyal Society, Vol. SXXV. 1883, p. 48.
CLOSED CIRCUIT BATTERIES, 89
of the jar if it is not disturbed, except that it slowly
diffuses upward toward the zinc.
These cells should be set up with well-diluted zinc
sulphate, extending at least an inch below the. zinc. If
water and crystals of copper sulphate alone are used,
the cell will not work at first; and as soon as the
copper salt reaches the zinc, either by diffusion or stir-
ring, the zinc turns black from the oxidation of the
reduced copper, and stalactites will soon be found hang-
ing from the zinc.
When the cell is properly set up, with copper in
copper sulphate and zinc in zinc sulphate, the chemical
reactions are the same as in the Daniell cell.
If the cell is left standing on open circuit, the copper
sulphate diffuses upward, as already explained, and
wasteful local action takes place. Besides, the cell
becomes foul much more rapidly than if the copper salt
were not allowed to reach the zinc. Hence this cell
always keeps in better condition if a closed circuit is
maintained through a high resistance when the battery
is not in use. Zinc then replaces copper in the copper
salt as fast as it diffuses upward. The zinc sulphate
formed must be occasionally drawn off and replaced
with soft water. So, too, crystals of copper sulphate must
be added from time to time to keep the solution satu-
rated. Care must be taken not to allow these crystals
to lodge on the zinc. It is better to add small quantities
at frequent intervals than to place too large a supply in
the jar at once.
When the water evaporates, the zinc sulphate crystal-
lizes round the jar, and then creeps up by capillary
action, crystallizing as it ascends, till it finally flows
slowly over the top. As a preventive, the tops of the
40 PRIMARY BATTEHIES.
jara may be dipped in hot paraffin, or a atrip of very
adhesive tape may be pasted round the rim, inside and
out.
28. The Gethiiu Battery. — The inventor of this form
of copper sulphate cell has sought to combine the ad-
vantages of a Dauiell with those of a simple gravity cell.
The cupric sulphate is
placed round the sheet
copper in the bottom
of the jar, as in the
gravi^ form ; while
a porous cup, in the
shape of a frustum of
a cone, is hung in the
top of the jar by means
of a stout rim, as shown
in Fig. 11. The zinc
has a broad, heavy
foot, and stands in the
porous cup. About
four pounds of coarse
crystals of CuSOi are
placed in the bottom
of the jar, and the jar
is about half filled with
_ „ wat«r. The porous cup
PIb- 11. — The aothiDBBsttery- . , . . .
With the zmc is then
put in position, and a weak zinc sulphate solution is
poured in. The battery is then ready for use. Its
E.M.F. is slightly over one volt, and its internal resist-
ance three ohms. Hence only one-third of an ampere
can be taken from it, even on short circuit ; and none
of this can be utilized, but all is expended in internal
CLOSED CIRCUIT BATTERIES. 41
heat. For energy in the external circuit, there must be
external resistance in addition to the internal; and
hence the current will be smaller, unless several cells
are coupled in parallel.
Three of these cells in series will keep a storage
battery charged so that it will inin a phonograph as
much as is required for a private ofiSce. The storage
cell in the ease tried had thirteen plates, six positive
and seven negative, each 60 square inches in area. The
primary battery was kept constantly connected with the
secondary.
The diffusion of the zinc sulphate outward through
the porous cup is noticeably greater than that of the
copper sulphate inward. The level of the liquid outside
the cup rises till the difference in hydrostatic pressure
counterbalances the difference in diffusive tendency.
29. Delany'i Modified Oravity CelL — Cells of the
DanieU type, in which copper sulphate is the depolar-
izer, have been of such great ser- -
vice when small but constant cur-
rents are required, that a brief
description of the Delany modi-
fication seems desirable.
It is shown in Fig. 12. The
CuSO, is enclosed in a straw-
board box, and the zinc in a
paper envelope. The box pre-
vents the CuSO, dust from dis-
solving at once, and diffusing so
as to reach the zinc. The copper '■•b- >'■
sulphate solution gradually ap- ei"ny'" «''« "■
pears by transfusion through the strawboard. The
copper of the element consists of heavy wire wound in
42 PRIMARY BATTERIES.
vertical bands about the strawboard box, and an insu-
lated wire rises from this to the top of the cell.
The paper round the zinc prevents spongy copper or
other material falling upon the copper. It is claimed
that no stalactites depend from the zinc, and that the
deposit on the zinc is easily removed without hacking
or scraping. Ordinary gravity cells often need to have
this process vigorously applied to them.
A band of rubber cloth is attached by a sticky sub-
stance to the inside of the rim of the jar to prevent the
crystallized salts creeping over. It is said to present a
complete mechanical obstruction to the climbing of the
zinc sulphate. It may, of course, be applied to any
other jar, first making sure that the rim is thoroughly
clean ; then after warming the sticky side of the cloth,
press firmly all round against the rim.
30. Sir WiUiam Thomson's Tray Battery. — Another
form of Daniell cell was designed by Sir William
Thomson, with a view of diminishing the internal
resistance. The cell is made in the form of a large
wooden tray, about 20 inches square, lined with lead
on which copper has been deposited by electrolysis or
during the action of the battery. The lead extends over
the outside at the four corners and down under the bot-
tom, for the purpose of making contact with the next
cell below.
The zinc is in the shape of a grate, as shown in Fig.
13, which represents five cells in series. At the corners
are feet turned upward. The lead of the cell above
rests on the upturned feet of the zinc, making a good
electrical connection on account of the weight of the
cell.
Copper sulphate crystals are spread evenly over the
CLOSED CIRCUIT BATTERIES. 43
bottom of the tray, and the zinc is made to rest on four
blocks of paraffined wood at the corners. A parchment
diaphragm is sometimes placed above the copper sulphate,
and a dilute solution of zinc sulphate, density 1.10, is
poured on this till it covers the zinc.
These cells or trays may be piled up to the extent of
ten. The internal resistance may be as low as 0.2 ohm.
The circuit must be kept closed to prevent copper sul-
phate reaching the zinc. To secure a fairly constant
current, the density of the zinc sulphate must not be
allowed to greatly exceed 1.1. Some of the liquid at the
Vig. 13. — eir WllliHiu TboiiiKin'i TiBf Balterj.
top must be withdrawn daily, and soft water must be
added in its place.
Sir William Thomson's cell was originally designed
to work the siphon recorder in submarine telegraphy.
31. Grove'i Battery. — The Grove battery consists of
a cleft cylinder of zinc immersed in dilute sulphuric
acid (1 : 12), and a thin plate of platinum in strong
44 PRIMARY BATTERIES.
nitric acid (HNOs) contained in a porous cup. The
nitric acid is a powerful oxidizing agent ; and, in con-
sequence of this property, it acts as an efl&cient depolar-
izer by oxidizing the hydrogen. The nitric acid is
easily decomposed, and the nascent hydrogen readily
abstracts oxygen from it. The electric chain may then
be represented as follows : —
Zn. I HjSO^ I H2SO4 II 2HN08 | HJ^O, | Pt.
' y ' * V '
>^^ >
After the first step in the chemical reaction this
becomes —
Zn..a| ZnS04 | H2SO4 || 2HNO3J HNO3 I H2O | Pt.
On one side zinc sulphate is formed as usual at
the expense of zinc and sulphuric acid; while on the
other a molecule of nitric acid loses one atom of oxy-
gen, becoming nitrous acid (HNO2). As the action pro-
ceeds, the nitrous acid may lose another atom of oxygen,
hyponitrous acid (HNO) remaining. Or further, the
nitric acid may break up entirely, according to the fol-
lowing reaction : —
3H.t.HN08 = 2H20H-NO.
The products are water and nitric oxide. This last
is a gas which takes up more oxygen on escaping into
the air, forming the red fuming nitrogen peroxide, NOj.
These fumes are highly corrosive, and are the most
objectionable feature of the Grove cell. When a large
current is taken from a Grove battery, the nitric acid
has the appearance of boiling, on account of the rapid
disengagement of the nitric oxide. The acid is carried
CLOSED CIRCUIT BATTERIES. 46
off as a spray, corroding the metallic connections and
vitiating the air. This battery should therefore be
placed in the open air or in a strong draught, and the
donnectors should be frequently examined and cleaned.
The zinc cylinders must be kept well amalgamated,
and the platinum plates should be heated to redness
occasionally to prevent their becoming brittle from some
unexplained cause. These cells must be taken apart
and washed with an abundance of water every time
they are used.
They have the advantages of high E.M.F. and low
internal resistance. The former is from 1.8 to 1.9 volts,
and the latter is about 0.15 ohm, with a cell 20 cm.
high and 9 cm. in diameter. Such a cell is therefore
capable of giving 12 amperes on short circuit, or through
an external circuit of no appreciable resistance.
Before the introduction of dynamo-electric machines
and the storage battery, forty Grove cells, requiring
only seven or eight pounds of nitric acid, served the
writer for many years whenever a brilliant arc light was
needed or projection experiments in spectrum analysis
were performed.
When the nitric acid becomes dilute by the process
of decomposition in the porous cup, the reaction may
be quite different from that represented above. The
acid may give up its oxygen entirely, with formation
of nitrate of ammonium. The action may be represented
by the following chemical equation : —
2 HNOs + 4 Ha = 3 H2O + NH^NOj.
The presence of the salt of ammonia in an exhausted
Grove cell can be demonstrated by testing the liquid in
the porous cup for ammonia in the usual way, by heating
46 PRIMARY BATTERIES.
with powdered lime and water. A saturated solutioD
of ferric chloride, to which 4 per cent of nitric acid
has been added, has heen recommended as an excellent
substitute for nitric acid in a Grove cell. The E.M.F.
is then intermediate between that of a Grove and that
of a Daniell.
32. BiuiMn's Battery. — Soon after the invention of
the Grove battery, Bunsen modified it by substituting
a prism of baked carbon for the platinum. This is an
advantage in point of economy. The E.M.F. is slightly
less than that of the Grove. The usual construction of
the Bunsen cell is shown in Fig. 14.
The chemical action in the Bunsen battery is pre-
cisely the same as in the Grove. The hydrogen is
CLOSED CIRCUIT BATTERIES. 47
intercepted by the nitric acid, and is thus prevented
from reaching the carbon prism by oxidation.
Another modification of the Grove cell consists in
substituting an iron plate for the platinum in strong
nitric acid. On account of the passivity of iron in con-
centrated nitric acid, it does not dissolve; and it is
strongly electro-negative. When the acid becomes
weak, however, by the decomposition due to nascent
hydrogen, the acid attacks the iron with disengagement
of corrosive fumes. On this account iron is not used
in practice for the negative plate.
33. The Bichromate Battery. — If the bichi-omate of
potassium or of sodium in solution is treated with sul-
phuric acid, chromic acid is formed. This compound
(CrOs) is not only rich in oxygen, but it gives it up
readily to nascent hydrogen. Hence the application of
bichromates as depolarizers.
An ordinary Bunsen cell may be set up as a bichro-
mate cell by placing the amalgamated zinc cylinder in
dilute sulphuric acid as usual, and filling the porous cup,
holding the carbon prism, with a solution of the bi-
chromate salt acidulated with sulphuric acid. Or, since
both solutions contain sulphuric acid, the porous cup
may be dispensed with entirely, both the zinc and the
carbon being immersed together in the strongly acid-
ulated bichromate solution. In this case the zinc is
usually placed between two flat plates of carbon, an
arrangement adopted simply to reduce the internal re-
sistance of the cell. The E.M.F. does not differ materi-
ally from that of the Bunsen.
Fig. 15 represents one of the forms of this cell which
has been much used, though it is not to be recom-
mended. The zinc is attached to a rod, a, by means of
PRIMARY BATTERIES.
which it can be drawn up out of the
liquid when the battery is not in use.
The carbon plates are fastened to a
metallic clamp, which is attached to the
hard rubber top of the cell. The top
of the zinc is covered with an insulat-
ing strip to prevent direct contact with
the carbons.
Many forms of " plunge " battery for
bichromate solutions have been devised.
These are usually arranged as a battery
of four or more cells, with the zincs
and carbons suspended from a frame,
*' bv means of which they may all be lifted
iromate Cell. ■' , , , , , - i
out of the liquid together by a wind-
Such a battery is shown in Fig. 16. It is a very
Flu. I*. — Plunge B»tt«ry.
CLOSED CIRCUIT BATTERIES, 49
convenient form for experimental work in physical dem-
onstrations.
If the current falls off because of the exhaustion of
the liquid in contact with the plates, it may be increased
again by lifting the plates, by stirring the liquid, or by
blowing air through, as is done in the Byrne batteiy.
One inventor gives a slow motion to the carbon plates
by means of a small electric motor. Gendron has
recently described a bichromate cell, in which the zincs
can be easily replaced without interrupting the current.
By a system of automatic valves the exhausted liquid is
withdrawn at the bottom, while a constant level is main-
tained by the supply.
The initial E.M.F. of a bichromate battery is a little
in excess of two volts per cell.
34. Chemical Eeactions in the Bichromate Battery. —
When a solution of bichromate of sodium or of potas-
sium is treated with sulphuric acid, a purely chemical
reaction takes place, resulting in the formation of
chromic acid. Thus: —
NasjCrjOy + H2SO4 = Na^SO^ + HjO + 2 CrOg.
The chromic acid, CrOg, is the useful agent to effect
depolarization by the oxidation of hydrogen. The pro-
cess is supposed to be represented by the following
reaction : —
6 H -I- 2 CrOs 4- 3 H2SO4 = 6 H^O + 012(804)3.
The final result is, therefore, the production of the
sulphate of zinc (at the positive plate), the sulphates of
sodium and chromium, and water. It will be observed
that, while all the oxygen atoms of a bichromate of
sodium molecule unite with hydrogen to form water,
only three of the seven are concerned with the removal
60 PRIMARY BATTERIES.
of the hydrogen displaced by the zinc. The other four
oxygen atoms unite with the hydrogen coming from the
four molecules of sulphuric acid, which take part in
the reactions written above. Only threensevenths of the
oxygen contained in the bichromate salt, therefore, are
useful in removing the polarizing hydrogen; and for
every three parts of sulphuric acid which are supplied
to act on the zinc, four more must be added to decom-
pose the bichromate and to release oxygen.
When potassium bichromate is used, a double sulphate
of potassium and chromium, K,Cr2(S04)4, crystallizes
out of the liquid as soon as it becomes saturated with
these salts. This is known as chrome alum. The crys-
tals attach themselves in a compact mass to the bottom
of the jar, and are difficult of removal.
35. The Advantages of Sodium Bichromate over Potas-
sinm Bichromate. — The advantages arising from the use
of the sodium salt in place of the corresponding one of
potassium, appear not to have been appreciated till quite
recently. But the sodium salt is to be preferred for the
f oUowing reasons : —
First. It contains a larger percentage of available
oxygen. The molecular weight of sodium bichromate
is 262.4, and of potassium bichromate 294.6. The two
molecules contain the same weight of oxygen. For
equal depolarizing capacity, therefore, about 11 per cent
less of the sodium salt is required than of the potassium.
Unless the cost of the sodium salt is more than 10 per
cent higher than that of the potassium salt, the former
is the cheaper.
Second. It is much more soluble. The potassium
bichromate must be dissolved by the aid of heat, and
not more than about 100 gms. to the litre will remain in
CLOSED CIRCUIT BATTERIES. 51
solution when the liquid cools. The sodium salt dis-
solves in the cold, and in any quantity desired. A
denser solution can therefore be used with two distinct
advantages in this respect alone. The first one veiy
evidently is that the battery does not need to be re-
plenished with fresh solutions so frequently. The other
advantage is not so obvious, but it becomes apparent
when attention is drawn to the fact that there is no
liberation of gas in this battery to stir up the liquid ;
and the exhausted solution in contact with the cg,rbon
plates is replaced by fresh portions only by diffusion,
unless the liquid is agitated by lifting the plates or by
other mechanical means. The denser sodium bichromate
solution is not so soon exhausted of useful oxygen, and
will therefore maintain a large current with a smaller
rate of enfeeblement.
Third. The double sulphates of sodium and chro-
mium, if indeed they are formed at all, do not crystallize
out as in the case of the potassium chrome alum, but
remain in solution. The cells are therefore easily
cleaned.
36. Directions for Setting up a Bichromate Battery. —
For the solution, Bunsen recommends the following
proportions : —
Bichromate of potassium . . . 77.5 gms.
Sulphuric acid 78.5 c.c.
Water 750. c.c.
The bichromate must first be dissolved by heating the
water to boiling. Time will be saved by crushing the
crystals in a mortar before putting them into the water.
After the solution has cooled, the acid may be slowly
added. The acid should be poured into the water, and
52 PRIMARY BATTERIES,
not the water into the acid. After cooling again, the
solution is ready for use.
Reference to the chemical action of this battery shows
that for every molecule of K2Cr207 used, seven mole-
cules of H2SO4 are needed, provided the depolarizer is
entirely exhausted of its oxygen. The molecular weight
of K2Cr207 is 294.6, and the seven molecules of H2SO4
weigh 686. Hence, to find the weight of actual acid,
corresponding to 100 gms. of the bichromate, write the
proportion —
100 : aj : : 294.6 : 686.
Whence x = 232.8.
But sulphuric acid of density 1.8 contains 86 per cent
of acid. Hence about 271 gms. of 86 per cent acid are
required to furnish the 232.8 gms. of actual acid. This
is equivalent to 150 c.c, density 1.8.
But since the salt in solution cannot all be utilized to
effect depolarization, a residue always being left in the
spent liquor, the amount of acid may be reduced. It
is better to add a small quantity of fresh acid occasion-
ally rather than to supply too much at firat.
If sodium bichromate is used, 200 gms. may be dis-
solved in a litre of water, and to this should be added
150 c.c. strong acid. When the battery begins to show
signs of exhaustion, an additional 25 to 50 c.c. per litre
may be added. For complete exhaustion of the oxygen
from 200 gms. of sodium bichromate, about 600 gms. of
86 per cent acid would be required. This includes the
quantity necessary to form the chromic acid, and to act
on the corresponding weight of zinc.
If the sodium salt is powdered, it may be put into the
water, and the acid added to the solution at once. Com-
CLOSED CIRCUIT BATTERIES. 53
plete solution will quickly take place, and the misture
is ready for use as soon as it cools.
37. The Fuller Bichromate CeE — The special object
in the design o£ the Fuller battery is the continuous
amalgamation of the zinc. It is shown in section in
Fig. 17. Tlie zinc, to which a brass rod covered with
gutta pereha is attached, is placed
in a porous cup, and an ounce (30
gms.) of mercury is poured in. The
cup is then filled with water, and
is placed in the glass or earthen
jar containing the solution of bi-
chromate and acid and the carbon
plate. The acid diffuses through
the porous cup fast enough to act
continuously on the zinc, which
has enough mercury surrounding
it to keep it well amalgamated. Fi|c.i7.-TiwFpiier
This insures minimum local action
and constancy of current, especially if the current is
small.
Many thousands of these cells have been in use in
the PosfroiEce installation in London, and have given
good satisfaction. Each cell is said to serve an entire
year by replenishing with acid ten times and potassium
bichromate five times. At the end of a year the battery
is dismounted, cleaned, and furnished with new zincs.
38. Chromic Acid as the Depolarizer. — Instead of em-
ploying either of the preceding bichromates for the sup-
ply of chromic acid, the acid may be used directly. It
may be obtained in the form of a powder, and is soluble
in the acidulated water.
Since one molecule of a bichromate furnishes two of
64 PRIMARY BATTERIES.
chromic acid, it will readily be seen that ten-thirteenths
as much powdered chromic acid is required as sodium
bichromate. The amount of sulphuric acid is only
slightly less. Experiment shows that 150 gms. per litre
make a very serviceable solution. The initial E.M.F.
is then 2 volts.
Another modification, known as the Ward and Sloane
battery, employs zinc in caustic soda and carbon in a
mixture of chromic acid, nitric acid, and common salt.
The proportions are as follows: To one-half gallon of
nitric acid add one and a half pounds of chromic acid
and one pound of salt. This will make one charge for
the porous cup of a cell 12 x 12 x 9 inches. The zincs
are the equivalent of twenty-four rods half an inch in
diameter, and the carbons are equivalent to fifty electric-
light carbons. The initial E.M.F. is 2.9 volts. Such a
cell has an internal resistance of one-tenth ohm, and
will give a current of 10 amperes for 30 hours; final
E.M.F., 2.3 volts.
; The following solution has been found by Mr. J.
W. Swan (British Association, 1889) to give the best
I results : — «
I
Nitric acid (density 1.42) . . 1 part by weight.
[ Chromic acid 3 parts "
; Sulphuric acid 6 " "
! Water 5 " "
The chromic acid is first dissolved in the water ; the
nitric acid is then added, and finally the sulphuric.
This solution requires ten parts of acid to five of
water. It is scarcely possible to avoid wasteful local
action with even well-amalgamated zinc in such a con-
centrated acid solution.
CLOSED CIRCUIT BATTEHIES. 55
The suggestion has recentlj been made to use with
bichromat«8 only enough sulphuric acid to decompose
the salts and release chromic acid, and then to add at
least as much hydrochloric acid as sulphuric. It is
claimed that there ia leas liability of crystallization and
less heat with increased steadiness of current.
39. The Partz Acid Gravity Battery. — This zinc-carbon
element possesses sevei-al points of novelty and exhibits
excellent qualities under ap-
propriate tests. It is, in fact,
the application of the grav-
ity principle to an acid de-
polarizer. For this purpose
a flat carbon plate, with sur-
face increased by means of
pointed cones, corrugations,
or holes, lies in the bottom
of the cell ; and a carbon
rod, with the proper taper at
the lower end to fit tightly
into a hole in the plate made
to receive it, leaJfc to the
positive terminal on top of
the cell. The zinc is either
a heavy cylinder where a *'
*' -^ Parli Add GtnirUF BMlery.
porous cup IS employed (P ig.
18), or a large horizontal plate in the form without
porous cup (Fig. 19). In the former case the cup ia
paraffined to a height of two inches from the bottom to
prevent entrance of the acid depolarizer.
The depolarizer is a sulpho-chromic salt, in which
sulphuric acid has been caused to unite with chromic
acid in an amorpho-crystalline state. It is supplied to
66 pujaiary batteries.
the cell when everything else is in place, by filling into
the vertical tube shown in the cut to the level of the
liquid in the cell. The salt slowly dissolves and dif-
fuses over the bottom so as to cover the carbon plate.
The excitant is either sulphate of magnesium or com-
mon salt. The internal resistance is somewhat lower
with the latter.
Whenever the cell shows a tendency to weaken and
faU, it is necessaiy only to add one or two tablespoon-
Vlg. Ift. — ParU Add GrailQ' Battery.
fuls of the sulpho-chromic salt through the tube to
restore the current to its normal value. After the spent
salts have accumulated to such an extent as to interfere
wdth the working of the cell, it is better to turn out the
contents, soak the carbons in warm water, amalgamate
the zincs, and set up again with fresh solutions.
Since the depolarizer is intended to remain in the
bottom of the cell, it is apparent that this battery must
be left as much as possible undistuibed.
CLOSED CIRCUIT BATTERIES. 57
The form of Fig. 19 is set up by dissolving 11 oz.
of magnesium sulphate in the required amount of
water and iilliug the vertical glass tube to the level
of the liquid with the sulpho-ehromic salt. One of
these cells was tested for E.M.F., internal resistance,
current, and polarization. The initial E.M.F, was 2,08
volts ; and in the course of an hour on a closed circuit
through one ohm external resistance it fell to 1.85 volts,
but recovered to 2 volts again in a few minutes after
opening the circuit. The internal resistance was 0.82
of an ohm, and the current about 1.04 of an ampere.
40. Taylor's Battery. — This is a zinc-carbon element
capable of maintaining a very large current with small
diminution of E.M.F, The carbon
rods, eight in number, are attached
to a well-shellacked wood cap (Fig.
20) and make contact with the
bmss plate shown on top. The
zinc plate has an active surface of
27.5 square inches, is thoroughly
amalgamated, and is wholly im-
mersed in the dilute sulphuric acid
(1 : 15). Contact is made with the
zinc plate by means of a heavily
amalEramated copper wire, shown in
n* no
the cut. As the E,M.F. between Twior-. B.ttary.
amalgamated zinc and amalgamated
copper is very small, the loss from this cause is inap-
On account of the thorough amalgamation of the zinc,
the loss due to local action on open circuit is small.
The initial E.M.F. is 1.9 volts, the current on short
circuit 10 amperes, and ttie internal resistance as low as
68 PRIMARY BATTERIES.
0.18 ohms. The cell shown weighs, charged, 10.5 Ibe,
(4T65 gms.), and has a capacity, it ia claimed, of 70
ampere-hours.
The depolarizing solution is one of the best suhati-
tutes for nitric acid, and is rich in oxygen.
41. The Copper Oxide Battery. — It has been remarked
that, in general, the best depolarizers are liquid. There
are, however, two exceptions which exhibit notable effi-
ciency. They are the oxide of copper and the chloride
of silver. Both of these solids
readily give up their non-
metallic element to nascent
hydrogen, and by reduction to
the metallic state become excel-
lent conductors.
The copper oxide cell appears
to have been introduced by
Lalande and Chaperon, and one
of the forms was that shown in
Fig. 21, The spiral of zinc is
immersed in a solution of caus-
tic potash or caustic soda, 30
or 40 parts to 100 of water.
""' **• The upper vertical p&rt of the
OoppwOiidBBxtwy. . J^'^ -i. 4. *
zmc ff, where it passes out of
the solution, is covered with a caoutchouc tube to
prevent local action at that point. The negative con-
sists of a cup of sheet iron containing the copper oxide
S. To this cup is riveted an insulated copper wire
which passes up through the cover and forms the positive
electrode. To prevent action upon the alkaline solution
by the carbonic acid gas of the air, it ia covered with a
thin layer of heavy petroleum oil. The height of the
glass jar is 15.6 cm., and the diameter 10.5.
CLOSED CIRCUIT BATTERIES.^ 69
The larger pattern of cell is that of Fig. 22. Here
the zinc Is a helix of rolled metal suspended from an
ebonite cover, which is held in place by means of flanges
and nuts. The cell is capable of furnishing 12 amperes,
and has a capacity of 540 ampere-houra.
The copper oxide battery, invented by Lalande and
Chaperon, has a capacity for work per unit weight
greater than any other,
either primary or second-
aiy. One kilogramme (2,2
lbs.) is able to furnish
255 X 101" ergs, or 188,060
foot-pounds. A disadvan-
tage is that only a part of
the iron surface, consti-
tuting the negative plate,
is provided with the cu-
pric oxide sufficiently near
'^ ■' Pig. »a. —Copper Glide BslMty.
to be of any service m the
removal of the hydrogen, which accumulates on all por-
tions of the inner metallic surface. The reduced copper,
too, is not in good contact with the surface of the iron
cell. The conversion of the alkali into a carbonate, by
absorption of carbon dioxide from the air, necessitates
the closing of the cell against admission of the air, or
else the use of the heavy petroleum oil. The larger cell
is closed, and has a relief valve of rubber tubing.
The chemical reaction taking place may be written in
the form already employed in several cases.
Before the first step —
Zn^ I 2NaOH | 2 NaOli" ' | 'cuO | CuO | Fe.
(JO - PRIMARY BATTERIES,
After the first step, this becomes —
Zn.., I Na^nO, | 2NaOH | H,0 | CuO | Fe— Cu.
Ziac displaces hydrogen from the caustic alkali, form-
ing sodium (or potassium) ziucate ; while the ejected
hydi'ogen, travelling with the cuirent, arrives at the
cupric oxide, from which it abstracts oxygen, and me-
tallic copper is thus reduced at or near the iron of the cell.
42. The EdiBon-Lalaiide Battery. — Recognizing the
good qualities of the copper oxide as a depolarizer,
Edison has devised a form
designed to meet the ohjec-
tions noted above. The
copper oxide is employed in
the form of a compressed
slab, which, with its connect-
ing copper support, serves
also as the negative plate.
Two of these plates are
enclosed in a copper frame,
on the longer arm of which
is the binding post. A
hard rubber safety plug in
the middle prevents the zinc
plate on either side from
making contact with the cop-
per oxide and copper sup-
porting frame. One, two, or
three of these copper oxide
plates are used, according to
the Size and capacity of the
cells. The weight of the oxide plate for a 15 ampere-
hour cell is 2 oz., and for a 300 ampere-hour cell 2 lbs.
CLOSED CIRCUIT BATTERIES. 61
Fig. 23 is a 300 ampere-hour cell complete. The
cover is porcelain, with small openings for the zinc and
copper terminals. Since this cover does not exclude
the air, the formation of a carbonate is prevented by
pouring on top of the solution of caustic potash (KOH)
a small quantity of heavy paraffin oil, so as to form a
layer about one-fourth of an inch deep. It is of vital
importance that this oil should not be omitted. If it
is not used, the life of the cell is reduced fully two-
thirds.
If the cell is required to furnish a strong current at
once, it should be short-circuited for ten or fifteen min-
utes the first time it is used. By this means enough
metallic copper is reduced to form a good conducting
surface, and the internal resistance of the cell falls to
its normal working value. Subsequent short-circuiting
should, of course, be avoided, especially because the
internal resistance is very low, and the large current
flowing causes a great waste of material in the cell. In
recent cells the device has been resorted to of reducing
a superficial film of copper on the oxide before it is sent
from the factory.
The 300 ampere-hour cell shown is Hi inches high
and 5f inches in diameter. Its internal resistance is
about 0.03 ohm, and its working E.M.F. about 0.7 volt.
It is capable of delivering 14 amperes. On open cir-
cuit there is practically no local action.
The zinc should be well amalgamated.
Pressed copper oxide plates have also been used
abroad in a cell having the form of Fig. 24, in which
the compressed plates, B^ are held in contact with the
sheet iron, A^ by rubber bands. The cell is closed to
prevent entrance of air, but has a relief valve, JST, for
62 PRIMARY BATTERIES.
the escape of accumulated gas. The small zinc sui^
face, Fi means relatively large internal resistance. The
plates are made by mixing cop-
per oxide with from 5 to 10
percent of magnesium chloride,
and heating the thick mass in
an iron mould.
43. The Chloride of Silver
Cell — Mari€ Davy appears to
have been the first to suggest
the use of silver chloride as a
depolarizer about 1860 ; but it
was brought into prominence
by the investigations of War-
ren de la Rue, who constructed
a battery of this kind contain-
ing 15,000 cells.
Fi,.«.-B«urywiih^co™p™«d ^hc elemeuts are zinc and
silver, and on the silver is cast
the silver chloride, which is readily reduced to metallic
silver by nascent hydrogen. The chloride of silver
is easily melted in a porcelain crucible, and may be
cast on a silver wire in a hard carbon mould. Silver
foil has sometimes been cast in the chloride to give
better conductivity. The exciting fluid of De la Rue's
battery is ammonium chloride, and contains 23 gms. to
one litre of distilled water. A denser solution dis-
solves silver chloride. The silver and its chloride are
surrounded with a small cylinder of vegetable parch-
ment paper (Fig. 25), to prevent short circuits internally,
and the zinc rod and silver wire are held in a parafBn
stopper. The silver wire of one cell is wedged into the
zinc rod of the next.
CLOSED CIRCUIT BATTERIES. 68
The following chemical action takes place : —
Zn^ r2 NH«Cl I 2NH«C 1 |" 2AgCl | Ag,=
Zn,_, I ZnCl, I 2NH(C1 | 2NH,C1 | Ag^^
This may be considered the normal action ; but where
the cell is worked hard, it may happen that the ammo-
nie chloride loses chlorine faster than it recovers it from
the silver chloride ; and the ammonium breaks up into
ammonia and hydrogen. The ammonic hydrate thus
formed is capable of dissolving silver chloride, with the
formation of ammonio-silver chloride. The hydrogen
may reduce silver chloride with production of hydro-
chloric acid. This acid increases local action. Under
such conditions gas may be liberated in the cell, and pro-
vision must be made for its escape ; or the cell must be
made veiy strong and must be securely sealed.
The initial E.M.F. of a silver chloride cell is about
1.1 volts. Its internal resistance falls very rapidly upon
64 PRIMARY BATTERIES.
first closing tlie circuit, on account of the reduced silver.
It polarizes but slightly, and recovers promptly. It is
employed chiefly for testing purposes; sometimes for
physicians' use. But it should never be put into service
requiring anything more than small currents. Upon
standing, the zinc is liable to become coated with a thin,
adherent film of the oxychloride of zinc, offering high
electrical resistance.
44. Hodifioations of the Silver Chloride Cell — The mod-
ifications thus far introduced consist in the substitution
of some other exciting liquid for the ammonic chloride.
Thus caustic potash or soda has been used by Scrivanoff.
The chemical reaction is then the same as with the
copper oxide cell, except that the hydrogen displaced
by zinc unites with the chlorine of the depolarizer, form-
ing hydrochloric acid. A secondary reaction is thus
possible, due to the action of the acid on the zinc.
There is then greater liability of local action than if
the cell were set up with sal-ammoniac.
The excitant may also be zinc sulphate. The dis-
placement process taking place is as follows : —
Zn^ I ZnS04 I ZnS04 I 2AgCl | Agy =
--v~
Zn^_i I ZnS04 | ZnS04 | ZnClg | Ag^+j.
In this case zinc chloride is formed at the expense of
silver chloride, and the energy appearing in an electrical
form may be represented as due to the difference be-
tween the heat of combination of zinc chloride and silver
chloride.
The initial E.M.F. with caustic potash is 1.64 volts;
with zinc sulphate, 1.16 volts ; and with zinc chloride
(Gaiffe), 1.01 volts.
CLOSED CIRCUIT BATTERIES, 65
It should be remarked that silver chloride is soluble
to some extent in the chlorides of the heavy metallic
elements. When the liquid contents of the cell contain
as much as one part of concentrated zinc chloride in ten
parts of water, the silver chloride is dissolved in quanti-
ties which are quite appreciable. Local action then
ensues, due to the displacement of silver by zinc, and
the zinc rod or plate quickly blackens.
The marked efficiency of silver chloride as a depolar-
izer is perhaps due to its slow or partial dissolving in
the exciting liquid, since liquid depolarizers are, in gen-
eral, more effective than solid ones. A weak solution
of ammonic chloride may not attack the solid silver
chloride. Hence local action does not take place so
long as these cells have not been placed in use ; but im-
mediately upon closing the circuit through them zinc
chloride is formed, and thereafter local action begins to
exhaust the silver chloride with blackening of zinc. So
that silver chloride cells that have been much used will
not stand on open circuit without waste. Moreover,
their internal resistance will increase if the zinc becomes
encased in the film of oxychloride before mentioned.
66 PRIMARY BATTERIES.
CHAPTER V.
OPEN CIRCniT BATTERIES*
45. The Leclanche Cell. — The present chapter will
Jl be devoted to open circuit batteries in which a solid
I depolarizer is used. At the head of this list stands the
I Leclanche cell, so called from the name of the inventor.
S Metallic oxides had been proposed as depolarizers pre-
* vious to the invention of Leclanche, but without prac-
c tical results. Thus, with zinc in dilute sulphuric acid
and platinum surrounded with the peroxide of lead in
t a porous cup, Beetz found an E.M.F. of 2.4 volts. Dur-
S ing 30 minutes short circuit this fell to 1.4, but recov-
i» ered after five minutes rest to 2.16. It is evident that
this high E.M.T". is due not only to the oxidation of the
h zinc, but to that of the hydrogen as well, both chemical
i processes contributing to the electromotive stress in the
i same direction.
I The chief disadvantage in the employment of lead
• peroxide as a depolarizer lies in the fact that the re-
i duced lead is converted into lead sulphate. This accu-
^ mulates on the negative plate and has the effect of
largely increasing the internal resistance of the cell.
It is worthy of note in this connection that one of
the more recent forms of storage batteries is composed
essentially of the elements used by Beetz ; namely, zinc
'^nd lead in an acid solution of zinc sulphate.
The depolarizer of the Leclanch^ cell is manganese
OPEN CIRCUIT BATTERIES. 67
dioxide (MnOj)- It is not used m a powder, but in
granules mixed with broken gas carbon to increase the
conductivity. The negative plate is baked carbon, and
is surrounded with the mixed manganese dioxide and
broken carbon, packed in a porous cup, which is finally
sealed with pitch, with two small vent tubes inserted.
The typical Leelanch^ cell, with its porous eup (Fig.
26), has a glass jar moulded with a lip, in which is
placed the zinc rod. The carbon plate is usually sui^
Fig. se.— Leclanch^ Cell.
mounted with a lead cap, cast on the carbon, and hold-
ing the binding post of the positive terminal. The cut
exhibits a new connection, designed to avoid corrosion
of the lead cap.
The size of the zinc rod, which never exceeds half an
inch (1.25 cm.) in diameter, indicates large internal re-
sistance, and shows that this cell is designed to furnish
only small currents through considerable external resist-
ance. The amount of energy held potentially in the cell
is represented approximately by the weight of the zinc.
68 PRIMARY BATTERIES,
The exciting liquid is ammonic chloride, the sal-
ammoniac of commerce. To set tip the cell, five
or six ounces of best sal-ammoniac are , dissolved in
water. Water, or water containing sal-ammoniac, is
also poured into the porous cup through one of the
vent tubes. If water alone is added, the cell must
stand for about 24 hours before use, to permit the
diffusion of the ammonium salt through the porous
cup, unless there are holes in it which allow the liquid
to pass in rapidly.
An incidental advantage of this cell is that the dif-
fusion of the liquid through the porous vessel, which
serves only to hold the depolarizer and the broken
carbon, is of positive utility ; while in two fluid cells
the diffusion of the two liquids through the pores is
an undesirable feature.
The initial E.M.F. of the Leclanch^ cell varies from
1.4 to 1.7 volts, and the internal resistance from about
0.4 to 2 ohms.
46. Chemical Seactions in the Leclanch^ Cell. — Theo-
retically no chemical reactions take place so long as
the circuit remains open, inasmuch as the cell contains
neither acid nor an acid salt. But when the circuit is
closed, zinc displaces ammonium from the ammonic
chloride, and the ammonium breaks up into ammonia
gas, which is set free and escapes after the liquid be-
comes saturated, and hydrogen which is oxidized by
the manganese dioxide. These chemical changes may
be represented by the following equation : —
Zn. 1 2NH4CI I 2NH4CI II 2Mn02 j C =
* , '
Zn,_i I ZnCla | 2NH4CI || 21^^11^ \ MnA | Kfi \ C.
OPEN CIRCUIT BATTERIES. 69
If the liquid is allowed to become supersaturated by
evaporation, a double salt of the chlorides of zinc and
ammonium is liable to crystallize on the zinc. This
reduces the E.M.F. and increases the internal resist-
ance. A small quantity of hydrochloric acid will usually
dissolve these crystals.
When a Leclanch^ cell is left undisturbed for some
time, it will be found that the zinc rod is eaten away at
the surface of the liquid, and that it is conical in shape,
with the larger end of the cone at the bottom of the
zinc. The excessive waste at the surface is doubtless
due to oxidation, but the coning is the result of a
peculiar local action sometimes seen in other forms of
battery.
The double chloride of zinc and ammonium gradually
settles to the bottom of the cell, becoming progressively
denser and denser as the bottom is approached. Now
zinc in a solution of ammonic chloride is positive to
zinc in zinc chloride; if the latter liquid contains ammo-
nic chloride also, the resulting E.M.F. is smaller, but
still appreciable.^ Hence local circuits are formed be-
tween the upper and lower portions of the zinc rod, the
upper portions playing the part of the zinc in a simple
voltaic combination.
The zinc plates of the copper oxide battery show a
similar thickening from the liquid surface downward.
The heavy zincate formed can be seen settling toward
the bottom of the cell, and local action sets in, as already
explained.
47. The Prism Leolanche Battery. — The prism form of
the Leclanch^ cell was devised for the purpose of dis-
pensing with the porous cup. The carbon plate is sus-
1 See Experiments of Chapter IX.
70 PRIMARY BATTERIES.
pended from the cover (Figs. 27, 28), and attached to
it by rubber bands are the two agglomerated prisms,
containing the depohirizer. They consist of 40 parts
granulated manganese dioxide, 52 parts granulated
carbon, 5 parts gum shellac, and 3 parts acid potas-
Tbe Frllm Lectaach^ BaKerf.
sium sulphate. The mixture is heated to 100° C,
and then compressed in moulds under a heavy press-
ure.
This form of Leclanch^ cell has not met the expecta-
tions entertained at its first appearance. It appears not
to be as efScient and durable as the original form, and
has not come into general use in this country.
• OPEN CIRCUIT BATTERIES. 71
48. The Closed Leolanohe Cell. — When an open Le-
clanch^ cell ia kept in a dry place the liquid evapo-
rates, and the solution becomes more concentrated, with
greater liability of crystallization at the surface and
consequent creeping of the salts upward toward the top.
To avoid this difficulty,
closed cells (of which
Figs. 29 and 30 are ex-
amples) have been de-
vised. In the former,
the cover is wood sat-
urated with paraffin and attached to the porous cup,
but removable from the outer jar. So also the zinc is
held loosely in the cover, and can be taken out. The
cover fits down on a shoulder in the top of the jar, and
a soft rubber ring makes it tight.
In the latter form the porous cup is made with a
flange (Fig- 31), which rests upon the top of the jar.
12 PRIMARY BATTERIES.
Both the jar and the flange are paraffined, so that a close
joint is made. The zinc passes through an opening in
the cell specially provided for it. This is made tight by
a piece of soft mbber tubing enclosing
the rod at the point where it passes into
the jar.
Two or three other modifications of
details may be noted in these cells. In
the one of Fig, 29, the porous cup,
which is unusually large, has in the
bottom three large holes covered with
burlap. When the cell is set up, the
sal-ammoniac solution enters at once,
and the cell is ready for use.
The porous ns-si.
/ 17' 01 s CloHd LeElHnch^ Cell,
cup (Fig. SI)
has a small hole in the bottom
to admit the liquid, and two
holes, shown in the cut, on
either side of the carbon at
the top. The carbon has a
special connection by means of
a bolt and lock nuts, which
serve their purpose satisfac-
torily.
A stop in the bottom of the
glass jar prevents contact be-
tween the zinc and the porous
cup. The two water marks on
the jar serve as a convenient
Fig. Sa — The Micropbone Cell. , ,' . ,„. t^ ■, i,
guide in filling. Each cell re-
quires 4 oz., or 120 gms,, of sal-ammoniac.
Both of these types of battery show an unusually
OPEN CIRCUIT BATTERIES. 73
high E.M.F., and have done excellent service in the
hands o£ the writer.
49. Leclanclie Cells with Carbon Cap. — It is entirely
practicable to dispense with the unglazed porous cup,
and to make a carbon cylinder serve as a receptacle for
the manganese dioxide. Two such cells are represented
in Figs. 32 and 33. Both of these are loosely covered,
to prevent evaporation, and have the depolarizer en-
closed by carbon. The zinc of the latter is a cleft
FIK. 33. — SaiDBOD Batter;. Fie. 34. — Zinc and Carbon.
cylinder (Fig. 34), and the carbon cup is corrugated
to secure a larger surface. Both the polarization and
recovery of these cells are not so rapid as in other
forms of Leclanch^ cells, but they are more nearly con-
tinuous or uniform. A marked feature is the low inter-
nal resistance. It is only slightly over 0.3 ohm, and is
no lower in the second than in the first, though the zinc
cylinder has so much larger surface than the rod. The
intervening distance is greater in No. 33, thus offsetting
the larger surface.
74 PRIMARY BATTERIES.
With an external resistance of 5 ohms, the loss of
potential in the interior of these cells is only 0.07 or
0.08 of a Tolt, or about 5 per cent of the total E.M.F.
of the cell. They show, therefore, high commercial effi-
ciency.
50. Leolandie Cdl with Agglomerated Carbon. — In the
cell shown in Fig. 35 the manganese dioxide appears to
he incorporated with the carbon in
the pastCi and an agglomerate is
thus produced by baking. This cell
is efFectively closed, and the zinc is
insulated by a special glass sleeve
passing through the earbou cover.
A lug on the zinc rod fits into a
corresponding socket in the glass,
and serres the double purpose of
holding the zinc up from the bottom
of the cell and preventing its turn-
ing round when the connecting wire
Fig. 3a.-ceii with Ag. ig screwed fast to the negative tei^
glomerited Ciihon. , , °
minal.
The agglomerated carbon cylinder has a long cleft on
either side, and the zinc rod hangs in the centre. The
glass insulator holds the zinc somewhat rigidly, and
prevents any contact between it and the carbon.
This cell exhibits the same pecuharities of moderate
but progressive polarization and good recovery as those
of the last section. It has a somewhat higher internal
resistance, which is, however, less than that of the ordi-
nary LeclanehS element.
51. Roberta' Peroxide Battery. — The elements are
amalgamated zinc, carbon suiTounded with an agglom-
erate of peroxide of lead, and a solution of chloride of
OPEN CIRCUIT BATTERIES, 75
sodium, to which is added a small quantity of bichro-
mate of sodium. The E.M.F. is 1.8 volts.
The agglomerate is made by adding minium (red
lead) to powdered permanganate of potassium xftlid
hydrochloric acid, in quantity sufficient to form It semi-
liquid paste. By the combined action of the acid and
the permanganate, the PbgOs is converted into lead
peroxide (Pb02). The paste is then introduced into
a mould containing a carbon electrode ; and when after
a few minutes it has set, it is withdrawn from the
mould and dried at the temperature of the air. By this
means a mass is obtained as dense as carbon.
The bichromate is added to the exciting liquid for the
purpose of converting the chloride of lead in the agglom-
erate into an insoluble chromate. The partly soluble
chloride would form a deposit of lead on the zinc.
In the action of the battery, zinc displaces sodium
with the production of zinc chloride and sodium hydrate.
Hydrogen is released in the formation of the hydrate,
and this abstracts oxygen from the lead peroxide.
The internal resistance of such cells is large on
account of the presence of insoluble lead salts.
52. The Sulphate of Mercury Battery. — Mari^ Davy
first proposed the use of the sulphates of mercury as
the depolarizing agent. For commercial purposes the
acid sulphate is used, containing probably both the mer-
curic and the mercurous salts. These solids are only
slightly soluble, and are therefore slow-acting depolar-
izers. The cell has various forms, but always contains
zinc as the positive plate, and carbon, surrounded with
the mercury salt, as the negative.
The form in which it is most used ia for medical pur-
poses. The carbon is at the bottom of a moulded rubber
76 PRIMARY BATTERIES.
caae. Oa this is placed the mercurial salt with a little
water. The amalgamated zinc plate is laid on top and
is brought into contact with a platinum wire in the
body of the rubber cell, and connection is thus made
with the electrode. Usually two such cells are mounted
together in series. The E.M.P. is about 1.45 volts.
53. The Fitdi "Chlorine" Battery. — In Mr. Fitch's
original battery the
depolarizer was one
of the chlorides of
mercury ; but in the
process of improve-
ment the chloride
has been replaced
by the chlorates of
potassium and so-
dium. The excitant
is composed of the
chlorates of potas-
sium and sodium
and sal - ammoniac,
" mixed in their
proper combining
proportions." Two
Flg.36.—ThB Filch Battery. ■; ,
forms, shown in
Figs. 36 and 37, differ only in the extent of carbon
surface exposed, and therefore in their internal resist-
ance. The internal resistance of the form with carbon
cylinder ia about 0.35 ohm when the current flowing is
0,2 ampere, or with an external resistance of 5 ohms.
Each package of the excitant weighs 145 gms., or 5 oz.
About three-quarters of this is ammonic chloride, the
remainder being the chlorates.
OPEN CIRCUIT BATTERIES. 77
The larger cell requires four packages of excitant,
each equal to the above. By accident, three of these
cells were left on a closed circuit of 75 or 80 ohms
for 2375 hours in
long-distance tele-
phone service. This
is about 20 ohms per '
volt. During this
three months ser-
vice, their efficiency
had not decreased
sufficiently to be
noticed in using the
transmitter.
When this cell is
exhausted by use,
clean thoroughly the
jar, the carbon, and
the cover ; and after
drying, replace the
zinc with a new one
and supply a fresh
solution of the ex- F.g.37.-Tb. Fi«=b B..wy.
citant. The battery
is then again ready for extended service.
In case of accidental short-circuiting, extreme cold,
or very hard service, crystals of spent residue may
form on the zinc and carbon. These may be removed
by adding to each cell 1 oz., or 80 gms., of hydro-
chloric acid. More than this should never be added
at one time, and then only when the accumulation oa
the platea demands it. Otherwise local action will take
place on account of the presence of the acid.
PRIMAnr BATTERIES.
CHAPTER VI.
BATTI1RIE& WITH017T A DEPOIl&RIZBR.
54. The Smee Cell.— The oldest battery of any prac-
tical value without a depolarizer is the Smee (Fig. 88).
The positive plates of this cell are zinc, enclosing be-
tween them, with proper insulation, a negative of thin
silver, corrugated and covered
with platinum in a very finely
divided state. The excitant or
electrolyte is dilute siilphuric
acid; and the purpose of the
roughened surface of the silver
is the mechanical dislodgement
of the hydrogen as fast as it is
released at the negative plate,
since hydrogen is found to be
much more easily detached from
a rough surface than from a
smooth one.
The silver plate may be pre-
FiK 38.-Tiie Smee Ceil. P^^e^ ^s f 0II0W8 : Obtain thick
silver foil and roughen the sur-
face lightly with fine glass-paper, or by brushing over
with strong nitric acid. Unless the surface is rough-
ened the platinum black will not adhere. Connect the
silver plate, by means of a copper wire, with a small
slip of zinc, and insert the silver in a vessel of dilute
BATTERIES WITHOUT A DEPOLARIZER. 79
acid, to which has been added a few drops of platinic
chloride. The zinc slip should then be merely touched
to the dilute acid at a point remote from the silver.
The slight current thus produced will be sufficient to
decompose the platinic chloride, and the platinum will
gradually deposit on the silver and color it. Then add
more of the platinum salt, and insert the zinc deeper
into the liquid. Gradually increase the current till the
surface of the silver plate is covered with a black coat-
ing of finely divided platinum.
The platinic chloride may be prepared by dissolving
scrap platinum in a mixture of two parts hydrochloric
acid to one of nitric acid, and gently warming for some
time. For the above use it is not necessary to drive off
the acid or to crystallize the salt.^
A negative plate for the Smee cell has been formed
of copper, with the surface roughened by electro-deposi-
tion, then plated with silver, and finally platinized. It
is said, however, that the silver plating is liable to be
porous, and that the acid in time works through to the
copper. Also, that the copper dissolves at the edges and
is deposited again on the silver.^
55. The Sea Salt Battery. — A battery which is said to
have done good service has been made with sea salt and
powdered alum, in the ratio of five parts to two, dis-
solved in water, as the excitant. The elements were
zinc and carbon, the latter having a very large surface.
Zinc chloride and zinc sulphate are formed, and hydro-
gen is set free, with formation of sodium and potassium
hydrates.
Exactly what part the alum takes in the reactions is
uncertain and obscure. But such cells are capable of
1 Sprague's Electricity, p. 92. 2 phii. Mag., May, 1840..
80 PRIMARY BATTERIES.
intermittent service for certain classes of work requir-
ing only small currents.
56. The Law Battery. — In this battery, and in others
of similar design, reliance is placed upon a large carbon
surface to e£fect depolarization mechanically. The nega-
tive consists of a double cleft cylinder of carbon, with
the zinc rod hanging well within the cleft (Fig. 39).
The carbon has a surface of about 145 square inches,
and the internal resistance is 0.4 ohm when the current
flowing is 0.2 ampere. The cell
is effectively closed by an insu-
lating cover, so made that by a
partial turn it locks down tightly
against a soft rubber ring. The
jar is of flint glass, annealed, and
its capacity is one and one-third
quarts, or one and a half litres.
Sal-ammoniac is the excitant, and
each cell takes one litre of the
ng.3».-The Law Bat«ry. solutlou Containing 150 gms., or
5 oz., of the salt.
A renewal of an exhausted cell requires only a new
zinc rod and a fresh solution of sal-ammoniac. The
spent solution should always be thrown out, and the
double carbon cylinder should be thoroughly soaked in
water and then exposed to the sun and air, to remove
the absorbed salts.
This cell is neat, clean, durable, and efficient. For hard
work it polarizes more continuously than a Leclanch^
cell, but for light currents the polarization is not suffi-
cient to be noticeable. The initial E.M.F. is 1.37.
57. The Diamond Carbon Battery. — The negative of
this cell is composed of seven rods of soft carbon, 5.5-
BATTERIES WITHOUT A DEPOLARIZER. 81
inches long and five-eighths of an inch in diameter, set
into a soft metal top and secured by a set screw, in the
manner shown in Fig. 40. Tlie metal top is cast round a
porcelain insulator through which passes the zinc rod.
The zinc is kept from falling too low by an iron cross-
pin, and a rubber ring closes the annular opening in the
i-.
lie. 40. — "UamoDd" Carbon Brnttsry.
porcelain round the zinc. Another rubber ring at the
bottom of the zinc prevents contact with the carbons.
The tops of the cells are covered with paraffin or bees-
wax ; the inside of the cover and the upper ends of the
carbons are also paraffined. Care should be taken not
to allow any of the solution of the sal-ammoniac to get
on the cover, otherwise the crystallization and creeping
82 PRIMARY BATTERIES.
of the salts produce a short circuit, and the cell exhausts
itself on apparently open circuit,
i The internal resistance is only about 0.25 ohm, with
t ohms external resistance ; the polarization is continu-
ous and progressive, as in all cells of this class, but the
recovery is very good. The initial E.M.F. is 1.36 to
1.39 volts.
58. Cylinder Carbon Batt«riea. — In addition to the
cylinder carbon battery already described, attention may
be drawn to two others (Figs.
41 and 42). In the former, the
carbon cylinder and cover of
the jar are made in one piece,
and the cylinder in both is
cleft for free diffusion of the
sal-ammoniac solution. The
oval form o£ the Laclede (Fig.
42) has no advantage, except
increased carbon surface. The
connection with the binding
post is made in both cases in
such a way as to render corro-
sion by capillary ascent of the
liquid quite remote. A greater
Fig.4i.-Tbeo,H»d«ceu. danger in all these cells arises
from careless handling after
they are set up, during which the liquid splashes up
against the top and over the porcelain insulating the
zinc. The initial E.M.F. of all carbon cells without
depolarizer appears to be about the same, — between 1.3
and 1.4 volts. They quickly drop below this value with
a current of two-tenths of an ampere, and subsequently
rise but little above a single volt. The ease and cheap-
BATTERIES WITHOUT A DEPOLARIZER. 83
ness with which they may be restored to nearly theit
initial efficiency after exhaustion constitute a strong
commendation in their favor.
p:-.
Wf. «.— ThB ladedo Batterr.
59. Hie Oaasner Dry Battery. — A larfje part of the
most recent batteries appearing as candidates for public
favor are of the ao-called dry type. They contain the
excitant in the form of a paste, the composition of
■which is in most eases a secret. Their eonveo^^^ce
commends them to those having no technical kno^^^l^dge
relating to batteries, and they are very useful in situa-
84
PRIMARY BATTERIES.
tioiis precluding the use of unsealed cells with liquid
electrolytes. But their store of available potential
energy is, in general, smaller than that of batteries
containing a larger quantity of fluid.
One of the oldest cells of the dry type is that of Dr.
Gassner (Fig. 43). The zinc, composing the positive
element, is the containing vessel. It is usually covered
with paper, or is enclosed in
a paper box. The negative
element is carbon, and it occu-
pies about one-half the space
in the cell.
The paste, which is filled in
between the zinc and the car-
bon in the Gassner cell, has
the following composition :
"Oxide of zinc, 1 part, by
weight ; sal-ammoniac, 1 part,
by weight; plaster, 3 parts, by
weight; chloride of zinc, 1
part, by weight; water, 2 parta,
by weight. The oxide of zinc
in this composition loosens
and makes it porous, and the
greater porosity thus obtained facilitates the interchange
of the gases and diminishes the tendency to the polai--
ization of the electrodes."
The initial E.M.F. of this cell varies but little from
1.3 volts. ,It polarizes very rapidly on so low an ex-
ternal resistance as 5 ohms ; while the internal resist-
ance, which is different for cells of different size, is very
irregular during the working of the cell, probably on
account of the slow and irregular diffusion of the prod-
ucts of the chemical action.
Vis- *3. ~ Qunier D.
BATTERIES WITHOUT A DEPOLARIZER. 85
Such cells should be employed for intermittent ser-
vice, where the circuit is kept closed for short periods
only. In such situations they will doubtless prove effi-
cient and durable. Their convenience, particularly in
the hands of unskilled persons, is much in their favor.
Meserole's composition for a dry battery is the fol-
lowing : —
Charcoal, 3 parts ; mineral carbon or graphite, 1 part ;
peroxide of manganese, 3 parts; white arsenic oxide,
1 part; a mixture of glucose and dextrine or starch,
1 part ; hydrate of lime, dry, 1 part — all by weight.
These are intimately mixed and worked into a paste
of proper consistency with a solution composed of equal
parts of a saturated solution of chloride of ammonium
and common salt, to which are added one-tenth of the
volume of a solution of bichloride of mercury and an
equal volume of hydrochloric acid. The fluid is added
to the dry mixture gradually, and the mass is well
worked to insure uniformity.
86 PRIMARY BATTERIES.
CHAPTER VII.
STANDARDS OF ELECTROMOTIVE FORCE.
60. Latimer Clark*8 Standard Cell — The original
Latimer Clark normal element was described for the
first time in a paper read before the Royal Society,
June 19, 1873.^ The metallic elements are pure zinc in
zinc sulphate, and pure mercury in contact with mer-
curous sulphate (Hg2S04). The mercury was placed
in the bottom of the cell, and contact was made with it,
either by passing a platinum wire down through a small
glass tube in the cell itself, or else through one blown
on the cell near the bottom.
The zinc sulphate solution was made by boiling an
excess of pure zinc sulphate crystals in distilled water,
and decanting the clear solution off from the crystals
after cooling.
On the mercury was poured a thick paste, made by
boiling mercurous sulphate with the solution of zinc
sulphate, saturated in the manner just described. Into
this paste dipped a zinc rod, or else a plate of pure
zinc rested on its surface. Special stress was placed on
the boiling of the paste with zinc sulphate solution for
the purpose of expelling the air.
The cell was imperfectly sealed with a paraffin
stopper.
1 Philosophical Transactions, 1874.
STANDARDS OF ELECTROMOTIVE FORCE, 87
The normal E.M.F. of this cell, according to Clark,
was 1.457 volts. But this was on the basis of the
British Association (B.A.) unit of resistance. Now
the unit of E.M.F. varies directly as the unit of resist-
ance. If, therefore, the true ohm, which is represented,
according to our latest knowledge, by the resistance of
a column of pure mercury of one square millimetre
cross-section, and 106.3 cm. long at 0** C, is 1.014 times
the B.A. unit, then the true volt is also 1.014 times the
B.A. volt. Hence, to reduce 1.467 B.A. volts to true
volts, divide by the above ratio. The result is 1.437
true volts. This is only 0.002 volt higher than the later
value assigned by Lord Rayleigh, as the result of his
extended observations.
61. Lord Eayleigh's Form of the Clark Element. — The
original Clark cells exhibited certain abnormal and
irregular values both of E.M.F. and temperature co-
efficient. A thorough investigation of the Clark cell
was therefore undertaken by Lord Rayleigh, and the
results were published- in the "Philosophical Transac-
tions of the Royal Society," Part II., 1885, under the
title of " The Clark Cell as a Standard of Electromotive
Force." This paper was supplementary to one published
in the same place in 1884 on "The Electro-Chemical
Equivalent of Silver, and the Absolute Electromotive
Force of Clark Cells." Only a brief summary of results
of this very important investigation can be given here.
The E.M.F. of a Clark cell may be too high (1) be-
cause the paste is acid ; (2) because the zinc sulphate
solution is not saturated. The first fault will cure
itself in the course of a month or so.
The E.M.F. may be too low (1) because the cell has
become dry ; (2) because the solution is supersaturated ;
88 PRIMARY BATTERIES,
(8) because the mercury is not pure. The cell loses
liquid because of imperfect sealing. Paraffin cracks
away from the glass. Lord Rayleigh recommends
marine glue. Supersaturation results from heating the
solution or the paste. The strong solution will then
cool without any deposit, or will throw down an abnor-
mal hydrate. The presence of crystals does not prove
that the solution is not in the state of supersaturation,
unless it can also be proved that these crystals are those
of the normal hepta-hydrated salt. The addition of a
few crystals of the normal zinc sulphate will always
cause the excess of salt held in solution at a given tem-
perature to crystallize out.
Respecting the presence of other metals in the mer-
cury, it is sufficient to notice only that of zinc. Zinc
opposed to pure mercury, without the presence of
Hg2S04, gives an uncertain E.M.F. of about 1.186
volts. But when the mercury contains one part of
zinc in 5,900,000, the E.M.F. falls to 0.513 volt ; and
with one part zinc in 200,000 it becomes only 0.124
volt.i
With zinc opposed to pure mercury in a zinc sulphate
solution, the E.M.F. is not constant from hour to hour,
and is altered by the passage of a minute quantity of
electricity which would be insufficient to produce the
least effect upon a cell provided with mercurous sul-
phate. So marked is the action of the mercurous
sulphate in repurifying the mercury, that Lord Ray-
leigh suggests that this may be its principal office
in the Clark cell; and he attaches the greatest im-
portance to purity of mercury. " It is clear," he says,
1 ** On the Electromotive Force of Mercury Alloys," Journal So-
ciety Telegraphic Engineers, Vol. VIII, 1879.
STANDARDS OF ELECTROMOTIVE FORCE. 89
" that the mercuroua sulphate has the property o£
freeing the mercxiry from the smallest CQDtamination
with zinc."
Lord Rayleigh's cell (Fig. 44) is made as follows :
A small tube has a platinum wire sealed through the
closed end. On this is poured enough pure mercury, dis-
tilled in vacuo, to cover
the platinum effectively. ^ „^
The paste which covers
the mercury is prepared ^^i^
by rubbing together in
a mortar 150 gms. mer-
curous sulphate, 5 gms. g^Zu^ ^».'»>'*
zinc carbonate to neu- "J 03,'"°*
tralize acid, and as much
zinc sulphate solution,
saturated by standing '** hm
in a warm place, as will
make a thick paste.
After the carbonic acid rn.44.-R.yi.ighsu«brdCeu.
gfts has escaped, this paste is poured into the tube
through a small funnel, care being taken not to soil the
sides of the tube. After adding a few crj'stals of zinc
sulphate to insure saturation, the cleaned zinc rod, with
a copper wire soldered to its upper end, is passed down
into the paste, and is' held in position by a piece of cork
which nearly touches the paste. Finally, enough hot
marine glue is poured in to cover the zinc and cork, and
to leave only the wire projecting. The figure shows the
cross-section of such a cell. In no stage of the process
is heat applied to the paste. In this particular, Lord
Rayleigh's procedure is in marked contrast to that of
Latimer Clark.
90 PRIMARY BATTERIES.
The E.M.F. of a Clark cell, constructed as above,
Lord Rayleigh found to be 1.435 true volts. This is
equal to 1.438 legal volts, corresponding with the legal
ohm, or to 1.455 B.A. volts at 15° C.
Using a silver voltameter as a secondary standard,
the writer found a Clark cell, made in Berlin after
Latimer Clark's directions, to have an E.M.F. of 1.437
legal volts at 15° (strictly 1.434 at 18° C).
The value of the tempemture coefficient was also
investigated by Lord Rayleigh. It was found to vary
considerably for different individual cells ; but for cells
with saturated solutions the following equation can lead
to no appreciable error: —
E = 1.435 {1 - 0.00077 (t - 15) } :
t is the temperature of the cell.
Latimer Clark found a temperature coefficient of 0.06
per cent per degree C. for temperatures within 10° on
either side of 15. For higher temperatures he observed
a diminution of the coefficient; so that for the whole
range of observations, extending up to 100° C, the
coefficient was 0.055 per cent per degree C.
62. A Standard Clark Cell with Low Temperature
Coefficient. — The objections to Lord Rayleigh's form of
the Clark normal element are: (1) the temperature
coefficient is high and apparently variable ; (2) it is not
constructed in such manner as to keep the zinc and
metallic mercury out of contact ; (3) the contact of the
zinc and the mercurial salt permits of local action
whereby zinc replaces mercury.
Respecting the first objection, the method to be pur-
sued in reducing the temperature coefficient is suggested
by the fact, now well known, that the E.M.F. decreases
STANDARDS OF ELECTROMOTIVE FORCE, 91
with an increase in the density of the zinc sulphate
solution. Hence, if the solution is saturated at 30° or
40**, upon a lowering of temperature the excess crystal-
lizes out with a decrease of density. The reverse pro-
cess takes place with rise of temperature, with the
additional disadvantage that time is required for the
diffusion of the redissolved salt. The temperature
coefficient in such a cell is therefore made up of two
parts : one a real temperature effect, the other a second-
ary change resulting from a variability in the density
of the zinc sulphate solution. A rise of temperature
lowers the E.M.F. by increasing the density of the
solution in addition to the direct primary effect of the
temperature change.
The slowness of diffusion when the temperature rises
makes the coefficient for a rapid rise of temperature
smaller than for a slow one. Thus Professor Threlfall,^
investigating Clark cells made in accordance with Lord
Rayleigh's directions, found the coefficient to be 0.000402
for a rapid rise of temperature from 21° to 34° C. This
is less than half the value found by Lord Rayleigh
between the same temperatures.
The magnitude of the temperature coefficient depends,
then, upon the temperature at which the zinc salt is
saturated ; and, because of diffusion, upon the rapidity
of the temperature change. To obviate these difficulties
the zinc sulphate should be saturated at some definite
temperature lower than any at which the cell is to be
used. The temperature selected by the writer is that
of melting ice.
The following table exhibits the observed and calcu-
1 Philosophical Magazine, November, 1889.
92.
PRIMARY BATTERIES.
lated values of the E.M.F. of a cell, set up with such a
solution, in terms of a Rayleigh cell at 20"* C. : —
Temperature C.
Observed.
Calculated.
o
8.3
1.0108
1.0106
8.5
1.0103
1.0105
9.3
1.0104
1.0102
11.8
1.0093
1.0092
13.8
1.0084
1.0085
15.0
1.0080
1.0080
18.1
1.0069
1.0068
19.4
1.0064
1.0063
19.9
1.0062
1.0061
20.3
1.0060
1.0059
20.8
1.0054
1.0057
21.1
1.0057
1.0056
21.6
1.0054
1.0055
22.4
1.0050
1.0052
23.3
1.0048
1.0048
25.1
1.0044
1.0041
26.4
1.0035
1.0036
30.2
1.0019
1.0022
33.1
1.0014
1.0013
39.1
0.9991
0.9989
41.7
0.9980
0.9979
50.4
0.9949
0.9947
52.7
0.9939
0.9940
The Rayleigh cell was always very near 20® C, and
the reduction to that temperature was made by means
of the coefficient 0.00077.
The equation for the E.M.F., derived from the above
observations, is —
E,=^E,,\1- 0.000387 (t - 15) + 0.0000005 (t - 15)* J.
STANDARDS OF ELECTROMOTIVE FORCE. 93
The calculated values of the second column were all
obtained by this formula. The change for one degree C.
is the following linear function of the temperature : —
- 0.000386 -h 0.000001 (t - 15).
The coefficient ranges from 0.00040 at 0° to 0.000376 at
25% and 0.000361 at 40^ C. At the highest observed
temperature of the table it was only 0.000348. The
1.010
N,
""
—
^"
^
«v
N
V
X
\
\
1.005
\
V
V
V
Pm
^
N
»
\
>■
pq
S
\.
1.000
>;
^
s
k.
—
,
\
s^
N
N
»
\
Sk
.995
X
V
Ti
m
>«1
qX
U.1
et
•«
N
^^^
^^^
^^,
^^^
_
—
Nr
10 20 30 40 50
Fig. 45. — Relation between E.M.F. and Temperature.
curve of E.M.F. with temperatures as abscissas is clearly
concave upward (Fig. 45), indicating a fall in the tem-
perature coefficient with rise of temperature. Lord
Rayleigh's cell showed a considerable increase in the
coefficient with rise of temperature, the sign of the
second term in his equation expressing the relation
between E.M.F. and temperature being negative.
The other two objections urged against the usual
form of Clark cell are founded chiefly ou the local
94 PRIMARY BATTERIES,
action taking place when the zinc and mercurial salt
are in contact. Zino replaces mercury to some extent
when in contact with a salt of mercury. With the
oxide of mercury this action is very marked, resulting
in reduction of the mercury and oxidation of zinc. The
same replacement process goes on with mercurous sul-
phate, zinc sulphate being formed at the expense of
zinc and mercury sulphate, while the zinc is amalga-
mated with the reduced mercury. A progressive change
in the density of the solution ensues, entailing perhaps
a rise in the value of the temperature coefficient.
It may be noted, further, that if the cell contains
crystals of zinc sulphate, the liquid at the surface of the
mercury salt in an undisturbed cell is likely to be denser
than it is even a few millimetres higher up, because the
zinc sulphate crystals form at the bottom of the liquid:
Bearing in mind that zinc in dilute zinc sulphate is
positive to zinc in a relatively denser solution, it is easy
to see that a voltaic couple is thus formed of one metal
and two solutions of different densities. That this is
actually the case is proved both by experiment^ and by
the deposit of zinc on the zinc rod just at the surface
of the mercurous sulphate. Upon dismounting and
opening one cell, which was perhaps a year old, it was
found that zinc had been removed from the rod at the
surface of the liquid, and some of it had been deposited
again upon the rod at the surface of the mercury salt in
a solid frill, which was not easily detached. This action
is analogous to the transfer of copper from one plate to
another in electrical connection with it, when the two are
immersed in a solution of copper sulphate, and the tem-
perature at one plate is kept higher than at the other.
1 See Chapter IX.
*
STANDARDS OF ELECTROMOTIVE FORCE. 96
The obvious remedy is to insert a porous partition
between the mercui-ous- sulphate paste and the zinc in
zinc sulphate solution.
For cells not intended for transportation, plaster of
paris, mixed up with a somewhat dilute solution of zinc
sulphate, answers perfectly. Its effect on the E.M.F.
appears to be nitl But if much disturbed it is liable to
break up after a few months. A
slip of cork is better if the cell is
to be roughly shaken, as in trans-
portation. The separation of the
zinc from the mercury salt increases
the E.M.F. about 0.4 per cent, or
from 1.435 to 1,440 true volts at
15° C. Since mercurous sulphate
is almost insoluble in concentrated
zinc sulphate, the separation of the
zinc from the mercury salt appears
to present a complete mechanical
obstacle to local action. This view
is confirmed by observations on
cells two years old.
To prevent accidental short cir-
cuits, it is desirable to mount a
standard cell with a high resistance ^'^- *^-
.,, ., fr-L- i CatharuClBtk SOindanl Coll.
in series with it. Ihis resistance
of about 10,000 ohms, consisting of plumbago on glass,
is mounted in the case (Fig. 46), and is, therefore, always
in circuit with the cell. It can give rise to no error so
long as zero or condenser methods are employed.
63. The Oxide of Kercnry Standard CeU. — This normal
element was described by M. Gouy in the " Journal de
Physique," Tom. VII., 1888, p. 532. M. Gouy employs
96 PRIMARY BATTERIES,
the oxide of mercury instead of the sulphate as a de-
polarizer. He further makes use of a 10 per cent solu-
tion of crystallized zinc sulphate, of density 1.06, in
place of a saturated one.
M. Gouy finds that the negative polarization of his
cells, due to closing the circuit, does not amount to
one one-thousandth of the E.M.F. after the cell has been
agitated and left standing for a short time.
On the other hand, the positive polarization, arising
from a reverse or charging cun*ent, persists longer than
the negative. It can be gotten rid of by closing the
circuit for a short time to produce negative polarization,
from which the cell rapidly recovers. The reverse cur-
rent undoubtedly forms some mercurous sulphate, which
gives a higher E.M.F. as a depolarizer than the oxide ;
and, while it lasts, produces an apparent polarization in
the positive sense.
The E.M.F. of this cell is 1.390 legal volts at 12° C,
and the change due to temperature is 0.0002 volt per
degree. The formula for the E.M.F. is then
Et = 1.390 - 0.0002 (t - 12) .
This is equivalent to a temperatui*e coefficient of
0.000104, or only about 0.01 per cent per degree C
The E.M.F. of this cell is said to increase with in-
crease of density of the zinc sulphate solution.
To prevent local action, the zinc is not allowed to
come in contact with the mercuric oxide. For use in
which high internal resistance is of no consequence, the
zinc rod is placed in a glass tube having in it a small
hole near the lower end. If it is necessary to decrease
the internal resistance, the zinc is enclosed in a linen bag.
Detailed directions are given for the preparation of
STANDAHDS OF ELECTROMOTIVE FORCE. 97
zinc sulphate and mercuric oxide ; also for tlie purificft-
tion of zinc and mercury.
64. Sir WiUiam Thouuoa's Standard Daniell Cell —
Some form of DanicU cell has long been used as a
standard of E.M.F,, partly because its polaiization is
small, and partly because its E.M.F. is near unity. To
insure constancy, some provision must be made to pre-
vent, or at least to greatly retard, the mingling of the
two sulphates. Thus Raoult's cell consists of two glass
vessels, one containing zinc in zinc sulphate, and the
other copper in copper sulphate. When in use the two
vessels are connected by an inverted U-tube, filled with
zinc sulphate solution, and closed at both ends with a
piece of thin bladder.
The normal Daniell element of Sir WiUiam Thomson
(Fig. 47) consists of a rather low glass jar, with a plate
of zinc in saturated zinc sulphate solution at the bottom.
Above is suspended the copper plate ; and the copper
sulphate, which is a half-saturated solution, is introduced
through the funnel, connecting by a rubber tube to a
98 PRIMARY BATTERIES.
siphon which terminates in a pointed horizontal tube at
the surface of the zinc sulphate. By filling the funnel
and gently raising it, the copper sulphate will flow over
the surface of the saturated zinc sulphate, so that the
surface of separation between the two liquids will be
clearly defined. Upon the termination of the experi-
ment the funnel is lowered and the solution is run out.
It should be used but once. Just before making quan-
titative use of the cell a feeble current should be sent
through for a short time to freshly coat the copper plate.
The E.M.F. of a cell thus set up has been found to
be 1.072 true volts at about 15** C. The temperature
coefficient is small, but appears not to have been care-
fully determined. Dr. Fleming found it to be about
one-fifth of the variation of the Rayleigh-Clark cell
between 0** C. and 20° C. ; ^ but Mr. Preece found a
greater variation, amounting to 9 parts in 1000, for
one-half the range of temperature, or between 17** and
28°.2 If Mr. Preece is correct, the temperature co-
efficient of the normal Daniell cell within the above
range is quite as high as that of the Rayleigh form of
Clark element. Mr. Preece's method was scarcely sensi-
tive enough to admit of a good determination of the
variation of E.M.F. with temperature.
65. Lodge's Standard Daniell Cell. — A wide-mouthed
bottle (Fig. 48) is provided with a cork, through which
passes a large test-tube R with a small opening at the
bottom. The zinc rod Z is held in this tube by a cork.
A small test-tube e is fastened to It by an elastic band.
This tube contains the solution of copper sulphate, and
into it dips a gutta-percha covered copper wire, bared at
1 Philosophical Magazine, August, 1885, p. 136.
2 Proceedings Royal Society, Vol. XXXV. 1883, p. 48.
STANDARDS OF ELECTROMOTIVE FORCE. 99
the lower end and furnished with a fresh deposit of
electrolytic copper. The insulated wire passes through
a cork in the small tube. This tube is immersed in the
zinc sulphate solution contained in the bottle ff up to a
point near its top.
Lodge'H BtwldRrd Danlell Cell. Flerolng'B Sundurd DBUlell Cell.
The two sulphates are by this device kept entirely
separate, and the electric connection between them is
established by means of the moisture covering the glass.
The internal resistance of the element b enormously
high, and the cell is applicable, only to zero methods or
comparisons by means of a condenser.
66. Fleming's Standard Daniell Cell — The form of
Daniell cell shown in Fig. 49 was specially designed
100 PRIMARY BATTERIES.
by Dr. Fleming as a standard of E.M.F.^ It consists of
a U-tube 8 inches long and f inch in diameter, provided
with side tubes, glass taps, and reservoirs as shown.
To fill the cell, the tap A is opened, and the tube is
filled with the denser zinc sulphate solution. A is then
closed, and the zinc rod is secured in the left-hand
branch by means of an air-tight rubber stopper P. The
tap C is now opened, and the liquid falls in the right-
hand branch only; and if the tap B is opened at the
same time, the copper sulphate solution will flow in
gently as the level of the zinc solution sinks in this
branch. The operation may be so conducted that the
surface of separation between the two solutions will
remain quite sharp, and will gradually sink to the level
of the tap (7. All the taps are then closed, and the
copper rod is inserted in the right-hand limb.
When the surface of contact ceases to be sharply
defined by reason of diffusion, it is only necessary to
draw off the mixed liquid at the level of the tap (7, and
to supply fresh solutions from the reservoirs above. The
extra tubes, L and iff, are for the purpose of holding the
electrodes when not in use, each in its own solution.
The exact value of the E.M.F. of a Daniell cell is
dependent upon the density of the solutions and the
condition of the zinc and copper surfaces. Thus
Increase in density of the CUSO4 solution increases E.M.F.
Increase in density of ZnS04 solution decreases E.M.F.
Oxidation of the copper surface increases E.M.F.
Oxidation of the zinc surface decreases E.M.F.
Moreover, for an equal increment or decrement of
density of both solutions the increment and decremjent
1 Philosophical Magazine, 5 S., Vol. XX. p. 126.
— -,
STANDARDS OF ELECTROMOTIVE FORCE. 101
of the E.M.F. are so nearly equal, that for equi-dense
solutions, within limits, the E.M.F. is independent of
the absolute density of either.
It is of the utmost importance that oxidation of the
copper surface should be carefully guarded against.
Even slight oxidation, indicated by brown spots, raises
the E.M.F. by as much as 4 parts in 1000, while a film
of dark brown oxide may affect the E.M.F. as much as
2 per cent. Since rolled copper sheets or drawn wire
probably enclose more or less oxide mechanically, it has
been found necessary to freshly electroplate the copper
surface immediately before use. Raoult found that
copper foil gave a higher E.M.F. than electro-deposited
copper by about one two-hundredth ; and he attributed
it to the oxides of copper enclosed in it. If a newly
electroplated copper rod is left in the copper sulphate
solution, it is gradually oxidized ; and the oxidation is
more rapid if the rod is exposed to the air and contains
even a trace of the copper sulphate. The rod should be
electroplated with a thin film of copper immediately
before it is transferred to the standard cell for use.
If a chemically pure zinc rod is used, it is immaterial
whether it is amalgamated with pure mercury, or is
freed from oxide on the surface by slight rinsing in
dilute sulphuric acid before placing it in the sulphate
of zinc.
For general use Fleming recommends two standard
solutions of each salt. First, a solution of copper sul-
phate, saturated at 15° C, and of density 1.2, and a
solution of zinc sulphate of the same density. Second,
a solution of the copper salt, of density 1.1 at 15% and
one of the zinc salt, of density 1.4 at the same tempera-
ture.
102 PRIMARY BATTERIES,
If equi-dense solutions are used, with the precau-
tions already described respecting the surfaces of the
zinc and copper rods, the E.M.F. is very close to 1.102
true volts.
If, however, copper sulphate of density 1.1 and
zinc sulphate of density 1.4 are used, then the E.M.F.
of the cell is 1.072 volts. These last solutions corre-
spond with those employed by Sir William Thomson in
his standard form of gravity cell.
If the cell is allowed to stand an hour or so after the
freshly electroplated copper pole is introduced into it
before measuring the E.M.F., then its value will be
about 0.003 volt higher than the above, provided the
zinc retains a bright untarnished appearance. But the
smallest deposit of copper on the zinc, due to the diffu-
sion of the copper salt into the zinc sulphate, lowers the
E.M.F. 2 or 3 per cent.
The many precautions required to insure a normal
E.M.F. in a standard Daniell cell, on every occasion of
its use, are more than an offset to a negligible tempera-
ture coefficient in comparison with that of a Clark cell,
particularly if the latter is reduced to 0.038 or 0.039
per cent.
67. The Chloride of Lead Standard CeU. — MM. Bailie
and F^ry have proposed ^ the use of a salt of lead as a
depolarizer. The best results were obtained with the
chloride. It has one of the disadvantages of the Daniell,
but in an inferior degree ; that is, the deposition on the
zinc of the metal contained in the depolarizer. But
with proper precautions, the formation of this metallic
deposit may be greatly retarded.
The cell is mounted as follows: Powdered lead
1 Journal de Physique, Tome IX. p. 234.
i
STANDARDS OF ELECTROMOTIVE FORCE. 103
chloride, precipitated from a warm solution and of
crystalline texture, is introduced into the tube A (Fig.
50) which encloses a lead wire, forming the negative of
the element. The positive is a plate of zinc, amalga-
mated and immersed in a solution of chloride of zinc, of
density 1.157.
When the circuit is closed zinc is dissolved, and
chloride of lead is reduced.
The E.M.F. decreases with the
concentration of the zinc chloride
solution. With the above density,
made by dissolving 17.2 gms. pure
zinc chloride in 100 c.c. distilled
water, the E.M.F. is exactly one-
half a volt. Dr. Fleming's standard
Daniell cell was taken for com-
parison.
The variation of E.M.F. with
temperature was found to be almost
negligible, amounting to only 0.005
volt in 46° C.
The solution of zinc chloride ^*«^- ^3",^^^^^^^^^
should be made neutral by agitation
with zinc oxide, since the presence of free acid aug-
ments the electromotive force.
The polarization, though greater than in the Daniell
cell, is still very small, and the cell recovers promptly
and exactly its normal value.
68. To Measure the E.M.F. of a Standard Cell. — In the
absence of means of making an absolute determination
of the E.M.F. of a standard cell, the silver voltam-
eter may be resorted to as a secondary standard. Of
this method. Lord Rayleigh remarks : " It will be seen
104
PRIMARY BATTERIES,
that in this way any one may determine the E.M.F. of
his standard battery with a very moderate expenditure
of trouble, and without the need of any special
apparatus." ^
The method of making the determination is shown in
Fig. 51. The main battery -B is a storage cell, and in
series with it is a carefully adjusted resistance SR of
10 legal ohms at 14® C, made of platinoid wire im-
mersed in kerosene; also a silver voltameter V^ and a
second resistance
B
iron
of heavy
wire for the pur-
pose of adjusting
the current to the
proper value. The
standard cell B' is
placed in a derived
circuit at the ter-
minals AC oi the
10-ohm coil. In
circuit with it is
a sensitive "long
coil " galvanome-
ter (r, and a carbon resistance, HR^ of 100,000 ohms. A
balance is effected between the E.M.F. of the standard
and the fall of potential over the 10-ohm coil by vary-
ing the auxiliary iron resistance, and by greater or less
immersion of the vertical silver plates of the voltameter
in the silver niti-ate solution. If any small change
occurs in the current during the deposition of the
silver, the balance may be maintained perfectly by
changing slightly the depth of immersion of the silver
1 Philosophical Transactions, Part II. 1884, p. 453.
Fig. 51. — E.M.F. Measured by Silver Voltameter.
STANDARDS OF ELECTROMOTIVE FORCE. 105
plates. For this purpose the voltameter is provided
with a rack-and-piniou movement for the plates.
All the conditions for a balance being ascertained,
the gain plate is carefully washed, dried, and weighied.
It is then replaced, and the circuit is kept closed for a
sufficient time to secure enough gain in the kathode
plate to weigh accurately, the balance being carefully
maintained as described during the entire time. The
gain plate is again removed, and the amount of silver
deposited is determined. This gives the value of the
mean current through the 10-ohm coil. Then by Ohm's
law, E— OR ; and since both current (7 and resistance
B are known, E is in this manner determined.
Example.
Temperature of standard cell, ^(15^5 + 15^7) = 15^6 C.
Temperature of 10-ohm coil, ^16°-^ + 1^**-'^) = 16°-6^ C-
Eesistance of 10-ohm coil at 16°.65 = 10.00583 legal ohms.
Weight of silver plate after deposit, ... 29.99292 gms.
Weight of silver plate before deposit, . . 29.79942 "
Weight of. silver deposited, 0.1935 "
Time of deposition, 20 minutes.
1 ampere deposits 4.0246 gms. per hour. ,
Hence the current equals
0.1935 H- i(4.0246) = 0.14424 amperes,
and
E = 0.14424 X 10.00583 = 1.44324 legal volts at 15°.6 C.
Reducing to 16° by the formula ^
1.4432 = ^ [1 - 0.000386 {t - 15)],
the E.M.F. of the standard equals 1.4435 legal volts.
PRIMARY BATTERIES.
CHAPTER VIII.
MIBCBLLANBOUS BATTESIB8.
69. Qrove's Oaa Battery. — The polarization cnrreiit
obtained from a water voltameter, and due to the oxygen
and hydrogen clinging to the two platinum plates, sug-
gested to Grove the possibility of prolonging this cur-
rent by supplying a suf-
ficient quantity of the
two gases in contact
with platinum. The po-
larization current soon
exhausts the films of
oxygen and hydrogen
on the two respective
plates. By extending
the strips of platinum
so that they are partly
in the liquid and partly
in the gas of each tube
of the voltameter siip-
plied with water acidu-
lated with sulphuric
acid, density 1.2, Grove
succeeded in producing
riB.5a.-G™v«'>GuB<iiwry. coutinuous currents of
sufficient intensity to decompose water and to produce
a brilliant spark in broad daylight between two carbon
MISCELLANEOUS BATTERIES. 107
points. For this latter purpose he employed fifty-
pairs.
The figure exhibits the form of gas battery preferred
by Grove. F'is a three-necked Woulflfs bottle. In the
two outer holes are fitted two glass tubes by means of
rubber stoppers. Each tube is open below and contains
a piece of platinum foil ending above in a platinum
wire, which is sealed into the top of the tube. The
entire apparatus is filled with acidulated water through
the middle opening jB, and a current is then passed
through till one tube H is filled with hydrogen, and
the other half-filled with oxygen. If now the battery is
removed, and the terminals at P and JVare connected
by a conducting circuit, a current flows from the oxygen
tube to the hydrogen through the external circuit.
In order to increase the surface of the liquid in con-
tact with the platinum and exposed to the gas. Grove
covered the foil with pulverulent platinum by Smee's
method of electrolytic deposit. The liquid then rises
along the roughened surface by capillary action.
The hydrogen in this cell plays the part of the zinc in
a voltaic element. The current through the cell is from
the hydrogen to the oxygen — the reverse of the decom-
posing or charging current.
In modern nomenclature this is a storage battery.
The effect of the charging current is to decompose sul-
phuric acid primarily and water as a secondary reaction ;
and the accumulation of the products of the electrolysis
in the two tubes is a storage of potential energy. When
this potential energy is converted into the kinetic energy
of a current, all the processes are reversed, the current
with the others. In the same way, when energy is
stored in the potential form by lifting a weight from the
108 PRIMARY BATTERIES.
earth, the running down of this energy by conversion
into the kinetic variety involves a reversal of the mo-
tion of the weight.
In the electrolytic process the chain of molecules may
be represented as follows : —
■^ — %
H,0 I H2SO4 I H3SO4 I H,0.
* — , — '
< «
After the first step in the electrolysis this becomes —
Ha I H2O I H2SO4 I H2SO4 I 0.
The oxygen and hydrogen are now at the two ends of
the chain; and, leaving out the water as unessential,
the chain of the gas battery may be written —
Ha I H2SO4 I H2SO4 I O;
» >
and this becomes, after the first exchange of atoms
among the molecules —
H2SO4 I H2SO4 I H2O.
Hydrogen is in both cases transferred in the direction
of the current, which is shown by the arrow. In the
discharge process the oxygen may equally well be sup-
posed to suffer a transfer in the opposite direction,
though it is simpler to conceive of the motion of the
hydrogen only. The operations of the electrolytic
process are then strictly reversed in the recombining
process.
The tubes of the gas battery may be filled with the
two gases obtained by any other method than elec-
trolysis, with no difference in the result. <
MISCELLANEOUS BATTERIES. 109
If one tube is filled with hydrogen and the other with
acidulated water, a current is still obtained, and hydro-
gen gradually disappears on closed circuit. Grove
showed that the current in this case was due to the
oxygen absorbed from the atmosphere.
Similar results were obtained with other gases, not-
ably hydrogen and chlorine; also with one gas and a
liquid whenever chemical reaction was possible between
the two.
70. TTpward's Chlorine Battery. — The electrodes are
zinc and carbon, the former immersed in water contained
in a porous cup, and the latter in water saturated with
chlorine gas. The space between the porous cup and
the carbon is filled with broken retort carbon. Each
cell contains several zincs and carbons joined together
in multiple.
Since the chlorine is both the active exciting agent
and the depolarizer, the liquid about the carbon is kept
saturated with the gas, which passes into the porous cup
by diffusion, while the zinc chloride formed diffuses
outward. The cell must be closed air-tight to prevent
the escape of chlorine.
Each cell consists of a glazed vessel, with an inlet
tube near the bottom and an outlet near the top. A
glazed cover, with the requisite provision for the passage
through of the two electrodes, closes the cell tightly.
The chlorine, made from chloride of lime (CaOClj), is
stored in a glazed earthenware cylinder provided with
inlet and exit tubulures. The cells and the reservoirs
are connected together in series, the top of the reservoir
to the bottom of the first cell ; the top of this cell to
the bottom of the second; and the top of the second
back again to the reservoir. Each cell is further pro-
110 PRIMARY BATTERIES.
vided with a draw-off stone tap for removal of the zinc
chloride formed in the action of the cell.
The E.M.F. is 2.1 volts and very constant. Large
cells have been built by Woodhouse & Rawson for
charging storage batteries, and they are said to furnish
a current of 150 amperes on short circuit.
71. Powell's Thermo-Electro-Cliemical Battery.^ — Differ-
ences of potential have often been observed between
two plates of the same metal in a solution of a salt of
the same, when one plate is at a higher temperature
than the other. Thus two zinc rods in sulphate of zinc
are at a different potential if their temperatures are
different, the one of higher temperature constituting
the positive electrode (negative plate) of a voltaic couple.
This property has been applied to the construction of
a thermo-chemical couple with copper plates in copper
sulphate solution. A horizontal plate is placed in the
bottom of the cell, and a well insulated wire leads out,
preferably through a glass tube. Another copper plate,
with a copper tube attached to its centre, is suspended
so that its under surface touches the surface of the solu-
tion. Half-a-dozen small openings at the bottom of the
copper tube convert it into a rose burner. Gas is con-
ducted in through the tube, lighted at the openings, and
the small flames heat copper wires riveted to the copper
plate. The transfer of heat to the plate, and so to the
liquid, is thus increased.
Under these conditions, a current flows from the warm
to the cold plate through the external circuit, and
copper is transferred from the cold plate to the warm
one through the solution. In other words, the cold
plate performs the same function as the zinc in a simple
1 London Electrical Review, Vol. XX. p. 2.
MISCELLANEOUS BATTERIES. Ill
voltaic element.^ The energy concerned in the transfer
comes from the heat applied. The combination is thus
both a primary (heat) battery and an electrolytic cell.
The potential energy transformed is in this case repre-
sented by the illuminating gas.
The E.M.F. is small, only about 0.035 of a volt with a u^
difference of temperature of 50° C. between the upper
and the lower plates.
A small addition of sulphuric acid, which is of utility
in an electrolytic cell for copper sulphate, reduces the
E.M.F. of the thermo-chemical battery to zero. Copper
nitrate may be used in place of the sulphate.
Note. — The inventor of this battery describes it with the current
flowing through the cell from the warm plate to the cold one, and says
expressly that copper is transferred from the top to the bottom
(Electrical Review, Vol. XX. p. 2, London). But if the reader will
consult the next chapter, he will find an account of tests on this point,
with a table of E.M.F. 's at different temperature differences.
72. A Battery Absorbing Oxygen from the Air.^ — When
copper is alternately exposed to the air and immersed in
an aqueous solution of ammonia, it oxidizes, and the
oxide dissolves as a blue solution of ammoniacal cupric
oxide. If the copper remains immersed in the solution
at a considerable depth, the supply of oxygen that can
reach the copper plate is very limited, and cuprous
oxide is formed and dissolved.
If now an aerating plate of platinum foil or platinum
sponge is supported on the liquid surface, and connected
by a wire with the copper, a current flows through the
liquid and the wire, and the process of oxidation and
solution is greatly hastened. The platinum plate or
1 See Chapter IX.
2 Proceedings Rpyal Society, Vol. XLIV. p. 182. —
112 PRIMARY BATTERIES.
sponge condenses oxygen, which is gradually transferred
to the copper.
The current rapidly runs down if its density is more
than one micro-ampere (millionth of an ampere) per
square centimetre of the aerating plate. The E.M.F.
may be from 0.5 to 0.6 of a volt. The addition of
common salt or of sal-ammoniac reduces the internal
resistance and increases the E.M.F.
With a thin layer of spongy platinum as the aerating
plate the E.M.F. may be as high as 0.8 of a volt.
Similarly, if a platinum plate is immersed in a solu-
tion of ferrous sulphate or sulphurous acid, and an
aerating plate is placed on the surface of some dilute
sulphuric acid in another vessel ; , and if the two vessels
are connected with a siphon or a piece of moistened
candle wick, and the two plates are joined by an electric
conductor, the oxygen condensed by the aerating plate
will be transferred to the oxidizable solution in the
other vessel, with the formation of ferric sulphate or
sulphuric acid, and at the same time a current will flow
through the circuit.
73. Minchin's Seleno-Alumintun Cell. — Professor Min-
chin ^ constructs a cell sensitive to light in the following
manner : Two small clean plates of aluminum are taken,
and a thin layer of sensitive selenium is spread over
one of them. Fine platinum wires are then attached to
both plates, and they are immersed in presence of each
other in a small glass cell containing acetone. Alcohol
— preferably methylic — answers very well, except that
in a few days the plates become covered with a gelati-
nous deposit of aluminate of alcohol.
The selenium must be treated by heating and care-
1 Philosophical Magazine, Vol. XXXI. p. 207.
MISCELLANEOUS BATTERIES, 113
fully keeping it near the melting-point for some time,
till it assumes a very dark brown color. It has then its
most sensitive surface.
When a cell, constructed as described, is exposed to
light, an E.M.F. is at once developed, and the sensitive
seleno-aluminum plate is negative towards the insensi-
tive one, i.e. as copper to zinc.
This photo-electric cell is sensitive to all parts of the
spectrum, with a maximum in the yellow near the bor-
der of the green. The variation in sensitiveness through-
out the entire visible spectrum is about 30 per cent.
74. Shelf ord Bidwell's Dry Battery. — This cell, which
grew out of an investigation into the sensitiveness of
selenium to light, has thus far only a scientific interest.
On a plate of clean copper is spread a layer of copper
sulphide. The sulphide is then compressed in a vise
between the copper plate and one of polished steel. The
steel plate is next carefully removed, and a thin layer
of silver sulphide is spread over the compressed copper
sulphide. Finally, a plate of silver is pressed down
upon the sulphide and the cell is complete.
The copper plate constitutes the positive electrode,
the current flowing through the cell from the silver to
the copper. The chemical action consists in the reduc-
tion of the sulphide of copper with deposition of copper
on the copper plate, and the simultaneous formation of
an equivalent amount of the sulphide of silver.
The cell is entirely analogous to the Daniell, with
copper and silver in their sulphides in place of copper
and zinc in their sulphates.
With copper and silver separated by copper sulphide
only no current was obtained; but when free sulphur
was njixed with the sulphide, the cell became active.
114 PRIMARY BATTERIES.
75. Jabloclikoffs Battery. — Carbon is attacked by
nitrates in a state of igneous fusion, while iron is not.
Hence a vessel of cast iron, cylindrical in form and filled
with fused nitrate of potassium or sodium, serves at the
same time as a receptacle and as an unattacked electrode.
An iron wire helix serves to hold the coke and to con-
duct to the external circuit. If the nitrate is maintained
in a state of fusion, the cell will have an E.M.F. of from
one to two volts.
It has been observed that if an aqueous solution of
the salt is used instead of the fused nitrate, the poles
are reversed, or the iron is the negative electrode and
acts like zinc in a simple cell.
76. Battery with Two Carbon Electrodes. — This was
devised by Tommasi and Radiguet in 1884. At the
centre of a cylindrical glass jar is placed a carbon rod,
covered with a thick layer of peroxide of lead, the
whole enclosed in a linen bag.
This enclosed electrode is placed in a carbon tube
pierced with holes; the two electrodes are then put
into the glass jar and filled around with fragments of
retort carbon, and a concentrated solution of chloride
of sodium added to chloride of calcium. This latter salt
serves to retard very much the evaporation of the water.
The carbon rod with the coating of lead peroxide is
the positive electrode.
The E.M.F. is from 0.6 to 0.7 of a volt. No action
takes place on open circuit, but since polarization sets in
rapidly on closed circuit, the cell can be used only for
applications requiring an intermittent current. For
such purposes it has a very long life. Some of these
cells, after remaining in service for several years, operate
absolutely as well as the first day they were set up.
BATTERY TESTS. 115
CHAPTER IX.
BATTERY* TESTS.
77. What a Systematic Test Includes. — The most ob-
vious quantities to be measured are the E.M.F. and
internal resistance. While a high E.M.F. is desirable
for most purposes, a low E.M.F. is no indication that a
battery may not be admirably adapted to its intended
work. So low internal resistance is a commendable
feature, because, oaBterls paribus, low internal resistance
means high efficiency ; but if a battery is to be used on
a circuit of high resistance, its own resistance is rela-
tively of less account. For large currents, low internal
resistance is a necessity.
It is further very desirable to know the rate, progress,
and total amount of polarization that takes place when
a cell is kept on a closed circuit of known resistance for
a definite period. The results of a test to determine
such data respecting polarization can all be expressed
graphically in the form of a curve.
So also the promptness and extent of the recovery
from polarization are equally essential objects of investi-
gation, and the results can be expressed in the same
manner as the polarization.
These data, together with the potential difference at
the terminals or electrodes, when the battery is on
closed circuit, furnish all that is needed to compute the
internal resistance and the current.
116 PRIMARY BATTERIES,
An efficiency test can be made only by working a
battery to exhaustion. This is not practicable for one
of relatively large internal resistance and rapid polariza-
tion. For open circuit cells many plans have been
devised to secure continuous intermittent test service
extending over long periods. But none of these is so
satisfactory as to place a battery in actual service and
wait for results.
Another important object of inquiry is the amount of
depreciation and local action taking place on prolonged
standing on open circuit. This is applicable strictly to
open circuit cells only.
The practised eye of the observer with experience
will not overlook many details of mechanical construc-
tion, which are as important to the satisfactory working
of a battery as its electrical features.
78. Theory of the Method of Measuring E.M.F. and
Internal Resistance. — The E.M.F. is measured by com-
paring it with that of some standard which is known.
The standard employed in the following tests was the
author's form of the Latimer Clark cell, having an
E.M.F. of 1.44 true volts, or 1.444 legal volts, at 15° C.
For ordinary battery tests a rapid method of comparison,
accurate to one-half per cent, is all that is required.
The condenser method is the only one that admits of
sufficient rapidity, and it possesses the required accu-
racy. For this purpose, a standard mica condenser,
divided into fractions so as to admit of using from 0.05
to one microfarad, and a sensitive reflecting galva-
nometer of from 5000 to 7000 ohms resistance, are
required. Also the proper charge and discharge keys,
and an ordinary circuit-closer. *
The condenser is then charged with the standard cell
1
i
BATTERY TESTS. 117
and discharged through the galvanometer, and the deflec-
tion noted. The same process is repeated with the
cell to be tested. The ratio of the deflections produced
is the ratio of the electromotive forces of the standard
and the cell in question ; for the deflections are at least
approximately proportional to the quantities of elec-
tricity discharged through the galvanometer, so long as
those deflections are not large and not widely different ;
and the quantities are proportional to the electromotive
forces charging the condenser, the capacity of which
remains constant.
To obtain the internal resistance, we must know the
total E.M.F. of the cell, and the difference of : potential
between its terminals when the circuit is closed through
a known external resistance. If, now, it is assumed that
the potential difference at the terminals can be measured
so soon after closing the circuit that no polarisation has
set in, then the total E.M.F., previously measuted, is the
whole fall of potential over the resistance of the entire
circuit, while the difference of potential at the battery
terminals represents the fall over the known external
resistance, which must contain no source of E.M.F. If,
therefore, J& and H' represent total E.M.F. and terminal
potential difference, r and jB the internal and external
resistance respectively, then —
E:E'::r+ R:E.
Hence E -E' :E' ::r: B,
and r = i?^^:^.
E'
Since M is known, and ^ and ^' have been measured,
r is also known for flie given conditions of external
resistance and current*.
118
PRIMARY BATTERIES.
There is reason to believe that the resistance, and
probably the electromotive force, of a battery depends to
a certain extent upon the current flowing through the
battery, and upon the rate of diffusion of the products
of the chemical changes taking place. The resistance,
and generally the electromotive force, varies also with
the temperature of the battery.
All that can be positively affirmed of the value r,
obtained as described, is that it satisfies the equation
K
B
r3
K'
Fig. 53. — Diagram of Battery Tests.
expressing the relation between 5, -B, and JE'. Still, it
is true that for widely different values of B, the value of
r ascertained by this process will enable us to compute
with considerable accuracy the potential difference \B'
at the terminals available to produce a current through
a known external resistance M.
In Fig. 53 are shown diagrammatically the conneC'
tions of the apparatus for making the measurements
BATTERY TESTS. 119
described. The condenser is at 0, the galvanometer at
Q^, the battery at B^ and the charge and discharge key
at K. When the key makes contact with the upper
point, the battery is disconnected, and the condenser is
in the discharge relation to the galvanometer. When
the key K is depressed, the galvanometer is cut off
from one side of the condenser, the battery charges the
condenser, and as soon as the key again makes contact
with the upper point, the condenser discharges through
the galvanometer. This operation requires only a frac-
tion of a second. It is repeated several times, first with
the standard cell, and then with the battery to be tested.
To measure the internal resistance, the battery is
closed by the key K' through a suitable resistance 22,
which must be known. The key JS?. should be closed
only long enough to charge and discharge the condenser
by means of key K. A little practice will enable the
experimenter to accomplish this within a second, pro-
vided he is supplied with suitable keys. It would not
be difficult to have these operations performed mechani-
cally.
79. To Obtain Data for Cnrves of Polarization, Eecovery,
Internal Eesistance, and Current. — For initial electro-
motive force and internal resistance, proceed as de-
scribed in the last section. Then close key K' and at
the end of two minutes charge and discharge the con-
denser to obtain potential difference between the ter-
minals. At the end of four minutes open key K' long
enough to quickly and expertly charge and discharge
the condenser. This may be so quickly accomplished
that there will be no appreciable recovery of the battery
from polarization.
If the galvanometer needle can be brought to rest in
.y.
120 PRIMARY BATTERIES.
two minutes, these operations are repeated alternately
every two minutes for an hour. The key K^ is then
opened permanently, and the total E.M.F. is measured
every two or four minutes for an hour longer to follow
the recovery from polarization. If practicable, the
operations may be repeated at shorter intervals at the
beginning of both the polarization and the recovery,
when the rate of change of electromotive force is the
greatest.
With accurately ruled square paper, the electromotive
forces may then be read off by means of a single straight
line, which is drawn on the paper as follows : —
Let the vertical lines or ordinates represent electro-
motive forces, and the horizontal ones the deflections of
the galvanometer. Since the E.M.F. is zero if the deflec-
tion is zero, the origin is one point of the required line.
Then if the E.M.F. of the standard cell is 1.44 volts,
and the corresponding deflection obtained by charging
the condenser with it is D; and if d is the deflection
corresponding to an electromotive force ^ to be found,
then —
lM:E::D:d,
and E = 1.44-|.
It is evident that the relation between H and d is the
equation of a right line, and the constant of the equa-
tion is 1.44/i). Assume any value of c?, as 100 ; the
corresponding value oi JS is 144/ D. Lay off this value
of ^ as an ordinate corresponding to an abscissa of
100, and a second point is obtained. The straight line
drawn through this last point and the origin is the one
required. Then for any other deflection of the galva-
BATTERY TESTS. 121
nometer, find the vertical line passing through the num-
ber on the axis of abscissas, and follow this line up to its
intersection with the straight line just drawn. The
number on the horizontal line passing through this inter-
section, at the point where it meets the axis of ordinates,
is the E.M.F. sought. In other words, we have only
to find by means of the oblique straight line the ordi-
nate corresponding to any abscissa which represents a
deflection.
If the square paper is divided into small sections,
each small space may represent a hundredth of a volt.
The horizontal scale of deflections may be chosen in any
convenient manner.
To find the curve of internal resistance, it is necessary
to plot first the curves of polarization and terminal
potential difference, with the times of observation as
abscissas. Then the short vertical lines between the
two electromotive force curves represent the loss of
potential, JE — W^ in the battery itself. Substituting
in the formula for r of the last section, the series of
internal resistances for the entire time of the test is
obtained.
For the current curve, divide the terminal potential
differences at the successive times of observation by the
constant external resistance iJ. These quotients, laid
off as ordinates, will give the points of the curve, x^ "
80. Test of a Typical Leclanche Cell. — The entire data
of such a test as has been described are contained in the
table. The column headed t gives the time in minutes
from the beginning of the test, when the circuit was
closed; d and d' are the deflections observed when the
condenser was charged, first with the battery circuit
open, and then closed through 5 ohms ; ^-*»^ JiP
122
PRIMARY BATTERIES.
W
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BATTERY TESTS,
123
have the significations already given to them ; while
and r are current in amperes and internal resistance in
ohms respectively. The E.M.F. of the standard was
1.443 legal volts.
The same results are expressed graphically in the
curves of Fig. 64, all of them being drawn to the same
scale, except the internal resistance as indicated.
JO
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Hin. 20 40
Fig:. 54.— Test of Leclanch^ Cell.
60
The polarization curve shows a very rapid fall of
electromotive force for the first four minutes, and quite
a steady decrease up to three-quarters of an hour. The
recovery curve shows an almost equally rapid rise of
124 PRIMARY BATTERIES.
electromotive force for the first four minutes after open-
ing the circuit. It continues to increase up to the end
of the hour, when it is still a quarter of a volt below its
initial value. The recovery curve is plotted back from
the end of the polarization curve toward the left, so as
to exhibit more plainly the depression of the voltage at
the end of the two hours test.
The terminal difference of potential is more nearly
constant in valuej after the first steep incline than the
total E.M.F. ; and the shortening of the intercepts be-
tween the two curves shows the decrease in the internal
resistance during the hour.
The current after the first two minutes exhibits great
steadiness for an open circuit battery. The fall to the
end of the hour is only 0.016 ampere, or a little less than
8 per cent.
The initial resistance of this particular cell is high,
but it falls more than 50 per cent during the hour.
Other individual cells made at the same factory show
an internal resistance as low initially as 0.8 of an ohm.
81. Test of Leclanch^ Cell with Depolarizer Enclosed in
Carbon Cylinder. — In cells of this character the depolar-
izer is not favorably located to accomplish its purpose,
since the current leaves the outside of the carbon cylin-
der rather than the inside where the manganese dioxide
is placed. It is exceedingly doubtful if the depolarizer
is of much value in this relation to the carbon surface
unless it is a soluble salt and diffuses through the
liquid.
The large area of carbon surface is an offset, however,
to the unfavorable location of the manganese dioxide.
A large carbon surface diminishes polarization. It has .
been found as a result of many experiments that reduc-
BATTERY TESTS.
125
tion of zinc surface does not exercise so notable an effect
on the current strength as the reduction of carbon sur-
face. Hence the practice of employing zinc rods of
small surface area, and carbon plates, rods, and cylin-
der of much greater superficies.
Attention is called to the slower rate of polarization
of this cell. Fig. 55, as compared with Fig. 54, on first
as
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Fig. 56. — Test of Cell with Carbon Cylinder.
60
closing the circuit. The polarization is more continu-
ous, but not so precipitate. The recovery is of the
same character. This feature in the polarization curve,
which may be called the " characteristic " of a battery,
is advantageous in cells which are designed for service
requiring ordinarily the closing of the circuit for only
126
PRIMARY BATTERIES,
a few seconds intermittently. The polarization is less
for short intervals than with cells having a steeper
polarization curve at the beginning.
The terminal potential curve runs nearly parallel with
the total E.M.F. curve, and the vertical intercepts be-
tween the two are short. With 5 ohms external
Min. 20 40
Fig. 66.— Test of Another Carbon Cylinder Cell.
60
resistance, the uniform value of M for all these curves,
unless another value is given, the internal loss of
energy in this cell is only 6.2 per cent, the internal
resistance averaging about 0.33 of an ohm. The cur-
rent fell from 0.26 to 0.2 of an ampere during the entii*e
hour.
Fig. 56 illustrates another cell of the same general
BATTERY TESTS.
127
characteristics, but of a different manufacture and smaller
zinc surface. Both have the black oxide of manganese
enclosed in a carbon cylinder, and both show polariza-
tion and recovery curves of the same character, though
the recovery of the latter is less marked. It has a higher
E.M.F. and a slightlj'^ smaller internal resistance. The
energy wasted internally averages about 6.7 per cent.
/.o
V
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Fig:. 57. —Zinc-Carbon Cell without Depolarizer.
60
It is to be borne in mind that these particular values
are derived from individual cells, and do not represent
the average obtained from a number of the same type.
82. Test of Zinc-Carbon Cell without Depolarizer. — The
curves of Fig. 57 are derived from an investigation of a
well-known type of battery employing ammonic chloride,
but no depolarizer whatever. The polp '■ ' ' "^^ '*s some-
128
PRIMARY BATTERIES.
what more pronounced at the start, but has the same
progressive character as in the two preceding cases.
The internal resistance exhibits marked irregularities,
and is higher than would be anticipated, considering the
extent of carbon surface. The current is nevertheless
quite regular and has a mean value somewhat above 0.2
of an ampere.
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60
Fig. 58. — Curves from a Dry Cell.
83. Test of a " Dry *' Cell. — A dry cell has the excitant
in the foi-m of a semi-fluid or porous, pasty mass. In so
far as polarization depends upon diffusion the dry cell
may be expected to show a more marked and persistent
depression of voltage when placed on an external resist-
ance of no more than 5 ohms. Such anticipations are
abundantly justified by the curves of Fig. 58, derived
from a test oi one of the best known cells of this class,
The E.M.F. fell to less than one-half its initial value in
BATTERY TESTS.
129
the hour, and its recovery during the following hour
was quite leisurely. The semi-liquid electrolyte admits
of only slow diffusion, even though ingredients may be
added to make the mass porous. The internal resist-
ance of this cell was not large, but was irregular, and
the current fell during the test to less than half its
initial value, because of the great drop in potential.
/•o
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20 40
Flgr. 59. — Second Dry Cell.
60
Another type of dry cell, not described in the preced-
ing pages, furnished the curves of Fig. 69. It must be
admitted that this cell makes a showing comparing
favorably with zinc-carbon cells set up with a liquid
electrolyte.
The polarization is leisurely, the internal resistance
only three-tenths of an ohm, and the current averages
fully 0.2 of an ampere. —
180
PRIMARY BATTERIES.
Two of these same cells were subjected to a test in
which for four months continuously they actuated a
relay-sounder of about 100 ohms resistance every second,
by means of a seconds-pendulum. They showed no
perceptible deterioration in that time, though on two or
three occasions the clock was stopped for several houra,
during which time the circuit remained closed. On
T
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Fisr. 60. — Corves from Chloride of Silver CelL
starting the clock, the relay-sounder again operated
without readjustment. After standing idle for seven
months these same cells were again put to the same
service in time measurements, and they are still as
efficient as ever after three months' additional use.
84. Test of a Silver Chloride Cell. — The curves (Fig.
60) obtained from a small silver chloride cell, made by
BATTERY TESTS, 131
the author, serve to illustrate a number of points. The
chloride was cast in a carbon mould on a silver wire,
leaving a veiy small surface of silver exposed to the
exciting liquid, which was dilute ammonic chloride.
The internal resistance on first closing the circuit,
before any metallic silver had been reduced from the
chloride, was 79.7 ohms. It fell during the hour to one
ohm, the scale of the resistance cui-ve being one-tenth
as large as that of the others. This precipitate fall of
resistance is due to the reduction of silver from the
chloride, which converts a poor conductor into the best
one known.
Coincident with this fall of internal resistance is the
rise of potential difference at the terminals and of the
current. The latter does not rise above 0.17 of an
ampere. The cell was a small one, with about two
square inches of zinc surface. The polarization of the
silver chloride cell is so slight as to justify its inclusion
in the list of constant current batteries ; for the E.M.F.
is nearly constant, and the drop in internal resistance
causes the current to increase in intensity instead of the
reverse. The recovery from polarization is extremely
prompt, and occurs within the first two or three min-
utes after opening the circuit. The initial value of the
E.M.F. is not regained, but the final loss is less than
0.05 of a volt.
85. Efficiency Test of Copper Oxide Battery. — An effi-
ciency test consists of two parts : —
First, the determination of the total quantity of elec-
tricity obtained by the consumption of a definite weight
of zinc, compared with the quantity of electricity required
to deposit the same weight in electrolysis.
Second, a comparison of the useful energy in the
182 PRIMARY BATTERIES.
external circuit with the internal energy as heat waste
in the cell itself.
For the first, the zinc must be weighed at the beginning
and end of the test, and the whole number of ampere-
BATTERY TESTS. 133
hours must be determined. This gives the quantity
of electricity obtained by the consumption of a known
weight of zinc. The quantity required to deposit the
same weight of the metal can be calculated from the
electrochemical equivalent of zinc.
For the second part of the test, the internal resistance
must also be measured at intervals during the run.
Then the energy lost as heat in any circuit of resistance
r is (7V ; for energy is the product of current and elec-
tromotive force, and by Ohm's law electromotive force
is Or. Hence energy is CV. The external resistance
being known also, the external energy is calculated in
the same way.
The curves of Fig. 61 express graphically the results
of such a ''test made with great care by Mr. A. E. Ken-
nelly in €he Edison laboratory. Four 300-ampere-hour
cells were taken at random from the stock. They were
joined in series in a circuit of 0.8 of an ohm external
resistance. The total run was 108 hours. The external
energy increased quite up to the middle of the time,
because of the continuous reduction in internal resist-
ance.
The following are details of the computation : —
Weight of zinc before test .... 10,017 gms.
« « « after " .... 8,567 "
Total loss 1,450 "
Loss calculated from output . . . 1,444 "
Loss by local action 6 "
Mean current 2.76 amperes
« E.M.F 2.8 volts
Total quantity in ampere-hours . . 298
134 PRIMARY BATTERIES.
The ampere-hours are the product of the mean current
and the time, or 2.76 x 108 = 298.
Taking the electrochemical equivalent of zinc as
0.0003367, the calculated loss is as follows : —
298 X 3600 X 0.0003367 = 361 gms. per cell.
361 X 4 = 1444 gms. for 4 cells.
The quantity 0.0003367 is the weight in grammes
deposited by one coulomb, — an ampere for one second.
Hence ampere-hours must be multiplied by 3600 to
reduce to ampere-seconds or coulombs.
In reading the figures* at the left of the diagram, all
except those relating to current must be divided by 4
to reduce to the values for a single cell.
An efficiency test of this same type of cell, conducted
by the author, showed curves approximating much more
closely to straight lines than those of the diagram. The
E.M.P., current, and internal resistance were even more
constant after the first few hours than those represented
above. The total output for a single cell was 390
ampere-hours.
A 15-ampere-hour cell tested to exhaustion gave 10.1
ampere-hours and 7.5 per cent loss of zinc by local
action. This cell had been standing a long time with
the undissolved alkali exposed to the air.
86. Testing Battery Designed for Small Lamps. — The
following method for testing primary batteries, designed
for lighting small incandescent lamps, is recommended
by Mr. I. Probert.
It may be impracticable to measure exactly the cur-
rent flowing by the direct use of the ammeter, as the
resistance of the latter, though low, is usually sufficient
to materially reduce the current when the instrument is
BATTERY TESTS.
135
inserted in the circuit. The present method is said to
overcome this difficulty entirely. The illustration shows
the connections.
The battery jB to be tested is joined up to the lamp
(which has a voltmeter V across its terminals), the
switch S being turned to the position shown. Under
these conditions the battery works directly on the lamp,
and the voltmeter V gives the voltage between the
lamp terminals. In order to determine the current, the
switch S is turned to the position shown by the dotted
B' *-
£
- *
Flgr* 62. — TesUng Battery for Current.
lines; this brings into circuit the auxiliary battery
(preferably small portable accumulators, as they have
a low resistance), the ammeter J., and the electrolytic
resistance E, The current from -B, though reduced
by the resistance of the ammeter, is reinforced by the
auxiliary battery B^ ; and by adjusting the distance be-
tween the plates of the electrolytic resistance the current
can be adjusted to the greatest nicety, until the deflec-
tion of V is the same as it was previous to the turning
of the switch S, Hence the ammeter A no^ " ^ "^ the
136 PRIMARY BATTERIES.
current which, under the former conditions, was flowing
through B. The observation being taken on J., the
switch S is turned back to the position shown in the
figure, and the battery B continues to work under
the practical conditions.
<: 87. Analysis of the Temperature Coefficient of a Battery.
— If the temperature coeflScient is a purely thermo-
electric effect, then it should be susceptible of analysis
by a measurement of the thermo-electric power of the
two metal-liquid pairs. If, for example, the thermo-
electric power of zinc— zinc sulphate and copper— cop-
per sulphate can be measured separately, then their
algebraic difference should represent the temperature
coefficient of the Daniell cell, except so far as it may
depend upon the thermo-electric power of the liquid
pair, zinc sulphate-copper sulphate, which is the only
other contact of dissimilar substances in the cell.
So, also, if we combine the results obtained by measur-
ing the thermo-electromotive force of zinc— zinc sulphate
and mercury— mercurous sulphate in zinc sulphate, the
result should be the temperature coefficient of a Clark
standard cell.
The meaning of thermo-electric power may perhaps
be explained with advantage. If two junctions are at
two temperatures fi and t\ of which
/O __ ^1 "t" ^2
2
is the mean ; and if H is the E.M.F. of the pair under
these conditions, then
The thermoelectric power at f = •
88. To Determine the Thermo-Electric Power of Zinc—
Zinc Sulphate. — For this determination it is necessary
BATTERY TESTS.
137
to have two contacts of zinc and a solution of its
sulphate so related that one can be kept at a constant
temperature, while the other is brought to successive
different temperatures. Two stout glass tubes, about
four inches (10 cm.) long and three-quarters of an inch
in diameter, were connected near the tops by a narrow
glass tube 10 inches (25 cm.) long. This will be
called the " experimental cell." It was filled with zinc
sulphate solution saturated at zero, and two zinc wires
about a foot in length were suspended so as to dip half
\
LC
A
B
Flsr. 63. — Diagram Showing Method of Measuring Tbermo-Electric Power.
or three-quarters of an inch into the liquid. The
immersed ends were slightly amalgamated. Two ther-
mometers were hung from a convenient support so that
their bulbs dipped into the solution at the same depth
as the zinc wires. The liquid filling the small tube
served to make the electrical connection between the
two limbs.
The electromotive force was measured by the follow-
ing method: R and R' in Fig. 63 are two resistance
boxes of 10,000 ohms each. For most purposes R' may
be less than 10,000. They must be of the most exact
adjustment, or the errors, if any, must be known. They
are connected in series with a good Leclanch^ cell of
138 PRIMARY BATTERIES.
higher E.M.F. than a standard Clark cell. Two Daniell
cells would perhaps answer as well, but they are not so
convenient.
The total resistanoe in the circuit must be kept at
10,000 ohms, partly in M^ and the remainder in R'. In
a derived circuit from AB^ the terminals of i2, are
placed in series a Clark cell SO^ the experimental cell
-&(7, and the long coil galvanometer 6r. It is better to
include a resistance of 10,000 or 20,000 ohms besides in
this circuit. The standard cell must be so connected
that its positive is joined to the same terminal as the
positive of the main circuit Leclanch^ cell LO. A key
must be placed in both circuits, preferably a double
successive contact key of the style used with a Wheat-
stone's bridge. The first points coming in contact close
the main circuit ; increased pressure brings the second
pair of contact points together, closing the derived
circuit. When the pressure is relieved, the derived
circuit opens first, and finally the main circuit.
The adjustment consists in changing the resistance in
the two boxes, keeping their sum 10,000, till the closing
of the circuit does not cause the galvanometer needle to
swing. ^ A balance then subsists between the E.M.F. of
SC and the fall of potential in the main circuit over the
resistance between A and B, The cell UO is not in-
cluded in the derived circuit in this first balance. The
E.M.F. of the standard cell being known, the fall of
potential over a single ohm in the main circuit is then
known.
The galvanometer employed was a Thomson reflecting
instrument, astatic, and having a resistance of 7000
ohms. A change of a single ohm from li to iJ', or the
reverse, when the balance is nearly effected, is perfectly
BATTERY TESTS. 189
evident in the swing of the mirror. In fact, when a
balance has been secured, if the key is kept closed for
two or three seconds, the polarization of the main circuit
Leclanch^ cell is always evident in the overthrow of the
balance.
The next step is to include the experimental cell in
the circuit as shown in the figure. Both limbs are sur-
rounded with broken ice, and their temperature is
nearly or quite the same. It is usually necessary to
change the resistance 22 by a small number of units,
perhaps two or three, in order to restore the balance.
One of the limbs is then heated by successive stages,
using a bath of warm water. The temperature is
allowed to become as nearly stationary as possible, and
a balance is again brought about as before. If the
resistance R must be increased to bring the galva-
nometer needle to zero, then the E.M.F. of the experi-
mental cell is so directed as to place the cell in series
with the standard cell. If R must be diminished to
secure a balance, the experimental cell EC is in opposi-
tion to the standard. The closing of the key therefore
indicates at a glance which pole of EO is positive. For
if EO is in series with SC^ the galvanometer needle
will swing in one direction ; if in opposition fo 8C^ it
will swing in the other direction ; and the direction of
the swing always indicates to the operator whether R
must be increased or diminished to effect a balance.
With zinc in zinc sulphate the heating of one limb
always produces an E.M.F. tending to make the zinc in
the cold the positive plate, or to produce a current from
cold to hot through the ceU. . The zinc in the cold limb
acts like the zinc of a simple voltaic couple.
The table gives the data of one series of experiments.
140
PRIMARY BATTERIES.
TABLE I.
Temp. C.
Left Limb.
Temp. C.
Right
Limb.
Temp.
Difference
(corrected).
Resistance
in/^to
Balance.
Change in
E.M.F. in
Legal Volts.
B.M.F. per
Degree C.
0.6
9.8
14.4
19.0
27.8
37.6
47.3
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.2
9.2
13.8
18.4
27.2
37.0
46.7
9141
9183
9199
9225
9269
9320
9377
• • *
0.00660
0.00911 ?
0.01319
0.02011
0.02812
0.03707
• • •
0.00072
0.00065 ?
0.00072
0.00074
0.00076
0.00079
The observation marked doubtful I had reason to
think included an error in making the balance.
•OJO
•020
'OiO
1
""
r
2,
e.
.
7
z
7
A
^
/
/
F
i^
/
/
/
/
/
■A
/
A
/
V
/
of
/
je
/^
V
y
B
/
>
/^
y
/\
^
>
<
y
y
'^
^
■^
ij
U
Jg
^
^
i^
P
¥^
^-
_^
.
(
Qo
1
0°
2
0^
3
0°
4
0°
5G
Fig. 64. — Thermo -Electric Power of Zn— ZnS04 and Cu-CuSO^.
The mean theimo-electric power for a temperature of
23^85 C. is therefore 0.00079.
BATTERY TESTS.
141
89. Thenno-Electric Power of Copper— Copper Sulphate.
— The apparatus was set up in precisely the same man-
ner as before, with a solution of chemically pure copper
sulphate of density 1.11. Two freshly electroplated
copper wires were used as electrodes to dip with the
thermometers into the solution. The current produced
on heating one limb was found to have the same direc-
tion as in the case of zinc sulphate, viz. from the cold
limb to the warm through the liquid. The copper in
the cold acts like the zinc of a simple voltaic cell. The
table following contains all the data.
TABLE II.
Temp. G.
Left Limb.
Temp. C.
Right
Limb.
Temp.
Difference
(corrected).
ResiBtance
in Rto
Balance.
Change in
E.M.F. in
Legal Volts.
E.M.F. per
Degree C.
0.6
0.4
0.2
9129
...
...
5.1
0.4
4.5
9145
0.00252
0.00056
9.6
0.4
9.0
9166
0.00582
0.00065
16.1
0.2
15.7
9189
0.00944
0.00060
21.7
0.6
20.9
9216
0.01369
0.00066
32.1
0.5
31.4
9260
0.02061
0.00066
38.4
0.5
37.7
9295
0.02611
0.00069
44.7
0.6
43.9
9330
0.03162
0.00072
49.4
0.6
48.6
9353
0.03524
0.00073
The mean thermo-electric power for the copper-
copper sulphate couple is therefore 0.00073 for the mean
temperature of 26'' C. The results for both zinc and
copper immersed in their sulphates are plotted in the
curves of Fig. 64, in which curve A refers to zinc and
zinc sulphate, and curve B to copper and copper sul-
phate. The total E.M.F.'s in legal volts, due to heating
one limb, are plotted as ordinates, and tb*^ J' tf »*^uces of
142 PRIMARY BATTERIES.
temperature as abscissas. It will be noticed that both
are slightly concave upward, indicating an increase of
the thermo-electric power with rise of temperature.^
90. Application to a Daniell Cell — Since both zinc
and copper, each in a solution of its sulphate, tend to
become negative when heated, or to play the r81e of
copper in a simple voltaic element, it is evident that
they will exhibit the same phenomenon when set up
together as a Daniell cell. When the entire cell is
heated, the E.M.F. tends to rise because of the effect at
the copper side of the couple, while the heating of the
zinc and its sulphate gives to the zinc the power of gen-
erating a counter E.M.F. Whether or not the E.M.F.
of the cell as a whole will rise or fall with rise of
temperature depends upon the relative thermo-electric
power at the two sides. The thermo-electric power of
Zn— ZnS04 is a little greater than that of Cu— CUSO4,
so that the voltage of the cell falls by a very small
coefficient per degree rise of temperature. The near
equality of the two thermo-electric powers explains the
small temperature coefficient of the Daniell cell.
A little consideration will show that if the Cu— CUSO4 ;
side of a Daniell cell alone is heated, the E.M.F. of the
cell will increase, while heating of the Zn— ZnS04 side
1 Since these experiments were made, the author has ascertained
that similar ones were made by Bouty in 1880 (Journal de Physique,
1880, p. 229). Bouty made use of a similar method, but employed
a Lippmann voltmeter for electromotive forces. His results are
0.0006947 and 0.0006886 for zinc and copper in their own salts
respectively, expressed as fractions of the E.M.F. of a Daniell. If it
is assumed that the Daniell had an E.M.F. of 1.08 volts, the results of
M. Bouty are 0.00075 for Zn-ZnSO^ and 0.00074 for Cu-CuSO^.
Considering the small electromotive forces to be measured and the
many disturbing causes, such as oxidation of the surfaces and convec-
tion currents, the results are in very good agreement.
BATTERY TESTS.
143
alone will cause a somewhatl greater decrease of E.M.F.
The relative coeflBcients in the two cases were measured
by setting up the experimental cell as a Daniell, making
use of the same solutions that were used in the preced-
ing determinations, and inserting in the small connect-
ing tube between the two limbs a plug of purified
asbestos to prevent intermixture of the two sulphates.
After a balance had been obtained with a Rayleigh
standard cell the experimental Daniell was substituted
for it. A comparison of the E.M.F. of the two was
thus made, and data secured to calculate the changes in
the voltage of the Daniell by the subsequent heating.
The resistance to balance the Rayleigh cell at 18°.7 C.
was 9134 ohms. The E.M.F. of the cell at this tem-
perature is 1.434 legal volts. Hence the fall of poten-
tial over one ohm is 0.000157 legal volt. This constant
is used to calculate changes in E.M.F. of the Daniell
under test.
TABLE III.
Temp. 0.
Zn-ZnS04
Limb.
Temp. 0.
CU-CUSO4
Limb.
Corrected
Temp. Dif-
ference.
Resistance
initio
Balance.
Change in
B.M.F. in
Legal Volts.
B.M.F.
per Degree C.
1.0
0.9
0.1
6935
...
• • .
10.8
0.8
9.9
6896
0.00612
0.00062
18.8
0.9
17.8
6864
0.01115
0.00063
29.6
1.2
28.3
6815
0.01884
0.00067
45.9
1.3
47.5
6709
0.03548
0.00075
1.5
1.8
0.3
6948
...
...
1.4
15.2
13.5
6994
0.00722
0.00053
1.4
26.4
24.7
7036
0.01382
0.00056
2.0
38.8
36.5
7087
0.02182
0.00060
1.4
40.0
38.3
7093
0.02277
0.00059
1.3
48.5
4''.9
7129
0.02842
0.00061
144 PRIMARY BATTERIES.
The foregoing table of results justifies the anticipa-
tion respecting the changes in E.M.F. ; for it will be
observed that heating the zinc end of the experimental
cell causes a marked diminution of the E.M.F., while the
opposite result follows the heating of the copper end.
The coefficients in this case are both smaller than
when each metal in its sulphate was used in both limbs
rig. eS. — Tliermo-Elec trie PowsrilromDanieU OIL
of the cell. The difference is small in the case of zinc,
but larger in the case of copper. Another series of
measurements, made by heating the copper end of the
chain, gave almost identical results.
The changes in E.M.F. resulting from beating one
limb are positive, and from heating the other, negative.
Both are plotted as positive ordinates in the figure in
order the better to compare them and exhibit the dif-
ferences. Curve A belongs to zinc, and curve B to
copper.
BATTERY TESTS. 145
91. Temperature Coefficient of a Daniell Cell — A com-
parison of the thermo-electric powera of Tables I and II
shows that the resultant effect upon a Daniell cell, due
to heating the cell as a whole, should be the difference of
the two thermo-electric powers, or 0.00079 — 0.00073 =
0.00006 volt per degree, if the effect of heating the junc-
tion of the two sulphate solutions is negligible.
To investigate this last question, an experimental cell
was made in which the connecting tube was curved so
as to include a long U, and the junction of the two sul-
phates was at the bottom of this U when the cell was
set up as a Daniell. After balancing in the usual man-
ner, the U-tube was placed in a hot water bath, by
which its temperature was raised from 17** to 52°, or
through a range of 35** C. No effect was produced
upon the E.M.F. of the cell; or if any, it was less
than one ten-thousandth of a volt for the entire range
of 35°.
Many difficulties were encountered in the attempt to
determine directly the temperature coefficient. They
appear to be due to small changes in the E.M.F. , occa-
sioned by oxidation. The expedient was finally resorted
to of setting up the experimental cell as a Daniell with
amalgamated zinc and oxidized copper wires and allow-
ing the apparatus to stand for fifteen hours. Consistent
results were then obtained with both rising and falling
temperatures. The whole cell, except the tops of the
tubes, was immersed in water. The following data
were obtained : —
Eesistance to balance at 16°.8 .... 6976 ohms
" " 57°. 6 .... 6956 "
« « 15°.2 .... 6975 "
t
146 PRIMARY BATTERIES,
A rise of temperature of 40°.8 caused a diminution of
20 ohms to balance, while a fall of 42''.4 increased the
requisite resistance 19 ohms. An independent balance
with a standard cell gave the fall of potential over a
single ohm as 0.0001561 volt. The mean value of the
change in voltage per degree was accordingly 0.000073
of a legal volt. This is in very satisfactory agreement
with the result calculated from the separate thermo-
electric powers.
The E.M.F. of the experimental cell was 1.09 legal
volts. Hence the temperature coefficient is 0.000073 -^
1.09—0.000067. The effect of a change of temperature
on the Daniell cell is practically negligible. It is smaller
than known disturbances which are assignable to other
causes.
92. Thermo-Electrio Power of Mercury— Mercnrous Sul-
phate. — Some chemically pure mercury was poured into
each branch of the experimental cell, and on this was
placed neutral mercurous sulphate free from the mer-
curic form. Both branches and the thin connecting
tube were then filled with a neutral solution of zinc
sulphate saturated at O'^C. Connection was made with
the mercury in each limb by sealing a long platinum
wire into a glass tube, leaving a short portion of the
wire exposed at the sealed end. This was pushed down
into the mercury on either side.
The cell so set up was then immersed in melting snow
and connected in series with the Rayleigh standard, as
shown in the diagram, Fig. 63.
By the standard cell the fall of potential over each
ohm in R was found to be 0.0001571 of a volt.
One limb of the experimental cell was then heated,
while the other was kept at 0° C, with the results shown
by the table.
BATTERY TESTS.
147
TABLE IV.
Temp. C.
Left Limb.
Temp. C.
Right
Limb.
Temp.
Difference.
Resistance
in R, to
Balance.
Total E.M.F.
in
Legal Volts.
E.M.F. per
Degree C.
0°
0°
0<^
9150
^^^^
^^^
8.30
Oo
8.30
9161
0.00173
0.00021
14.50
oo
14.50
9170
0.00314
0.00022
20.70
0°
20.70
9179
0.00456
0.00022
26.70
0°
26.70
9288.5
0.00605
0.00023
35.80
0°
35.80
9204
0.00833
0.00023
42.60
0°
42.60
9219 ?
0.01053
0.00025 ?
49.25
0°
49.25
9226
0.01163
0.00024
The observation next to the last is marked doubtful
because there was reason to suspect an error indepen-
dently of the failure of this observation to agree graphi-
cally with the others.
In Fig. 66 the results are plotted in the curve 5,
together with those derived from zinc— zinc sulphate.
These measurements include, of course, the effect of the
contact of the platinum with the mercury, as well as
that of ZnSOi and Hg2S04. But these were purposely
included with a view of analyzing the temperature
coeflBcient of the Clark cell.
The direction of the E.M.F. produced by heating the
Hg— Hg2S04 is such as to produce a current through
the cell from the cold to the warm limb, precisely as in
the preceding cases.
When therefore a Clark cell is warmed, there is a
tendency to make both poles positive. The effect at
the mercury or positive electrode is added to the whole
148 PRIMARY BATTERIES.
E.M.F. of the cell, while that at the zinc or negative
electrode is opposed to the E.M.F. of the cell, as a whole,
and must be subtracted. The algebraic result, then, is
the difference of the two tliermo-electric powers, and is
negative because the Zn— ZnSOt one is numerically
larger than the other. The difference is 0.00079 less
0.00024, or 0.00055.
These are the mean values of the thermo-electric
powers. If we apply them to the corresponding equEi-
1
Fig. 66.— Thetmo-Elecirlc Power o( Hg-Hg,aO, ind Zn-ZnSO,,
tion for the E.M.F, of a Clark cell, involving only the
first power of the temperature, we should write
E. = E\\-x{t-U)'\,
in which a; is to be found. In the author's cells, with
the solution used above and with the zinc separated
from the mercury salt, the E.M.F. is 1.44 true volts at
15° C.
Hence E, = 1.44 [1 _ a; (( _ 15) ].
BATTERY TESTS.
149
If the difference between the thermo-electric powers
at the two sides of the cell is the change in volts per
degree, then from the preceding equations
Whence
Ex =1A4:X= 0.00055.
X = 0.000381.
Now the thermal coefficient of such a cell is 0.000386
at 15° C. (see section 62). The agreement between the
two methods is closer even than one might anticipate.
93. The Experimental Cell as a Clark Cell — By simply
substituting an amalgamated zinc wire, dipping into the
zinc sulphate, for the platinum wire of the last experi-
ment, the experimental cell became a Clark standard.
Heating up the mercury side alone increased the E.M.F.
of the cell; heating the zinc side alone reduced the
E.M.F. The results are embodied in the table.
TABLE V.
Temp. C.
Zn +ZnS04.
Temp. C.
Hg +
Hg8804.
Temp.
Diiference.
Resistance
in Ji to
Balance.
Difference in
E.M.F. in
Legal Volts.
E.M.F. per
Degree C.
15.75
15.80
16.00
16.20
16.25
15.8
23.6
34.2
43.2
50.7
0.05
7.80
18.20
27.00
34.45
9202
9214
9232
9247
9262
0.00189
0.00472
0.00708
0.00944
0.00024
0.00026
0.00026
0.00027
16.25
23.25
31.60
40.00
49.40
15.7
15.7
15.7
15.7
15.7
0.55
7.55
15.90
24.30
33.70
9207
9169
9124.5
9088
9029
0.00598
0.01298
0.01872
0.02800
0.00079
0.00082
0.00077
0.00083
150
PRIMARY BATTERIES.
The thermo-electric power is in both cases slightly
higher than in the experiments on the same metals and
their salts separately. But the difference of the mean
values is 0.00056 as compared with 0.00055 of the last
section. The temperature coefficient of the Clark cell,
calculated as before, would be in this case 0.000388. It
must be admitted that both of these results differ from
the coefficient obtained by the ordinary direct method by
a quantity well within the errors of observation.
Fig. 61. — TbenoD-Bleetrlc Power fiom Olack Cell.
The curve A of Fig. 67 shows the effect of heating
the zinc end alone of the chain, and curve B the
mercury end.
All of these curves are slightly concave upward,
indicating a small increase of thermo-electric power as
the mean temperature increases. Since the mean tem-
perature in this last experiment was 33°, and in the
preceding ones on the same metals and metallic salts
separately was 24°, the higher values of the thermo-
electric power obtained in the present case are partly
due to the higher mean temperature.
BATTERY TESTS, 161
"^ 94. Electromotive Forces of Various Combinations. •. — A
number of questions have arisen in the preceding pages
of this book relating to the effect of amalgamation, of
concentration of the solutions, of wearing away of zinc
at and near the liquid surface, and the like. The
experimental cell heretofore described was brought into
service for the purpose of determining some of these
questions.
1. The experimental cell was set up with zinc sul-
phate solution, and was supplied with an amalgamated
zinc wire in one limb and an unamalgamated one in the
other. The amalgamated wire served as the positive
plate of a simple cell, or this wire was attacked by the
solution more than the other. Zinc then gives a slightly
higher E.M.F. when amalgamated.
2. A saturated solution of ammonic chloride was
introduced into one limb of the experimental cell, and a
6 per cent solution of the same into the other limb, with
amalgamated zinc wires in both.
Eesistance to balance Eayleigh cell at 18**.4 . . . 9148
" " with experimental cell in series, 8971
Difference 177
E.M.P. of exp. cell = 177 X 0.000157 = 0.028 volt.
The zinc in the dense solution acts like the zinc of a
simple voltaic element. The denser the liquid the
greater the tendency of zinc to replace the ammonium
of sal-ammoniac.
8. The following combination was then tried : —
Zn I ZnCla | NH4CI \ Zn.
< «
152 PRIMARY BATTERIES,
The result was an E.M.F. producing a current through
the ceU in the direction of the arrow ; or the zinc in the
amnionic chloride is the positive plate and negative
electrode.
Eesistance to balance experimental cell alone . . . 1710
" " Rayleigh cell at 18^4 . . . 9148
Hence E.M.F. of eXp. cell is 1710 x 0.000157 = 0.268 volt.
4. The next combination was —
Zn I ZnCla + NH^Cl | NH^Cl | Zn.
< «
The direction of the current was the same as before, but
the E.M.F. was reduced to 0.186 volt. Hence zinc in
a mixture of zinc chloride and ammonic chloride does
not replace the ammonium so actively as when the zinc
chloride is absent. For this reason the initial E.M.F.
of all ammonic chloride cells is higher than they ever
reach again after they have been on closed circuit, or
have done any considerable work.
Further, the double chloride settles to the bottom of
the cell, where, it .crystallizes when the solution becomes
concentrated. Hence also local action goes on, with
the zinc in the ammonic chloride solution at the top
playing the part of the zinc of a voltaic couple. The
rod or "plate is therefore eaten away more at the top
than at the bottom. The tapering of the zinc rods is
thus accounted for.
5. The experimental cell was set up as a chloride of
silver cell, with dilute ammonic chloride. The E.M.F.
was 1.08 volts. Upon heating the Ag— AgCl end, the
E.M.F. rose very perceptibly. Heating the Zn— NH4CI
BATTERY TESTS. 153
end, on the contrary^ caused the E.M.F. to fall. No
measurements were made.
6. The ammonic chloride solution was replaced by-
substituting a solution of ZnClj, made by adding four
parts of water to one of saturated zinc chloride. An
oxychloride of zinc formed and was filtered ofif.
The E.M.F. was 1.01 volts. Heating produced quali-
tatively the same effects as before.
95. Belative Value of Oxidants in Batteries. — Some
interesting and valuable experiments have recently
been made by J. T, Sprague, designed to test the rela-
tive merits of the more powerful oxidants used in
primary batteries. For this purpose he constructed a
cell with a small porous vessel, which held 400 fluid
grains with the carbon plate, so as to have the condi-
tions exactly the same for each^ substance. The exter-
nal circuit contained a large tangent galvanometer, and
a copper depositing cell to measure the total work
done by the 400 grains of each oxidizing agent.
The following are the measured resistances of the
liquids : —
Sp. qr. Resist.
1. Sulphuric Acid 1, water 12 vols. . . . 1.085 60.00
2. Sulphate of Copper, saturated .... 1.172 656.00
3. Potassium Bichromate, sat: sol. . . . 1.048 698.00
4. Same + j\ vol. U^O^ 1.139 70.00
5. Sodium Bichromate, sat-, sol. • . . . . 1.422 220.00
6. Same + i vol. HjSO^ 1.552 80.40
7. Chromic Acid, 1000 parts to water 1250, 1.353 48.70
8. Chromic Acid, 2250 grs. +lfL. oz. H2SO4, 1.454 57.90
^: Nitric Acid 1.375 5.69
These solutions containing the depolarizer were placed
in the porous cell ; the acid (1 : 12) to act on the zinc
164 PRIMARY BATTERIES.
was placed in the outer vessel. The chromic acid was
prepared by dissolving 1 lb. in 1 pint of water and
adding 7 fluid oz. of H2SO4.
The relative values of the oxidants was brought out by
plotting the results in curves. The potassium bichro-
mate showed a rapid loss by polarization. The small
amount of work possible with this salt justifies the
statement that this once valued oxidant is worthless
in batteries, compared with others now obtainable.
Bichromate of sodium and chromic acid are very
nearly equal in merit, but the acid does rather more
work. Both show an increasing current at first.
A solution in which chlorate of potassium is added
to chromic acid shows an increased effect in the early
part of the work.
The nitric acid curve, which extended over 30 hours
before sinking to the level of the others, when the
oxidant in all was exhausted, shows the vast superiority
of this over all other oxidants, so far as the production
of current is concerned.
The work in ampere-hours of each 400 grains was as
follows : • —
Nitric acid 16.0
Sodium bichromate 5.0
Potassium bichromate 1.7
Chromic acid ........ 6.4
In this comparative test the nitric acid did not fall to
the level of the rest, while the potassium bichromate
has too much credit, as the last six hours of its curve
belong to the zinc and acid alone.
j
BATTERY TESTS. 165
96. Hanganese Dioxide in Ledanche Cells. — The fol-
lowing contribution has recently been made by Mr.
Eugene Obach^ to the question, To what extent does
the manganese dioxide furnish oxygen as a depolarizing
agent in cells of the Leclanch^ type ?
Representative samples were taken from two different
brands of manganese peroxide and analyzed. No. 1 was
found to contain 16.09 per cent of available oxygen, and
No. 2, 15.55 per cent. Three medium Leclanch^ cells
of ordinary construction were filled with each of the
two brands of the dioxide, mixed with a suitable quan-
tity of crushed retort carbon. The cells were then
charged with the usual ammonium chloride solution,
and each was closed through a resistance of 100 ohms,
and the E.M.F. and internal resistance were measured
from time to time. The tests showed that the three
cells of each set were practically alike, and that there
was scarcely any difference between the two brands of
manganese.
For the first eight days the E.M.F. fell quite rapidly,
but after that much more slowly. After the lapse of
sixty-three days, when it had fallen to about one volt, the
circuit was interrupted, and the cells were allowed to
recover for a month. During this rest the E.M.F.
recovered fully 25 per cent, but rapidly fell to its former
value when the circuit was again closed, and then con-
tinued to fall at the same rate as before the interruption.
The experiment lasted 104 days, when the E.M.F.
had fallen to half its initial value. One cell of each
set was then selected, and the contents of the porous
pot were weighed and examined. From the data thus
1 London Electrical Review, May 15, 1891.
156 PRIMARY BATTERIES,
obtained, the weight of oxygen lost by the manganese
dioxide could be determined. Since the total electrical
output of the cell was known, the amount of oxygen
required to dispose of all the displaced hydrogen could
also be calculated.
The analysis of the manganese from the spent cells
gave 14.83 per cent of available oxygen for No. 1, and
13.18 per cent for No. 2. From the total weight of the
oxide of manganese present in each cell it was found
that No. 1 had lost 4.67 grammes of oxygen, and No. 2,
3.87 grammes. The oxygen required by the electrical
output, on the other hand, was 7.98 and 8.02 grammea
respectively. It thus appears that the manganese per-
oxide furnished not more than half the oxygen to effect
complete depolarization.
Two explanations are possible, with a probability that
both of them are to some extent correct. It can hardly be
doubted that an additional source of oxygen is the gas
occluded in the carbon, and dissolved in the surrounding-
liquid. With ready access of air, and with frequent
intervals of rest, it is possible that nearly enough oxygeu
may be supplied by the air to meet the requirements of
the cell.
It is also quite likely that when such cells are worked
hard, a considerable portion of the hydrogen evolved
escapes with the ammonia in the gaseous form. This
must occur in carbon cells without a depolarizer, and
probably does so when the depolarizer is an insoluble
solid, as in the instance described.
GROUPING OF CELLS. 157
CHAPTER X.
GROUPING OF CELLS.
97. Activity and Efficiency, — Before deciding upon the
best grouping of a given number of cells to accomplish
a definite result, it is necessary to consider whether this
result is to be attained with maximum activity or with
maximum economy. The conditions of working will be
different according as the one or the other is to be kept
prominently in view.
Maximum activity involves the most rapid conversion
of the energy applied into the energy of an electric
current ; maximum economy requires conditions so
arranged that the work may be performed with the
least loss ; or, in other words, that the ratio of the work
done to the energy expended shall be as large as pos-
sible. Maximum activity means that the work shall be
performed with the greatest celerity; maximum effi-
ciency means that it shall be done with the greatest
economy. In the one case energy is sacrificed to time ;
in the other, time is sacrificed to economy.
98. Application of Ohm's Law to a Single CelL — Let
E be the E.M.F. of the cell, r the internal resistance,
and R the external. Then, by Ohm's law, the current
from a single cell will be
If r is negligible in comparison with iJ, the current
equals approximately — . The current is inversely as
XV
158 PRIMARY BATTERIES.
the external resistance. In any case, whatever the
value of r, the greatest current that a single cell can
yield flows through it when the external resistance is
zero. The current then equals — . But this condition
r
involves the expenditure of all the energy in heating the
cell. To obtain the greatest proportion of energy in the
working circuit, the internal resistance should be made
as small as possible. How large an internal resistance
will be consistent with good economy depends upon the
external resistance employed. Thus, with an external
resistance of 4.5 ohms and an internal of 0.5, the loss
internally is 10 per cent of the whole.
99. Cells in Series. — A battery of n similar cells may
be grouped in several ways. When connected in series,
the positive terminal of one cell is joined to the negative
of the next, and its negative to the positive of the preced-
ing. Thus the zinc and carbon, or metal composing the
negative plate, of two adjacent cells are in metallic con-
nection, the negative terminal of the first cell in the series,
and the positive of the last being the only disconnected
ones. These, therefore, constitute the terminals of the
battery thus grouped in series, and the external circuit
extends through the conductor from the one main ter-
minal to the other.
So arranged, the total E.M.F. of the battery is the
sum of the E.M.F.'s of the several similar cells, or nJE;
and the entire internal resistance is n times that of a
single cell, since the current must pass in succession
through the several cells. Hence
If now the external resistance is small compared with
GROUPING OF CELLS. 159
the internal, then R is negligible in comparison with
nr.and 0=^ = ^. nearly.
nr r .
Under such conditions, an infinite number of cells in
series cannot maintain a larger current than a single cell
on short circuit.
If, however, r is negligible in comparison with B^
then increasing the number of cells increases the cur-
rent in nearly the same i-atio. For then
C=^ = n| nearly.
100. Grouping in Parallel or Multiple Arc. — The only
effect of such a grouping is to reduce the resulting
internal resistance to ^/«th that of a single cell. The
positive terminals are all joined together, and the
negatives likewise. These multiple terminals then
constitute the main ones of the battery. All the cells
side by side contribute equal shares to the output of
the battery. The effect is precisely the same as if the
n cells were replaced by one larger cell, with plates n
times the area of those in the smaller cells. The cross-
section of the liquid conductor is increased ?irfold, and
the internal resistance is reduced in the same propor-
tion. Hence t?
T
n
The E.M.F. is in no way greater than that of one cell.
But if now R is negligible in comparison with r, or
even with -, then
n ^ E E
r
n
GROUPING OF CELLS, 161
Ir
constant value. But — is the internal resistance of the
m
battery in multiple series, and R is the external resist-
ance. Hence the internal should equal the external
resistance for a steady current of maximum value.
This is also the condition for maximum activity for a
fixed resistance R. The efficiency may be said to be
50 per cent, since half the energy is wasted internally
and half externally, if none is stored up by electrolysis
or by a motor mechanism.
103. Orouping of a Battery for Quickest Action.^- In
the preceding topics it has been assumed that the battery
is to work with a steady current ; and it has been found
that to obtain the greatest current from a given number
of cells, with a constant external resistance, the battery
should be so grouped that the internal resistance shall
be as nearly as possible equal to the external. But this
is no longer true if the circuit contains an electro-magnet
which is to be worked rapidly ; such, for instance, as a
vibrating bell or a Wheatstone's automatic transmitter.
Such a circuit is said to possess the property of self-
induction. Any change in the current flowing through
it, either of increase or decrease, invokes an electro-
motive force of self-induction, which is always so di-
rected as to oppose the change going on. When the
circuit is closed, the E.M.F. of self-induction is opposed
to that of the applied E.M.F., and its effect is to increase
the time required for the current to grow to its maxi-
mum steady value. On opening a circuit, the self-
induction prolongs the flow of the current, and manifests
itself by the bright spark at the break. To take into
account this property of a circuit, a term must be intro-
duced into the expression for a current, additional to
162 PRIMARY BATTERIES.
those required by Ohm's law. This term depends for
its value, at any instant after closing the circuit, upon
what is called the coefficient of self-induction, denoted
by L, The E.M.F. of self-induction is the product of
this coefficient and the time rate of change of the cur-
rent. Expressed in symbols the current then is
C7. = |(l-e-).
Here (7, is the value of the current at the time t after
closing the circuit, R is the entire resistance, and e is the
base of the Napierian system of logarithms, 2.7183. This
is known as Helmholtz's equation.
If this equation is examined, it will be ttpparent that,
at the time ^, the value of the current falls short of its
maximum by a factor depending upon the second term
in the parenthesis. Whenever this factor becomes zero,
the current will have the value assigned to it by Ohm's
law. The effect of the self-induction in delaying the
arrival of the current at its maximum value is expressed
by this negative exponential term. The ratio ^/b is
called the time-constant of the circuit. It is the time
required for the current to rise to 0.632 of its final
value. This time will be longer, the larger the value of
L\ or, conversely, the larger L is the smaller will be
the current at any time t after closing the circuit.
The decimal fraction 0.632 is obtained in the follow-
ing manner. If in Helmholtz's equation t be made equal
to the time-constant Yu? ^^^
e
Substitute for e its value 2.7183, and we obtain 0.632.
Therefore jn
(7=:|x0.632,
XV
GROUPING OF CELLS. 168
or during an interval equal to the time-constant of the
circuit, the current will rise to 0.632 of its final value.
If, for example, i is 6 units and R 10 ohms, the time-
constant is one-half a second. In half a second the cur-
rent then rises to 0.632 of its maximum value. This
retardation in the growth of the current is due to the
presence of coils and magnets in the circuit ; the current
is retarded because it has to create magnetic fields.
Energy is stored up in these fields, and the resistance to
the work done on them is manifested as an opposing
electromotive force. As this opposition dies away, the
effective electromotive force increases, and the current
rises to its final value.
If now the current is to be worked with rapid inter-
ruptions, then it is desirable to reduce the time-constant
to as small diniensions as possible. With a given coeffi-
cient of induction, the time-constant is inversely as the
resistance. Hence this suggests the arrangement of the
cells in series ; for, while this arrangement diminishes
the final value of the current, it also diminishes the
time required for the current to rise to about two-thirds
of this value ; and it may easily follow that, for rapid
working, the series arrangement will give a larger cur-
rent in the short time during which the circuit remains
closed, than could be obtained by the rule for grouping
to get the greatest steady current.
An example will make this clear. Suppose twenty-
four Daniell cells are available, each of 3 ohms resist-
ance and 1 volt E.M.P. Let the external resistance be
6 ohms and the self-induction, i, 5 units (often now
called henrys). Grouped in series, the total resistance
of the circuit would be 77 ohms ; in parallel, 5.125 ohms ;
and in four series of six cells each, 9.5 ohms. This last
164 PRIMARY BATTERIES,
arrangement is the one indicated by the rule for maxi-
mum steady current. The current in the three cases
would have the final values 0.31, 0.195, and 0.63 of an
ampere respectively.
Let us now compute the time-constant of the circuit
in the three groupings. In the first it would be 5
divided by 77, or 0.065 sec. ; grouped in parallel,
0.975 sec; grouped for maximum current, 0.526 sec.
In these times the current would rise to ^OjS, 0.12, and
0.4 of an ampere for the three cases respectively. If now
the circuit were interrupted as often as every tenth of
a second, the current with the cells in series would rise
in this time to something over 0.2 ampere ; while the
best grouping for steady current would, in the same
time, give a current of only 0.109 ampere.
The current reaches 0.632 of its final value after an
interval in seconds equal numerically to the time-con-
stant of the circuit. At this instant the effect of the self-
induction is the same as if the entire resistance in circuit
had been increased 60 per cent. Hence the effect of
self-induction is often likened to a spurious resistance.
104. Coupling Together Dissimilar Cells. — It is permis-
sible to couple a battery in parallel or in multiple series
only when all the cells are of the same type. Cells not
differing much in E.M.P. or internal resistance may
always be joined in series without detriment to any of
them. If, however, the internal resistance of a cell is
so large in comparison with the current flowing through
the circuit in which it is placed that the fall of potential
in passing through it is greater than the E.M.F. of the
cell itself, then the addition of such a cell in series
diminishes the effective voltage of the circuit, and so
really diminishes the current.
GROUPING OF CELLS. 165
If, for example, the resistaDce of the cell interposed
in a circuit should be two ohms, and its E.M.F. one volt,
then with one ampere current flowing through the cir-
cuit, the loss of potential in passing through the resist-
ance of the cell would be two volts by Ohm's law, while
the E.M.F. added would be only one volt. Such a cell
contributes nothing to the production of a current. If
its internal resistance were one ohm, under the con-
ditions assumed, it would still contribute nothing to the
current, but its own E.M.F. would simply make up for
the loss due to its internal resistance. This can never
occur with cells of the same type and size, unless the
battery is on short circuit.
When dissimilar cells are joined in parallel by con-
necting together poles of the same sign, a short circuit
is always formed of every pair of adjacent cells or
adjacent parallel series. If now the E.M.F. of the one
cell or series is not exactly equal to that of the adjacent
parallel cell or series, then there will be an effective
E.M.F. equal to the difference of the two, which will
produce a current through the closed series of cells,
discharging the one, and charging the other as if it were
a storage battery. Thus, when the main circuit is open,
some of the cells may be running down, even when the
battery joined in multiple series or in parallel consists
of similar cells ; for some difference of E.M.F. always
exists among commercial cells of the same type.
166 PRIMARY BATTERIES.
CHAPTER XI.
THERMAL RELATIONS.
105. Oeneral ConsideratioiiB. — It has already been
pointed out in Chapter I. that a battery is a device for
the conversion of the potential energy of chemical
separation into the energy of an electric current. We
wish now to consider more specifically the relations
subsisting between the thermal energy of the chemical
changes taking place in a battery on the one hand, and
the current, electromotive force, and external work in
the circuit, on the other.
The basis of all such calculations is necessarily the
principle of the conservation of energy. This principle
is stated by Maxwell as follows : ^ —
^'The total energy of any material system is a
quantity which can neither be increased nor diminished
by any action between the parts of the system, though
it may be transformed into any of the forms of which
energy is susceptible."
This principle has been experimentally verified in
cases where the energy of the systems investigated takes
the form of heat, electricity, magnetism, etc. It is " the
one generalized statement which is found to be consistent
with fact, not in one physical science only, but in all."
To this statement of the law of conservation of
energy should be added that of the dissipation or
1 Matter and Motion, Art. 74.
*. ^
THERMAL RELATIONS. 167
degradation of energy, viz. that all energy tends
towai;d the form of uniformly diffused heat. And
since, by the second law of thermo-dynamics, heat can
be made to do work only by the transfer of heat from a
hotter body to a colder one, uniformly diffused heat is
energy in the unavailable form. Hence the available
energy of any physical system, which Professor Tait
calls Entropy, tends toward zero.
In any isolated system, such as a voltaic battery, with
its electric circuit and translating devices included
therein, energy is converted from one form into one or
more others, but without loss or gain of energy. The
proportion available for any useful purpose, however,
becomes progressively less. If the circuit, external to
the battery itself, is a simple non-inductive metallic
resistance, then the transformed energy which appears
first in the intermediate form of the energy of an
electric current is all finally expended in heating the
circuit. But if it contains an electrolytic cell, then a
part of the energy undergoing change of form is stored
up potentially in the chemical separation of electrolysis.
If the current actuates an electro-motor mechanism, a
part of the energy is transformed into mechanical work ;
and if electro-magnets or coils are included in the
circuit, some of the energy is stored up in the magnetic
field created by these coils and magnets. This portion
of the'transformation is a reversible one, and the energy
is restored to the circuit when the applied E.M.F. is
withdrawn. During the time that the current is rising
to its final value, work is done against the E.M.F.
of self-induction, and potential energy is accumulated
in the magnetic strain produced by extending the lines
of force about the circuit ; « while the current is falling
168 PRIMARY BATTERIES.
again to a zero*" value, the energy is restored to the
circuit to assume the final form of heat.
•^*
«•
In any cell the available energy is proportional to the
mass of sdnc or other metal composing the positive plate.
But tne same mass of zinc does not produce the same
amount of electrical energy in cells of difiFerent types,
because the chemical processes going on are different in
different cells. In every case, however, the total energy
at our disposal in a cell is dependent upon the chemical
changes taking place; and these are the sole source
of the energy, save in the exceptional cases in which
energy is supplied by light or heat.
106. Units of Foroe, Work, Activity, and Heat — In the
system of units now almost exclusively used in science,
the centimetre, the gramme, and the second are the
three fundamental units in terms of which all other
units are defined as derived units. Hence this is called
the C.G.S. or centimetre-gramme-second system.
1. The imit of force is the dyne. It is that force
which will give to a mass of one gramme in one second
a velocity of one centimetre per second. Gravity is
equal, therefore, to about 980 dynes.
2. The unit of work is the erg. It is the work done
by a dyne in producing a displacement of one centi-
metre in the direction of the force.
8. The unit of activity, or rate of doing work, com-
monly called power, is the watt. It is the rate of doing
work equal to 10"^ ergs per second. In engineering
practice the horse-power is commonly used as the unit
of activity when work is done on a large scale. It is
equal to 33,000 foot-pounds per minute, or 550 foot-
pounds per second. Reduced to the C.G.S. system, one
horse-power is equivalent to 746 X 10*^ ergs per second,
or 746 watts.
THERMAL RELATIONS. 169
4. The unit of heat is the calorie. It is the heat
required to raise one gramme of water from 0° C. to 1° C.
The calorie is connected with the C.G.S. system by
measuring experimentally the mechanical equivalent of
one heat unit. This determination is rendered neces-
sary by the fact that the Centigrade scale is independent
of the C.G.S. fundamental units.
The result of the laborious experiments of Joule is
tnat ^ calorie = 4.2 x 10' ergs (nearly).
More exactly, the heat required to raise a gramme of
water through one Centigrade degree, if applied me-
chanically, will do 41,695,000 ergs of work.
107. The Heat Equivalent of a Current — Since the
difference of electrical potential between two points of
a circuit is the work required to carry a unit quantity
of electricity from one point to the other, there being no
source of electromotive force between them, it follows
that when Q units are transferred, the work equals
W= Q(F- V%
where Fand V are the potentials at the two points.
But W= HJj where His the number of heat units ; J
is Joule's equivalent or 4.2 X 10^ ; and Q= C^ the
current, if the time taken is one second.
Also the difference of potential between two points,
under the above conditions, is equal to the E.M.F. re-
quired to produce the given current from one point to
the other. Therefore V- F' = J&.
Substituting these values, we obtain
ffj= CE = C^R (by Ohm's law).
Whence H=^^ = tt?^/
J 4.2 X 10'
170 PRIMABY BATTERIES.
Both (7 and R are taken in C.G.S. or "absolute" units;
but if C is measured in amperes, or 10"^ C.G.S. units,
and R in ohms, or 10^ C.G.S., then
H=. ^'f ^,-^5 = G^R X 0.24.
4.2 X 10'
Also when a current of amperes flows between two
points on a circuit having a potential difference of JB
volts, then electrical energy is converted into heat
between these points at the rate of OE watts, or CM X
10"^ ergs per second ; and the number of calories gener-
ated per second is
H^ cm X 0.24 = C^ X 0.24.
It follows 'that one watt is approximately equivalent to
0.24 calorie per second.
The first of the above formulae for the heat generated
per second is true for any homogeneous circuit or homo-
geneous parts of a circuit : it expresses a relation known
as Joule's law. The second formula is true only when
E
E is such that 0=^»
108. Heat Evolved in a Circuit with no Connter Electro-
motive Force. — When the circuit contains no source of
E.M.F. other than that of the battery itself, then by
Joule's law
H= C\R 4- r)0.24 =CEx 0.24 calories per second.
But the total activity in the circuit is
W=CE watts.
In this case all the energy transformed runs down into
the form of heat. In the circuit interior to the elec-
trodes the heat is
Hi = (7V X 0.24 calories per second.
THERMAL RELATIONS. 171
In the external circuit it is
H^ = C^R X 0.24 calories per second.
These conclusions were accurately verified by Favre by
determining, first, the quantity of heat evolved by dis-
solving 33 gms. of zinc in sulphuric acid ; and, second,
by determining the heat evolved by the consumption of
33 gms. of ^inc in a Smee cell closed with a homogeneous
conductor. These operations were conducted by intro-
ducing the vessel containing the zinc and acid into a
huge calorimeter, or instrument for measuring heat, and
observing the heat evolved. The Smee cell was subse-
quently introduced into the' same instrument. The first
operation produced 18,682 calories ; the second, 18,674,
a quantity almost identical with that due to the solution
of the zinc under the simple conditions not involving
an electric circuit.
By a further experiment, Favre measured separately
the heat evolved internally and externally as regards
the cell, and found the two quantities to be proportional
to the corresponding resistances.
If any diflSculty is found in understanding why the
heat evolved is proportional to the square of the current,
and not to its first power, it may be useful to consider
how the activity is affected by doubling the current
while the resistance of the circuit remains the same.
If we imagine the E.M.F. doubled by doubling the
number of cells in series, then the double current means
that twice as much zinc is dissolved in each cell per
second; and since the number of cells is doubled, the
mass of zinc dissolved in the whole battery becomes
four times as great as before. But the heat is also
increased four-fold by doubling the current. The mass
172 PRIMARY BATTERIES.
of zinc dissolved is a measure of the activity in the
circuit. The activity is quadrupled because its measure,
the product of C and J?, is quadrupled by doubling both
C and ^ simultaneously.
109. Counter Electromotive Force in a Circuit. — The
entire activity, or rate at which a battery is supplying
energy, may be represented in part by the heat evolved
in accordance with Joule's law, and in part by other
work done, such as the chemical separations in elec-
trolysis, the mechanical work of a motor, or in heat-
ing junctions of dissimilar substances by reason of the
E.M.F. arising at such a junction, and known as the
Peltier effect.
We may, therefore, write for the energy expended
in the circuit in time ^,
;fEt = q^Et -f A^.
The first term of the second member of this equation is
the heat waste ; and the second, the wprk done on the
external agent. This second quantity is in every case
proportional to the current, and A is the constant
required to express the activity other than that spent in
heating the circuit.
Dividing the equation through by Ct and transposing,
R is here the entire resistance of the circuit. It is
evident that the quantity — is of the nature of an E.M.F.
Moreover, it has the negative sign. It is therefore a
back or counter E.M.F. The effective E.M.F. pro-
ducing a current is the applied E.M.F. less the back
THERMAL RELATIONS. 173
E.M.F. arising from the fact that work of some kind is
done against a resistance. The only reaction that the
agent can offer to the work done upon it under the
electric pressure must be of the same nature as that
of the applied activity, viz. an E.M.F. This counter
E.M.F. is a necessary factor in every case in which work
is done by electricity.
110. Division of the Energy in a Circuit with Connter
Electromotive Force. — If E^ represents the counter
E.M.F., then the equation for the current becomes
But the heat waste in watts is, by Joule's law,
(7*^ = G{E - E') ^CE-^ GE\
Now CE is the total activity of the battery furnishing
the current. The heat generated in the entire circuit
of resistance R is less than this by the quantity CE'
watts. Hence the energy spent in doing work is the
product of the current and the counter E.M.F. The
ratio of the work done to the energy wasted in heat is
GE^ ^ E*
G{E - JS7') "" ^ - E''
It is evident, therefore, that the relative activity con-
cerned in the work done bears to the heat waste a larger
ratio the larger E^ becomes. But the larger E^ is the
smaller is the current. Maximum efficiency thus re-
quires a small current or small activity. It can easily
be shown that maximum activity involves an efficiency
of 60 per cent.
^-^111. Counter Electromotive Force of Electrolysis. — In
the general equation of section 109, -4.(7 represents that
174 PRIMARY BATTERIES.
portion of the energy expended in the entire circuit
which does not appear as heat in virtue of simple
ohmic resistance. In electrolysis this energy is ex-
pended in the work of chemical decomposition, or in
conferring potential energy upon the separated com-
ponents or ions of a chemical compound. The process
involves an increase in the intrinsic energy of the sub-
stance. Whenever the volume of the products of this
decomposition is greater than that of the electrolyte,
additional mechanical work is done in overcoming the
pressure. If the electrolyte is a liquid, and the products
are gases which fulfil Boyle's law, then for the same
temperature the product of the volume and pressure is
a constant. This product represents the mechanical
work done. Hence the electromotive force required to
effect the decomposition will be sensibly independent .of
the pressure of the liberated gas.
Let z represent the electrochemical equivalent of an
ion, and h the heat of combination of a gramme of this
ion with an equivalent mass of the other ion. The
electrochemical equivalent is the quantity of the sub-
stance electrolyzed by the passage of unit quantity of
electricity. Hence the quantity electrolyzed by current
C in time t is Czt ; and the energy expended is CzthJ.
But this is also represented by OA, Therefore
t
A
Now — has been found to be the value of the counter
t
electromotive force U'^ and zhJ is the mechanical
equivalent of the chemical action on one electrochemical
equivalent of the ion. This may be made to include
any mechanical work done in changing the molecular
THERMAL RELATIONS. 175
aggregation against pressure. Therefore the counter
electromotive force present in an electrolytic apparatus
is equal to the mechanical equivalent of the chemical
and mechanical actions involved in electrolyzing one
equivalent of the substance. These conclusions have
been verified by many experimenters.
112. Failure of a Cell to Effect Decomposition. — If the
counter electromotive force of the electrolytic cell is
greater than the direct electromotive force of the battery,
then electrolysis cannot take place. For in this case
C == — — — is negative, which means that the electro-
lytic cell would produce a current back through the
battery. Moreover, since CJE represents the energy
expended per second by the battery, and CH* the
activity necessary to do the work of electrolysis, it is
evident that the counter electromotive force cannot
grow to its maximum value, since the battery is deficient
in the necessary activity. It is for this reason that the
Smee cell cannot decompose water.
While these conclusions are correct as regards actual
decomposition, it is nevertheless true that any electro-
motive force, however small, will produce a current
through an electrolyte which obeys Ohm's law. Actual
decomposition does not take place till the electromotive
force reaches a finite magnitude determined by the con-
siderations already explained.
The theory of Clausius respecting the continuous
interchange of like atoms between different molecules
of a liquid serves to explain the flow of a current with-
out visible decomposition. Clausius supposes that the
same individual atom is at one time associated with an
atom of the opposite kind, and at another time with
176 PRIMARY BATTERIES.
another. In other words, decomposition and recomposi-
tion are continually going on in an electrolyte in an
irregular way, when no current is flowing; but the
application of an electromotive force serves to give this
process a definite direction. If the electromotive force
reaches a definite value for any given electrolyte, then
the accumulation of the ions in finite quantity upon the
electrodes gives rise to the counter electromotive force
of polarization. According to this view, which is
approved by Maxwell, the electromotive force of polar-
ization depends upon the deposit of the products of the
decomposition on the electrodes. But this deposit is
constantly tending to become free, by diffusing through
the liquid or escaping as a gas. If the decomposition
is so slow that the separated ions may pair again with
new partners, or disappear by diffusion, instead of
accumulating at the electrodes, then no visible decom-
position takes place. But a current of small magnitude
still continues to flow in accordance with the law of
Ohm. The density of the ions on the electrodes is
so slight that only a feeble state of polarization is pro-
duced. The dissipation of the ions by diffusion or
other means is then very small, and the strength of cur-
rent is really limited by this small rate of dissipation.
113. Calculation of E.M.F. from the Heat of Combina-
tion. — It has been shown in section 111 that the
counter electromotive force of an electrolytic cell equals
zhJ^ or the mechanical equivalent in ergs of the chem-
ical energy due to one electrochemical equivalent of
the substance electrolyzed. The same principles may
be applied to the calculation of the E.M.F. of a
battery ; for it may be provisionally assumed that the
total energy of the chemical reactions appears in the
THERMAL RELATIONS, 177
intefmediate form of the energy of an electric current
before assuming other forms, provided no local action
takes place which contributes nothing to the electric
energy. The chemical processes going on in the cell
involve a loss in the intrinsic energy of the mate-
rials. This loss represents the energy which takes the
form represented by the electric current. If this loss
can be calculated from thermal data, it may be placed
equal to the activity of the electric circuit.
If we suppose that only two chemical changes take
place, as in the Daniell cell, in which ZnS04 is formed
and CUSO4 is decomposed; and if z and z* are the
electrochemical equivalents of the two electropositive
ions, and h and h' their heats of formation for one
gramme of each (in the combinations in which they
appear in the battery) ; then, for a current (7, the loss of
chemical potential energy per second is
(Czh-Cz'h')JeTg^.
But the electrical energy developed is CE x 10^ ergs
per second. Therefore, equating the two,
CE X 10^ = C(zh - z'h') 4.16 x 10^
or JS7 = {zh - z'h') 4.16.
If 2 be used to represent "the sum of such terms
as," then we may write
-E7 = 4.16S2^,
so as to include all the chemical actions involving
thermal changes. This formula may be put into a more
convenient form for use in connection with tables giving
heats of combination.
The electrochemical equivalents are proportional to
J J
J J
178 PRIMARY BATTERIES,
the chemical equivalents of the substances ; that is, to
the relative weights of the substances which take part
in chemical reactions. If, therefore, we know the
electrochemical equivalent of hydrogen, the others may
be found by multiplying by their chemical equivalents.
The electrochemical equivalent of hydrogen in grammes
per coulomb is 0.00001036. If now the heat of combina-
tion of one chemical equivalent of an ion is jH", then
0.00001036 H equals zh. Consequently
E = 4.16 X 0.00001036 SIT,
or E = 0.000043 Sif.
It is only necessary then to find the algebraic sum of
the heats of combination for a chemical equivalent of
each ion taking part in the reaction in order to find the
E.M.F. in volts. If, in the formula, JS becomes unity,
then the number of calories corresponding to one volt
is the reciprocal of the constant 0.000043, or 23,200. In
this discussion the chemical equivalents used are half
atomic weights of bivalent substances, corresponding to
one of hydrogen, which is univalent. With this condi-
tion one volt is equal to 23,200 calories. If the chemical
equivalents used are the atomic weights of bivalent
elements, and double those of the univalent ones, then
a volt is numerically equal to 46,400 calories.
114. Application to the Smee Cell. — The chemical
action consists in the formation of zinc sulphate at the
expense of hydric sulphate or sulphuric acid.
Zn + H2SO4 = ZnSO* + H^.
Heat of formation of Zn, O2, SOj . 79,495 calories.
« " H2,02, SO2 . 60,920 «
.-. SH = 18,575 "
Therefore E = 0.000043 x 18,575 = 0.80 volt.
THERMAL RELATIONS, 179
These thermal values are from Thomsen's determina-
tions. Since ZnS04 is formed, and H2SO4 is decomposed,
the resulting heat of combination is the difference
between the thermal values of the two similar opera-
tions.
According to the determinations of Berthelot the
heats of formation are as follows : —
Heat of formation of Zn, 8,04. . 121,000 calories.
« " Hj, 8,04,. . 100,500 "
.-. SH = 20,500 «
Therefore E = 0.000043 x 20,500 = 0.88 volt.
115. Application to the Baniell Cell —
Eeaction, Zn + ZnS04 + CUSO4 = 2 ZnS04 + Cu.
One molecule of ZnS04 is formed every time one
molecule of CUSO4 is decomposed. After Thomsen we
have —
Heat of formation of Zn, O2, SO2 . 79,495 calories.
« " Cu, O2, SO2 . 55,745 "
.•.SH = 23,750 "
Therefore E = 0.000043 x 23,750 = 1.02 volts.
After Berthelot we have —
Heat of formation of Zn, 8,64. . 121,000 calories.
« " Cu, S, O4 . . 95,700 "
SH = 25,300 «
Therefore E = 0.000043 x 25,300 = 1.087 volts.
116. Application to the Bnnsen CelL —
Reaction, Zn + H2SO4 + 2 HNOa = ZnS04 -h 2 H2O + 2 NOj.
We have then to find the heat of combination of zinc
sulphate, water, and peroxide of nitrogen ; and from
180 PRIMARY BATTERIES.
their sum subtract the heat of formation of the decom-
posed sulphuric and nitric acids.
After Thomsen we have —
Heat of formation of Zn, Oj, SOj^ . 79,495 calories.
« « 2(H2, 0) . . 68,360 "
" « 2(N0, 0) . . 19,570 «
Total 167,425 «
Heat of formation of Hg, Oj, SO2 . . 60,920 calories.
« « 2 (H, NO, O2) . 63,185 «
Total 124,105 "
Therefore %H = 167,425 - 124,105 = 43,320 ;
and Ez=z 0.000043 X 43,320 = 1.863 volts.
117. Application to the Silver Chloride CelL — U we
assume the cell set up with a dilute solution of zinc
sulphate, then the result of the action taking place
when the cell is in operation is the formation of zinc
chloride and the decomposition of silver chloride.
Hence we have only to find the difference between the
heats of formation of the two chlorides.
From Thomsen's investigations these are —
Heat of formation of Zn, CI2 . . . 56,420 calories.
" " Ag2, CI2 . . . 29,380 "
.-. SH = 27,040 "
Therefore E = 0.000043 x 27,040 = 1.16 volts.
118. Helmholtz's Formnla for Electromotive Force. —
The direct measurement of the E.M.F. of a battery
rarely gives a result agreeing exactly with the value
calculated from the thermo-chemical data of the reaction
accompanying the work of the battery. Helmholtz has
THERMAL RELATIONS. 181
accordingly modified the formula from thermo-dynamic
considerations so as to express the E.M.F. by the
equation,
^ = 0.000043(7±T— ,
dT
in which C equals the heat of the reactions, E the
electromotive force, and T absolute temperature, or
temperature reckoned from a zero equal to — 273 of the
Centigrade scale. The last term of the equation ex-
presses a general relation which may admit of different
interpretations. We may suppose that the chemical
energy can be only partially transformed into electric
energy, whije the rest is directly converted into heat.
Or an explanation of the discrepancy may be sought for
in phenomena that tend to prevent the integral trans-
formation of the chemical energy.
An examination of this problem has been undertaken
by Chronstchoff and Sitnikoff.^ They have applied to
the solution of the problem the thermo-electromotive
force produced by the passage of a current at the con-
tact surfaces of liquids and metals in a battery. This is
known as the Peltier phenomenon. The expression for
this E.M.F. of thermal origin is identical with the final
term of the Helmholtz equation, which represents the
difference between the chemical heat and the voltaic
heat of a battery ; and the question arises whether they
are equivalent expressions for the same identical
quantity.
The results of their experimental investigation of the
problem raise a strong probability at least that this
explanation is the correct one. -One or two examples
must suffice to illustrate the application of this method
1 Comptes Rendus, Tom. 108, 1889.
182 PRIMARY BATTERIES.
to the explanation of the discrepancy existing between
the observed value of the E.M.F. and that calculated
from thermo-chemical data.
The thermo-electromotive forces of the metal-liquid
contacts were carefully measured by the experimenters
in each case.
1. Case in which the E.M.F. observed is greater than
the calculated value.
Pb I PbS04 I ZnS04 I CuSO* | Cu.
» >
E = 0.61 volts at 20** C.
E calculated from thermal values of CUSO4 and PbSO^
is 0.383.
The thermo-electromotive force for the system
Cu I CUSO4 I Cu,
between 0° to 50°, was found to be 0.00066 = — — •
For the system
Pb I PbSO^ I ZnS04 | PbS04 | Pb,
— = - 0.00011 volt.
dT
The value of T— is therefore 293x0.00077=0.225 volt.
dT
Then 0.383 -f- 0.225 = 0.608 volt.
This is almost exactly identical with the observed
value.
2. Case in which the observed E.M.F. is less than
the calculated value.
Zn I ZnS04 | PbS04 | Pb.
» >
E = 0.500 volt at 20° C.
THERMAL RELATIONS. 183
E calculated from thermo-chemical data of ZnS04 and
PbSO^ is 0.697 volt.
The thermo-electromotive force for the system
Zn I ZnS04 I Zn
was found to be 0.00076 volt per degree.
For the system
Pb I PbS04 I ZnS04 I PbS04 | Pb,
^ = _ 0.00011 volt as before.
dT
Hence T—= 293 x 0.00065 = 0.190 volt,
dT '
and ^ = 0.697 - 0.190 = 0.507 volt.
In this last example the authors of the paper appear
to have made an error in respect to the sign of — — for
(Jl JL
Pb - PbS04. The corrected value gives E= 0.473 volt.
The conclusion derived from all the experiments is
that the Peltier effect is of a nature to make up for the
discrepancy between the electromotive force observed
lirectly, and that calculated from the thermal values of
the chemical actions. The Peltier effect gives a value
of the same sign and of the same order as this differ-
ence.
INDEX.
PAGE
Activity and efficiency 157
" unit of 168
Advantages of sodium over potassium bichromate 60
Agglomerated carbon, Leclanch^ cell with 74
Air voltaic battery 23
" battery absorbing oxygen from Ill
Alloying, relative protection of 35
Amalgamation and local action 33
" effect of 34
Analysis of the temperature coefficient of a battery 136
Application to a Daniell cell 142, 179
" of Ohm's law to a single cell 157
" to a Smee cell 178
" to a Bunsen cell 179
" to a silver chloride cell 180
Arrangement to produce greatest current 160
Artificial electric organ 3
Baked carbon 46
Batteries without a depolarizer 78
** miscellaneous 106
Battery defined 1
'' primary and secondary 1
** gravity 38
" the Gethius 40
" Sir William Thomson's tray 42
" Grove's 43
" Bunsen's 46
** bichromate 47
** plunge 48
** Ward and Sloane , 54
" Partz acid gravity 55
" Taylor's 57
*' copper oxide 68
" Edison-Lalande 60
1 8iL
186 INDEX.
PAOB
Battery, chloride of silver 63
" open circuit 66
" prism Leclanch^ 69
" Samson 73
" Roberts' peroxide 74
" sulphate of mercury 75
" Fitch*' chlorine" 76
" sea salt 79
" Law 80
" diamond carbon 80
" closed carbon 82
" Laclede ; . . 83
" Grove's gas 106
" Upward's chlorine 109
" Powell's thermo-electro-chemical 110
" absorbing oxygen Ill
" Jablochkoflf's 114
*' with two carbon electrodes 114
" tests 1 15
Beetz 66
Behrens 4
Berlin Academy of Science 2
Bichromate battery 47
" ** chemical reactions in 49
" '* directions for setting up 51
" Fuller cell 53
Bidwell's dry battery 113
Blue vitriol 38
Bunsen's battery 46
Calculation of E.M.F. from heat of combustion 176
Calorie 17, 169
Carbon cup, Leclanche cells with 73
Carhart-Clark standard cell 95
Cells in series 168
Change in potential 19
E.M.F 19
Chemical changes 2
" reaction in the simple voltaic cell 10
" reaction in the Daniell cell 30
" reactions in relation to energy 32
** " in the bichromate battery 49
" " in the Leclanche' cell • 68
Chloride of lead standard cell 102
" silver cell 62
INDEX. 187
PAGB
Chromic acid ... — 49
** ** as the depolarizer 63
Circuit, simple battery 15
'* ^ electrolytic 30
Clausius, theory of 9, 175
Closed circuit batteries 27
" Leclanche cells 71
** carbon batteries 82
Compressed plates of CuO 62
Condensing electroscope 21
Conductor, electrolytic 21
Contact force , 21
Copper oxide battery 58
Coulomb 15
Counter electromotive force in a circuit 172
" " " of electrolysis 173
Coupling together dissimilar cells 164
Daniell, Professor , 28, 37
battery , 28, 29
cell, E. M.F. of 31
" polarization curves of 31
" defects of 36
" effect of temperature changes on 37
** temperature coefficient of 145
Data for polarization curves 119
Davy, Marie 76
Defects of the Daniell cell 36
Delany's modified gravity cell 41
Depolarizer 27
" efficient 28
" solid 28
" batteries without 78
Diagram of battery tests 118
Diamond carbon battery 80
Difference of potential 16
" " relation of E.M.F. to 18
" " between two points 18
Diffusion through porous cup 36, 68
" of zinc sulphate 41
" of the redissolved salt. 91
" slowness of 91
Directions for setting up bichromate battery 51
Dissimilar cells in parallel • 105
Dissipation of energy 166
188 INDEX.
PAGE
Distinction between open and closed circuit batteries 27
Division of energy in a circuit with counter E.M.F 173
Double sulphate of potassium and chromium 50
*< ** of sodium and chromium 51
Dry pile 4
** battery, Gassner 83
** ** Meserole's composition for 85
** " Shelford Bidweirs 113
Eidison-Lalande battery 60
Effect, Volta 21, 22
" of amalgamation 34
" Peltier 25, 183
Efficiency test of a copper-oxide battery 131
Electric pendulum 6
*' potential 15
** pressure , 17
Electrode, positive 8
'♦ negative 8
Electrolytic conductor 21
" circuit 30
** zinc 35
** process 108
Electrometer, quadrants of 6
Electromotive force 15, 17
** ** relation of, to difference of potential 18
*' " positive 16
** •* standards of 86
" " depends on materials 17
" •* effective 18
*' " seat of 20
«* " of the Daniell cell 31
*' " of the Clark cell 90
" " equation for , .90, 92, 96
** '* and temperature, relation between 93
♦* '* of standard Daniell cell 98
** ** measured by silver voltameter 104
** '* of various combinations 151
Electroscope, condensing 21
Element, simple voltaic 7
Energy of chemical separation 1
" expended 6
** chemical reactions in relation to 32
** conservation of 166
" dissipation of 166
Equation for electromotive force 90, 92, 96
--1
INDEX. 189
Jb ailare of a cell to effect decomposition J.76
Favre 171
Fitch " chlorine " battery 76
Formula for electromotive force 96
Fuller bichromate cell 53
Fundamental phenomena 7
** units 168
Galvani 3, 4, 21
Gassner dry battery 33
Greneral considerations 166
Gravity battery 38
" cell, Delany's modified 41
" battery, Partz acid 55
Grouping of cells 157
" in parallel or multiple arc 159
" in multiple series 160
" of a battery for quickest action 161
Grove's battery 43
" gas battery 106
rleat equivalent of a current 169
" evolved in a circuit with no counter E.M.F 170
'* of formation 23, 32
" of combination 17, 176
" reversible 26
** mechanical equivalent of 169
Helmholtz's equation 162
** formula for electromotive force 180
Hydrogen, accumulation of 11, 12
** nascent 12
** sulphuretted 25
" plays the part of zinc 107
Internal resistance, to obtain 117
Joule's equivalent 169
'* law 170
Laclede battery 83
Latimer Clark's standard cell 86
Law battery , 80
Ijeclanche cell 66
" ** chemical reactions in 68
190 INDEX,
PAGB
Leclanche cell with carbon cup 73
'* '• with agglomerate-carbon 74
Leyden jar 6
Local action 65
" " and amalgamation 33
Lord Rayleigh 87
** " form of Clark element 87
Loss of potential 19
Manganese dioxide 6G, 67
*• ** in Leclanche cells 156
Mechanical equivalent 169
Microphone cell 72
Minchin's seleno-alumiuum cell 112
Miscellaneous batteries 106
Modifications of the silver chloride cell 64
^Needle, aluminum 21
Negative pole 4
*' electrode 8
Nitrate of ammonium 46
Nitric oxide 44
Open circuit batteries 66
Origin of the voltaic cell 2
Oscillation, period of • 6
Oxide of mercury standard cell • • • . • 95
Partz acid gravity battery 55
Peltier effect 25, 183
" phenomenon 181
Pendulum, electric 5
Period of oscillation • • 6
Platinum black 78
pulverulent 107
finely divided 79
Polarization of a simple voltaic cell • 12
'* curve of a simple cell 13
progress of 13
curves of Daniell cell 31
** progressive ' 74
Positive pole 4
** electrode : 8
(I
ISVEX, 191
PAGE
Potassium bichromate 47. 50, 52
" sulphide 25
Potential, electric 15
difference of 15, 18, 20, 2a, 25, lti9
" practical zero of IG
** loss of 19
faUof 20
" slope of 21,23
Practical unit 15
" zero of potential 16
Preece,W.H 38
Prism Leclanche battery 69
Kack-and-pinion movement for plates 105
Reaction, chemical, in the simple voltaic cell 10
" " " Daniellcell 30
" ** " bichromate battery 49
" " Leclanche cell 68
" " in relation to energy 32
Reduction of copper 37
Relation of potential differences to external and internal resistance 20
" between E.M.F. and temperature 93
Relative protection of alloying and amalgamating 35
** value of oxidants in batteries 153
Removal of crystals of spent residue 77
\ Resistance, internal 20, 24
{ " external 14, 20, 24
; Reversible heat 26
) , Roberts* peroxide battery 74
\
1
. \
) I
;
I
1 \
Oamspn battery 73
Sea salt battery 79
Seleno-aluminnm cell 112
Simple voltaic cell 7
Smee cell 78, 171
Sodium bichromate, advantages over potassium bichromate 50
Standards of electromotive force 86
Standard cell, Latimer Clark's 86
" " with low temperature coefScient 90
'* " Carhart-Clark 96
" ** oxide of mercury >. 96
" " chloride of lead 102
" " to measure the E.M.F. of 103
•* Daniell cell, Sir William Thomson's 97
" Lodge's 98
(f
192 INDEX.
PAGE
Standard Daniell cell, Fleming's 99
*' solutions 101
Sulphate of mercury battery 75
Sulpho-ohromic salt 55
Systematic test, what it includes 115
± aylor's battery 57
Temperature coefficient, analysis of \26
•* *• of the Daniell cell , 145
" *' of the Clark cell 147,149
Test of typical Leclanche' cell 121
of Leclanche cell with depolarizer in carbou cylinder 124
of zinc-carbon cell without depolarizer 127
" of a " dry " cell 128
** of a silver chloride cell. 130
'* of battery for small lamps 134
Thermal relations 166
Thermo-electric power of zinc — zinc sulphate 136
** " " of copper — copper sulphate 141
** ** " of mercury— mercurous sulphate 146
Thermo-electro-chemical battery 110
Theory of the voltaic element 8
'* of Clausius 9, 176
Thomson, Sir William 6, 21
Thomson's contact apparatus 22
" tray battery 42
Time-constant 162
Typical Leclanche cell '. 67
" " " testof 121
Units of force, work, activity, heat 168
Upward's chlorine battery 109
Volta 3,4,21
" effect 21, 22
Voltaic cell, inconstancy of 11
" " origin of 2
'* " simple 7
** element 8
Volta's pile 3
Voltameter 104
Voltmeter 135
** electrostatic 18
" Lippmann 142
t
I
i
.. ..B
K
..147,11
C
12
E
e
II
a
n
a
%
....II
...J
.... j
....6,3
.... ■*
....^
,...lfi
... ff
...12
.21.2
. n
3
lOii
135
INDEX. 193
PAOB
IT ater marks in jar 72
What a systematic test includes 115
Work done 16
" positive and negative 16
Woulfif 's bottle 107
i^amboni 4
Zamboni's pile 4
I
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