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1X530.1,058 

1001 Questions and answBfs on physics or 




3 6105 04927 9859 







SCHOOL OF EDUCATION 
LIBRARY 



TEXTBOOK 
COLLECTION 



STANFORD V^^/ UNIVERSITY 
LIBRARIES 



IN THE SAME SERIES 



1001 QUESTIONS AND ANSWERS ON 

U. S. HISTORY. 

(Including the Federal Constitution and Amendments.) 

1001 QUESTIONS AND ANSWERS ON 
GEOGRAPHY. 

(Embracing Descriptive, Physical and Mathematical Geography.) 

1001 QUESTIONS AND ANSWERS ON 
GRAMMAR. 
(With Copious Illustrations, Parsing and Analysis.) 

1001 QUESTIONS AND ANSWERS ON 

ARITHMETIC. 

(Including nearly 300 Test Examples, with Solutions.) 

1001 QUESTIONS AND ANSWERS ON 
THE THEORY AND PRACTICE OF TEACHING 



1001 QUESTIONS AND ANSWERS ON 
PHYSIOLOGY AND HYGIENE. 

(Containing a Chapter on the Physiological effects of Alcohol 
and Narcotics.) 



1001 QUESTIONS AND ANSWERS ON 
ORTHOGRAPHY AND READING. 



1001 QT^ESTIONS AND ANSWERS ON 
GENERAL HISTORY. 



1001 QUESTIONS AND ANSWERS ON 
BOTANY. 



1001 QUESTIONS ANn Avcwirpc; ov 

TEST EXAMPLES IN ARITHMETIC. 



1001 QUESTIONS AND ANSWERS ON 
PHYSICS OR NATURAL PHILOSOPHY. 



HINDS, HAYDEN & ELDREDGE, Inc 
New York City 



• ipoi- 

QUESTIONS AND ANSWERS 



ON 



Physics or Natural Philosophy. 



A SCHOOLMASTER. 



HINDS, HAYDEN & ELDREDGE, Inc. 
New York City 



629025 

CoPYRiSmr, 1895, 
THIS BURROWS BROTHERS CXX 



PREFACE. 



The marked success of ihj previous volumes of the looi 
series has ded to the publication of this. It is hoped that it 
Tvill be of aid to the buyer ^ and of profit to the publishers. 

This is not one of the cases where " every conceivable 
question has been asked" and answered. In fact^ it was 
equally difficult to determine what to ask^ and what not to ask. 

It was thought that it would be helpful^ in the cases 
of some who study this book, to follow the order of some 
larger work which may be referred to for fuller informa- 
tion on some of the points that are herein necessarily briefly 
sketched. For this purpose^ Avery's " School Physics " was 
chosen. As that book is generally used in the schools of this 
country, it is easily accessible to the students of this volume. 



CONTENTS, 



CHAPTER I.— Matter. 

PAQB. 

A — Divisions of Matter, 5 

B — Properties of Matter, 8 

C — Conditions of Matter, 11 

CHAPTER II.— Mechanics. 

A — Motion and Force, . 13 

B — ^Work and Energy, 17 

C — Gravitation, Etc., 20 

D— Falling Bodies, 24 

E— The Pendulum, 27 

F— Simple Machines, . 28 

G — The Mechanics of L/iquids, 37 

H — The Mechanics of Gases, 41 

CHAPTER III.— Acoustics. 

CHAPTER IV.— Heat. 

CHAPTER v.— Radiant Energy. 

CHAPTER VL— E1.ECTRICITY and Magnetism. 

A — General View, 75 

B — Electric Generators, Etc., 95 

C — Electrical Measurements, 107 

D— Applications o** Elect' \city, 108 



CHAPTER L 
MATTER. 



A— Divisions of Mattbr. 

1. What is Natural Philosophy or F^ysics t 

Natural Philosophy or Physics is the branch of science 
that treats of matter, and of the forces operating upon 
it, and of the physical changes thereby produced. 

2. Whalns science f 

Science is classified knowledge. 

3. What is matter t 

Matter is anything that takes up room. 

4. Is this a real definition of f natter f 

It is not, for it simply tells something about matter in- 
stead of telling what matter really is. 

5. Why then is it given ? 

Because, in the present state of human knowledge, we 
can tell nothing of the real nature of matter and this 
does the next best thing ; it enables us to distinguish 
matter from that which is not matter. 

6. Give some other " definition " offnatter. 
Matter is anything that has weight 

7. What are the divisions of matter t 
Atoms, molecules, and masses. 

8. What is an atom t 

The smallest quantity of matter that can enter into com- 
bination. It is generally a part of a molecule. 

9. How many kinds of atoms are there? 

As many as there are " elements," of which we now know 
more than seventy. Others will probably be found. 

10. Can an atom be divided ? 

Atoms are considered indivisible, but the form or consti- 
tution of an atom is unknown. The atom is the chemi- 
cal unit of matter. 



6 MATTER. 

11. What is a molecule t 

It is the smallest quantity of matter that can have a sep- 
arate existence. 

12. How many kinds of molecules are there f 

As many as there are kinds of matter. The nature of 
the molecule determines the nature of the substance of 
which it is a part. 

13. Can a molecule be divided ? 

Yes, but. only by a chemical process. A molecule is the 
physical unit of matter. 

14. Give an illustration of a molecule, 

A particle of sugar so small that, if it is divided, none of 
Its parts will be sugar. 

15. What are the parts of a molecule called? 
Atoms. 

16. How many atoms are there in a molecule t 

Chemistry teaches that in the case of a very few of th« 
elements, the molecule consists of a single atom, but 
in almost every case, the molecule contains two or 
more atoms. The common sugar molecule has 45 
atoms ; the oxygen molecule has two atoms. 

17. What is a mass f 

A body of matter consisting of two or more molecules; 
it is not necessarily very massive. 

18. Give an illustration of a** mass " of matter, 
A minute dust speck, a mountain or a planet 

19. What is an element or an elementary substance f 

A substance, all of the atoms of which are alike, /. e,, one 
composed of a single kind of atoms. Silver is an ele- 
ment because all of its atoms are silver atoms ; it can- 
not by any known or conceivable process be " analysed " 
or decomposed. 

20. What is a compound substance ? 

One that contains more than one kind of atoms. One of 
its molecules is like its other molecules, but each mole- 
cule contains more than one kind of atoms. Thus, a 
dew-drop is composed of many similar water molecules. 



MATTER, 7 

but each of the molecules is composed of one oxygen and 
two hydrogen atoms. Hence, water in any quantity is 
a compound substance. Compound substances are in- 
numerable. 

21. What is a physical change f 

One that does not change the structure of the molecule 
of which the changed body is composed. 

22. Illustrate the meaning of ''^ physical change. ^^ 

A black iron nail may be made red-hot. The iron mole- 
cules jostle one another more vigorously than before, 
and the energy of their mutual thrusts we call heat. A 
certain kind of motion produced by such increased 
activity of the molecules we call light, but the mole- 
cules themselves are simply iron molecules, just as 
they were at first. The molecule is one thing; the 
motion of the molecule is a wholly diflFerent thing. 
By increasing the molecular motions^ we may even 
melt the iron, but that will not change the mole- 
cules ; it will simply be melted iron. Two such nails 
may be united by welding. The identical molecules 
are preserved, and with unchanged identity. With a 
file, some of these may be torn away from others, but 
each of the filings will be /V^w-filings, composed of un- 
changed iron molecules. Hence all of these are physi- 
cal changes. 

23. With this idea of** heat as a mode of motion ^^ give another 

definition of fnolecule, 
A molecule is an aggregation of material particles that 
do not part company during the journey of a heat vi- 
bration. 

24. State and illustrate some change of matter that is not a 

physical change. 
If the iron is treated with sulphuric acid, the iron atoms 
in each iron molecule will be torn asunder. Each iron 
atom will be joined to an atom of sulphur, and four 
atoms of oxygen, to make a molecule of iron-sulphate 
(green vitriol). We no longer have an iron molecule, 
and hence, no iron. Iron and iron-sulphate are essen- 
tially diflFerent. Changes that thus alter the constitu- 
tion of the molecule, and hence, the nature of the sub- 
stance, are chemical changes. 



8 MATTER. 

25. What objection is there to the term " Natural Philosophy f " 

There is a philosophy of mind as well as a philosophy of 
matter, and one pertains as mucli to Nature as the 
other. But the term "Natural Philosophy," with its 
restricted significance, is so completely worked into 
our languflge and literature that it would be difficult to 
change the usage. The term ** Physics " is preferable. 

B— Properties of Matter. 

26. What is a property of matter ? 
Some quality that pertains to matter. 

27. What is a universal property of matter ? 

A quality that pertains to all matter, a quality without 
which matter, as we know it, could not exist. 

28. What is a characteristic or accessory property of matter? 

A quality that pertains to some kind or kinds of matter 
and not to others, and that thus enables us to distinguish 
one substance from another. 

29. Name some of the universal properties of matter. 
Extension, impenetrability, indestructibility, weight and 

inertia. 

30. Name some of the characteristic properties of matter. 
Hardness, as of the diamond ; tenacity, as of steel ; brit- 

tleness, as of glass; malleability, as of gold; ductility, 
as of platinum. 

31. What is extension f 

The property of matter by which matter takes up room, 
i. e.y occupies space. 

32. To what does it refer? 

To length, breadth and thickness, a combination that is 
essential to the existence of matter. 

33. What is impenetrability ? 

The property of matter by which one body excludes 
another from the space in which it is. No two bodies 
can be in the same place at the same time. 



MATTER, 



34. What is indestructibility? 

The property of matter by which it defies annihilation. 
God created matter; He alone can destroy a single 
atom of it. 

35. What caution should be observed in this connection ? 

We should remember that there is a difference between 
disappearance and destruction. 

36. How can you illustrate this difference ? 

Water " boils away," but we all know that thoufrh it thus 
disappears, it does not cease to be. So a candle burns 
away and disappears. The candle is destroyed, but not 
an atom of the matter of which the candle was com- 
posed is destroyed. The hydrogen, which was part of 
the candle, burns [i, ^., unites with oxygen), and thus 
forms watery vapor that will condense to the liquid 
form, and perhaps help quench the thirst of a sheep 
that may jrield tallow for another candle. The carbon 
of the candle bums to carbon di-oxide (carbonic acid 
gas), that may feed the plant on which feeds the 
sheep that may yield more tallow for still another can- 
dle. Matter goes through almost endless transforma- 
tions, and appears in protean shapes. 

37. What says Shelley' s poem on " The Cloud? " 

' ' I pass through the pores of the ocean and shores ; 
I change, but I cannot die." 

38. What is weight ? 

It is the downward pressure of a body on or near the 
earth's surface, due to the attraction between that body 
and the earth. 

39. What does it measure ? 

It measures the force of gravity upon the body weighed. 

40. How is the term generally used ? 

As above defined, referring especially to terrestrial ob- 
jects. As a matter of fact, all matter has weight be- 
cause the attraction of gravitation is of universal ap- 
plication. 

41. What is inertia ? 

The property of matter whereby a body cannot change 
its condition of rest or motion. It is a purely negative 
property ; a quality of inability, and nothing more. 



MATTER, 



42. State a common erroneous notion concerning inertia. 

It is sometimes (unconsciously) held that rest is the natu- 
ral condition of matter, and that inertia means espe- 
cially the tendency of matter to remain in such condi- 
tion ; that t© overcome the inertia of a body means to 
put it in motion. 

43. What is the fact in the case ? 

Matter has no " natural condition '* either of rest or of 
motion. So far as we know, matter is nowhere at rest, 
everywhere in motion. But this motion is due not to 
any inherent tendency, but to the fact that the motion 
was communicated to it by some agency outside itself. 

44. State some of the consequences of inertia. 

On account of inertia, a body at rest cannot put itself in 
motion ; on account of inertia, a moving body cannot 
bring itself to rest or even change its rate of motion 
(velocity). Stopping a moving body is overcoming its 
inertia as truly as in giving motion to a body at rest. 

45. What is meant by the term, " body ? " 

A body is some definite, separate, portion of matter. The 
term refers only to matter, ** dead matter," or matter 
considered independently of the living, willing, or 
motive power of vegetable or animal existence. These 
are forces, mysterious forces, distinct from the material 
organism and acting upon it. With such vital forces, 
physics (at least, elementary physics) has nothing to do. 

46. What is hardness ? 

The property of some kinds of matter whereby they are 
able to resist being marked by scratching. The dia- 
mond is harder than glass, and will, therefore, scratch 
glass. Glass is harder than gold, and will, therefore, 
scratch gold. 

47. What is tenacity ? 

The property of some kinds of matter whereby they are 
able to resist being pulled asunder. Because an iron 
bar will resist a greater pulling force than a similar 
lead bar, we say that the former has the greater tenacity. 

48. What is brittleness ? 

The property of some kinds of matter whereby they are 
easily broken by a blow. 



MATTER, 



49. What is malleability ? 

The property of some kinds of matter whereby they may 
be rolled or hammered into sheets. 

50. What is ductility f 

The property of some kinds of matter whereby they may 
be drawn into wire. 

C— Conditions of Matter. 

51. In how many conditions does matter exist? 

Three or more. The three universally recognized condi* 
tions are the solid, liquid and aeriform (or gaseous). 

52. What is a solid f 

A body that has a strong tendency to retain its given 
form, like ice. It has little freedom of molecular motion. 

53. What is a liquid? 

A body, the molecules of which move easily among them- 
selves, and yet tend to cling together. Water is the 
most familiar illustration of a liquid. 

54. What is an aeriform body ? 

One in which the molecules move easily among them- 
selves and tend to separate from each other, like steam. 

55. How are aeriform bodies classified ? 
As gases and vapors. 

56. How do these differ ? 

Gases retain their aeriform condition at ordinary temper- 
atures and pressures, like oxygen or illuminating gas. 
Vapors take the liquid or solid form at ordinary tem- 
peratures and pressures, like steam. 

57. What is a fluid ? 

A body characterized by great freedom of molecular mo^ 
tion. 

58. What does the term include ? 
Liquids, gases, and vapors. 

59. Is there any other form of matter ? 

Mr. Crooke's experiments seem to show that there is a 
form still more tenuous than the aeriform, for which he 



12 MATTER, 



has proposed the name "Radiant." The Inminiferous 
ether, which pervades all space, is a form of matter 
more nearly imponderable than the gaseous. 

60. What would you call a fluid that is scarcely compressible? 
A liquid. 

61. What would you call a fluid that is easily compressible f 
A gas or a vapor. 

62. What would you call a body that has a definite form, of its 

own? 
A solid. 

63. What would you call a body that can not, of itself maintain 

a definite form f 
A fluid. 

64. Why may larger' bubbles be blown zvith soap-suds than with 

pure water? 
Because of the greater surface viscosity of the soap solu- 
tion. The 8U{)eriicial film of a liquid is highly viscous 
as compared with the interior; 1. ^., it is comparatively 
difficult to move or to break it. 

65. Slate another fa£l concerning the superficial films of liquids, 
A liquid surface is in a state of tension like that of a 

stretched membrane. 

66. What is capillary attraction ? 

The tendency of water and other liquids to rise above theii 
levels in fine tubes dipped into liquids that wet them. 

67. What happens if the liquid does not wet the tube ? 

The liquid, instead of being raised, will be depressed be- 
low its level. 

68. How can you illustrate your meaning ? 

• By plunging a clean glass tube into mercurv, or a greased 
tube into water, the liquid in the tube will be depressed 
below its level outside the tube. 

69. Give a familiar illustration of capillary attraction. 
The ascent of oil in a lamp wick. 



MECHANICS. 13 



CHAPTER IL 
MECHANICS. 



A— Motion and Porcb. 

1. What is motion f 
Change of position. 

2. What is velocity f 

Rate of motion ; it may be uniform or variable. 

3. What is acceleration ? 

Rate of change of velocity ; i. e,, the change of velocity 
per unit of time. 

4. What is force f 

Any cause that tends to produce any change of motion. 

5. What is momentum f 
Quantity of motion. 

6. How is it measured? 

By the product of the number of units of mass into the 
number of units of velocity. 

7. What is the unit of momentum called ? 

It has no specific name. We may compare the momenta 
of two moving bodies by the ratio between the two 
measuring products as above explained. The momentum 
of a body having a mass of 40 pounds and a velocity of 
15 feet per second is twice as ereat as that of a body 
having a mass of 10 pounds and a velocity of 30 feet per 
second. 

8. What is the first law of motion f 

Every body continues in its state of rest or of uniform 
motion, in a straight line^ unless compelled to change 
that state by some external force. 



14 MECHANICS, 

9. From what does this law result f 
From the inertia of matter. 

la What is centrifugal force? 

So-called centrifagal force is simply a confasing name for 
inertia, or the tendency of matter to obey the first law 
of motion. 

11. To what special act of obedience is the term applied? 

When a body is compelled to move in a curve, it always 
tends to pull away from the centre and to move in a 
straight line, tangent to the cnrve. 

12. Illustrate this tendency. 

Mud flying from a carriage wheftl, or water from a grind- 
stone. 

13. Do the mud and the tvater^ after pulling away from their 

circular paths^ move in straight lines? 
They do not, because they are continually pulled there- 
from by the force of gravity. 

14. Give the second law of motion. 

The effect of a force will be the same whether it acts alone 
or jointly with others. 

15. What name is given to the effect of two or more forces ^ 

acting jointly ? 
Resultant motion, which will be different from the effect 
of any one of the forces acting, and may be looked upon 
as the result of a single force called the resultant force. 

16. How is the resultant force determined? 

By what is known as the composition of forces. 

17. State one case of the composition of forces. 

When the given forces act in the same direction, the re- 
sultant equals their sum. 

18. Give an illustration. 

If a man rows a boat with a force that alone will produce 
a velocity of 4 miles an hour, down a stream that has a 
current of 3 miles an hour, the boat will move at the 
rate of 7 miles an hour ; 4-1-3=7. 



MECHANICS. 15 



19. State another case under the composition of forces. 

When the given forces act in opposite directions, the re- 
sultant equals their difference, its direction will be that 
of the greater force. 

20. Give an illustralion, 

Ifthe boat is rowed with the same force as before but 
against the same current, it will move up stream at the 
rate of one mile an hour ; 4 — 3=1. 

21. State another case under the composition of forces. 

When the given forces act at an angle with each other, the 
resultant may be found by a process known as the 
parallelogram of forces. 

22. Give an illustration. 

If the boat is rowed easterly with the same force as before, 
and the same current is flowing northerly, these two 
forces may be represented bv two lines, 4 inches and 3 
inches long respectively, and meeting at a right angle. 
Call the apex of this angle A. Consider these two lines 
as two sides of a parallelogram, and draw the other two 
sides. Draw a diagonal from A. By measurement, 
or mathematically, we may find that this diagonal is 5 
inches long. Its length and direction represent the 
intensity and direction of the resultant force. The 
boat will move in the direction thus indicated, and with 
a velocity of 5 miles an hour. 

•23. What is the third laiv of motion f 

Action and reaction are equal and opposite in direction. 

24. Give an illustration. 

When Columbus made the famous ^%% stand on end, the 
action of the ^^'g may have made a dent in the table. 
It is certain that the equal and opposite reaction of the 
table broke the shell. 

25. What is the law of reflected motion f 

The angle of incidence equals the angle of reflection. 

26. What is the angle of incidence? 

The angle included between the path of the moving body 
before reflection, and a line drawn perpendicular to the 
int of reflection. 



reflecting surface at the point k 



i6 MECHANICS. 



27. What is the angle of reflection f 

The angle included between the path of the moving body 
after reflection and the perpendicular drawn as above 
described. 

28. What very common error in this respect? 

To think that these angles are included between the two 
paths specified and the reflecting surface, instead of be- 
tween the paths and the perpendicular to that surface. 

29. How are forces measured? 

By comparison with some standard called a unit of force. 
There are two kinds of units of force, the gravity unit 
and the absolute unit 

3a What is the gravity unit of force? 

It is the weight of any standard unit of mass, as the gram, 
kilogram, or pound. When a force may be balanced by 
a weight of 100 pounds, we call it a force of 100 pounds. 
If a frictionless horizontal piston at the top of a steam 
boiler must be loaded at the rate of 100 pounds to the 
square inch to keep it in place against the force of the 
confined steam, we say that there is a steam pressure of 
100 pounds to the square inch. 

31. What is the absolute unit of force ? 

It is the force that, acting for unit of time upon unit of 
mass, will produce unit of acceleration. There are two 
such units m common use ; the poundal and the dyne. 

32. What is the poundal ? 

It is the force that, when applied for one second to one 
pound of maiter, produces an acceleration of one foot 
per second. It is called the F. P. S. (foot-pound-second) 
unit of force. 

33. What is the dyne ? 

It is the force that, when applied for one second to one 
gram of matter, produces an acceleration of one centi- 
meter per second. It is called the C. G. S. (centimeter- 
gram-second) unit of force. 

34. What is the numerical relation between gravity units and ab- 

solute units of force ? 
At the sea-level at New York City, the force of gravity 
g^ves to a falling (freely moving) body that weighs one 



MECHANICS, 17 



pound (or any other weight) an acceleration of 32.16 
Feet; consequently, at New York, a force of one pound 
equals 32.16 poundals. The same force produces an 
acceleration of 980 centimeters; consequently, at New 
York, a force of one gram equals 980 dynes ; a force of 
one kilogram equals 980,000 dynes. 

35. How may the acceleration be determined f 

By dividing the total velocity that the force has produced 
by the number of seconds that the force has acted. 

36. How is a force measured in absolute units t 

By multiplying the number of units of mass moved by the 
number that represents the acceleration produced. For 
poundals, the units used must be feet, pounds, and 
seconds (F. P. S.); for dynes, the units used must be 
centimeters, grams, and seconds (C. G. S.). 

37. What force is necessary to give a body weighing jo grams 

a velocity of 50 centimeters per second ^ by acting upon the 
body for two seconds f 
30X50-5-2=750, the number of dynes. 

B— Work and Energy. 

38. What is work? 

Work is the overcoming of resistance. The term implies 
a change of position and is independent of the time 
taken. 

39. What may we take as a type of work f 

The lifting of a bodv against the force of gravity, i, e,. 
against the ** pull *^ of the earth. 

4a How may we measure such work ? 

By considering both the weight of the body and the height 
which it is raised. 

41. How are work-units classified f 

As gravitation units and absolute units, with two in each 
class. 

42. What are the gravitation units of work f 

Th6 work expended in lifting one pound one foot against 
the force of gravity is called a loot-pound. The work 
expended in lifting one kilogram one meter against the 
same force is called a kilogrammeter. 



i8 MECHANICS. 



43. What are the absolute units of work ? 

The work done by one poundal in producing a displace- 
ment of one foot is called a foot-poundal. The work 
done by one dyne in producing a displacement of one 
centimeter is called an erg. 

44. What is the numerical relation between these units f 

A foot-poundal is equivalent to 421,402 ergs ; a foot-pound 
is equivalent to 32.16 times that many ergs. Since a 
force of one kilogram is equivalent to 9^,000 d3mes, 
and a meter to 100 centimeters, a kilogrammeter is 
equivalent to 98,000,000 ergs. In any case, the work 
done is numerically represented by the product of the 
number of utiits of force into the number of units of 
displacement Multiply the number of weight-units by 
the number of height units. 

45. A laborer with his hod of bricks weighs 300 pounds. How 

much work does he perform in carrying his load to the top 
of a building ^o feet high? 
300X50=15,000, the number of foot-pounds. , 

46. What is CLctivity ? 

The activity of an agent is the rate at which it can do 
work. 

47. Who/ is a horse-power? 

It is the most common unit of activity, and represents the 
ability to do 33,000 foot-pounds in a minute, or 550 foot- 
pounds in a second. 

48. How is horse- power computed f 

Multiply the number of pounds raised by the "number of 
vertical feet through which it is raisea, and divide the 
product by 33,000 times the number of minutes (or by 
550 times the number of seconds) required to do the 
work. 

49. What is the horse-power of an engine that can lift ^ tons 

J ': yards in half a minute ? 

10,000X33 

-=2a It is a 20 H. P. engine. ^ 



33,oooXj 
50. What is energy ? 

The power of doing work. 



MECHANICS. 19 

51. Name the two great classes of energy. 
Kinetic and potential. 

52. What is kinetic energy ? 

Euer^ of motion ; i, e.^ the power of doing work that a 
body has by virtue of its motion. 

53. What is potential energy f 

Ener^ of position ; 1. e.y the power of doing work that a 
body has by virtue of its position. 

54. Illustrate kinetic energy. 

The energy of running water, a falling pile-driver, a re- 
volving fly-wheel. 

55. Illustrate potential energy, ,. 

The energy of a head of water, a coiled spring, a drawn 
bow. 

56. How are these varieties of energy related ? 

They are mutually convertible. Either may be converted 
into an equivalent amount of the other. 

57. Illustrate this statement. 

It requires a certain amount of work to wind up a clock. 
When the clock is wound up, it has a store of potential 
energy. When the pendulum is Fet in vibration, the 
energy stored in the coiled spring or raised weight will 
perform an amount of work equal to that performed in 
winding up the clock. 

58. Give a further illustration, 

A ball is thrown vertically upwards with a certain velocity. 
Its kinetic energy lifts it to a certain height and, when 
the ball is at that point, its velocity is zero. It, there- 
fore, has no kinetic energy. All that it had at the start 
has disappeared, having been converted into an equiva- 
lent amount of potential energy. For, at this moment, 
the ball has a position of advantage from which it de- 
rives a power of doing work; it may be used as a 
weight to run machinery or in other ways. This energy 
of position maj be reconverted into its original form as 
energy of motion, for, if the body is permitted to fall, 
it will regain the velocity with which it started. At 
the middle point, going up or down, the energy of the 



20 MECHANICS, 

t>a^l is half kinetic and half potential, and at every 
point of the path, the sum of the two energies is a con- 
stant quantity. 

59. Hoiv is kinetic energy tneasured in gravitation units f 

We have the formula K. E. ="'^' , in which w represents 

the weight and z/ the velocity of the moving body, and ^ 
the acceleration due to gravity {i.e.y 32.16 feet or 9.8 
meters). Substituting in this formula values measured 
in feet and pouuds, we have the value of the kinetic 
energy in foot-pounds; using meters and kilograms, 
we have the value in kilogrammeters. See Avery *s 
" School Physics," page 88. 

6a Ho7v is kinetic energy mensured in absolute units f 

We have the formula, K. E.=>^mz/'. Measuring mass in 
pounds, and velocity in feet per second, this g^ves the 
energy in foot-poundals. Measuring mass in grams, 
and velocity in centimeters per second, gives the energy 
in ergs. See Avery's " School Physics," page 89. 

61. What is meant by the conservation of energy? 

When the universe was hurled into space bjr the hand of 
the Creator, it was endovred with a certain amount of 
energy. Like matter, energy may appear in many 
difiPerent shapes. The sum total of all these different 
forms of energy in the universe taken as a whole is a 
constant quantity, for energy, like matter, is indestruct- 
ible. 

C— Gravitation, ktc 

62. What is gravitation f 

The attraction that every particle of matter in the uni- 
verse has for every other particle. 

63. When the moon comes' between the sun and the earthy how 

does that affect the attraction between the sun and the 
earth f 
Such interposition of matter has no effect on the attract- 
ive force originally existing. 

64. What is the first law of gravitation f 

The attractive force (gravitation) between two bodies 
varies as the product of the masses of the two bodies. 



MECHANICS, 21 

65. Illustrate this law. 

If two bodies contain 5 and 10 pounds of matter respect- 
ively, the product may be represented by 5a \\ two 
other bodies contain 4 and 25 pounds of matter re- 
spectively, the product may be similarly represented by 
100. Acting at like distances, the attraction between 
the second two will be twice (=Vrf') as great as the at- 
traction between the first two. 

66. How would doubling the matter in one of the bodies affect 

their gravitation? 
It would double one of the factors, and thus the product, 
and thus the gravitation. 

67. How would doubling the weight of both of the bodies affect 

their gravitation ? 
It would double each of the two factors, and thus increase 
product and gravitation fourfold. 

68. What is the second law of gravitation f 

Gravitation varies inversely as the square of the distances 
between the centers of mass of the two bodies. 

69. What is the center of mass of a body ? 

The point about which all the matter of a body may be 
balanced. 
7a Illustrate the second law of gravitation-. 

Two bodies a foot apart (between centers) attract each 
other four times as much as do the same bodies when 
placed two feet apart. 

71. How would doubling the product of the masses and doub- 

ling the distance affect gravitation f 

It would ( ^ =syi J divide the attraction by two. 

72. How would trebling the quantity of ^natter in each of the 

two bodies and doubling the distance between them affect 
the gravitation f 

It would |3X3__^ \ increase it two and a quarter times. 

73. How does gravitation <ut f 

It is generally conceived of as Kpull^ but it is more likely 
to be a push. At the present time, the mechanical 
nature of the action is unknown. 



' \ 



MECHANICS. 



74. What is gravity f 

The attraction between the earth and a body on or near 
its surface. 

75. How is grainty measured f 

By the weight that it gives to a body. 

76. In what direction does it ctct ? 

Vertically downward ; i, e.^ toward the center of the earth. 

77. How is this direction easily illustrated f 
By a plumb line. 

78. Where does a body have the greatest weight f 
At the surface of the earth. 

79. How does carrying a body below the surface of the earth 

affect its weight f 

It decreases it as the distance from the center of the earth 
is decreased. 

80. Why is this? 

In descending, the matter left behind or above attracts it 
upward and neutralizes part of the attraction of the 
matter still below it 

81. How does carrying a body above the sutface of the earth 

affect its weight? 

It decreases it as the square of the distance from the 
center of the earth increases. 

82. How much will a pound of iron weigh 4^000 miles above 

the earth's surface f 

The distance from the center of the earth has been multi- 
plied by two, therefore its weight will be divided by 
the square of two which is four. It will weigh a quarter 
of a pound. 

S3. Considering the earth as a hollow sphere^ what would a 
body in its interior weigh f 

It has been mathematically demonstrated that a particle 
of matter within a spherical shell is equally attracted 
in all directions by the matter of the shell. Hence, 
a body would have no weight anywhere within the shell 
of such an earth. 



MECHANICS. 23 



84 Illustrate a method of finding the center of mass of a body. 
Drive a tack in a slate frame. Tie the middle of a string 
around the tack and a weight (plumb-bob) to one end of 
the string. Suspend the slate by the free part of the 
string and mark the direction of the plumb-line across 
the slate. Change the position of the tack and repeat 
the process. The intersection of the two lines thus 
marked on the slate "will approximately indicate its 
center of mass. 

85. Mow may this be proved f 

Place the point thus found upon the finger-tip ; the slate 
will balance. 

86. When is a body in stable equilibrium ? 

When it is so supported that when it is slightly displaced 
it seeks to return to its original position. 

87. Illustrate. 

A stick supported from its upper end; a pendulum; a 
hemispherical oil-can. 

88. When is a body in unstable equilibrium ? 

When it is so supported that when it is slightly displaced 
it seeks to fall further from its original position. 

89. Illustrate. 

A stick balanced on its lower end ; an ^^'g standing on 
its end. 

90. When ts a body in neutral equilibrium f 

When it is so supported that when slightly displaced, 
it seeks to move neither toward nor from its original 
position. 

91. Illustrate. 

A ball resting on a table. 

92. What is the line of direction f 

The path of the center of mass of a body when the body 
falls. 

93. Give another definition. 

A line drawn vertically downward from the center of mass. 

94. What is the base of a body ? 

The side on which it rests, or the surface bounded by 
lines joining its points of support. 



24 MECHANICS. 

95. When will a body stand f 

When its line of direction falls within its base. 

96. When will a body fall f 

When its line of direction falls without its base. 

97. How may the stability of a body be increased? 

By increasing its base or lowering its center of mass. 

98. How do we determine the height that a body is raised^ 
By the distance its center of mass is raised. 

99. Why is it easier to lift one end of a plank a yard than it is 

to lift the middle of the plank that distance f 
• In the first case, the center of mass is lifted only half as 
high as in the second case. 

100. What is the base of a sphere supported on a horizontal 
plane f 
A point. 

loi. What is the base of a cylinder supported on a horizontal 
plane? 
A line. 

102. Why is a sphere easily rolled on a horizontal plane ? 
Because such motion does not raise the center of mass. 

103. Why is a cylinder easily rolled on a horizontal plane? 
For the same reason. 

D-— Pai^wng Bodies. 

104. What is a jreely falling body ? 

One that moves under the influence of the force of grav- 
ity without any resistance. 

105. What is a constant force ? 

One that acts continuously and uniformly. 

106. Name such a force. 
Gravity. 

107. What kind of velocity does gravity give to a falling body f 
Uniformly accelerated velocity. 



MECHANICS. 25 

108. Why accelerated? 

Because the force that produces it acts continuously. 

109. Why uniformly accelerated? 

Because the force that produces it acts uniformly. 

no. What velocity will gravity give to a freely falling body 
each second? 
About 32.16 feet or 9.8 meters (=980 centimeters). 

111. What is the velocity of a body at the beginning of the 

first second of its fall? 
Zero. '^ 

112. What velocity zuill it receive during the first second? 
32 16 feet. 

113. What is this distance called ? 

The acceleration, or the increment of velocity due to 
gravity. 

114. How is it generally represented? 
By the letter g. 

115. What velocity will the falling body have at the end of the 

first second? 
32.16 feet (0+32.16=32.16.) 

116. Wtiat was its average velocity during that second? 



/0+32.T6 \ 

i6.o8 feet I =16.08. 



117. How far tuotUd it fall during that second? 
16.08 feet=>i g, 

118. yiow may you find the distance fallen ? 

By multiplying the average velocity per second, by the 
number of seconds it moves at that average rate. 

119. What Tvill be the velocity of a falling body at the begin- 

ning of the second second of its fall ? 
32.16 ft., or what it was at the end of the first second. 

120. What velocity ivill this force oj gravity give it during 

the second second ? 
32.16 ft., the same acceleration as before. 



s6 MECHANICS. 

121. What will its velocity be at the end of the second second? 
64.32 ft (32.16+32.16=5432.) 

122. What will be its average velocity for the second second f 
48,4 ft. /3^'6+64.3._^3^^ 



123. I tow Jar will it fall during this second second? 

48.21ft. 
Z24. Now far will ij^fall during both seconds f 

64.32 ft. (16.084-48.24=64.32.) 

125. How can you find the velocity of a falling body f 

Multiply the acceleration (32.16 ft.) by the number of 
8'coiidsit has been falling : v=gt. 

126. Ho7V can vou find how far a body ivill fall in any given 

second f 
Multiply the distance fallen in the first second (16.0S ft) 
by ' ne leas than twice the number of the second: 
/4=>i^(2/-i). 

127. How can you find how far a body will fall in a given 

number of seconds ? 
Multiply the distance fallen in the first second (16.08 ft.) 
by the square of the number of seconds : l=sy^gt^, 

128. What velocity will a body have at the end of the tenth 

second f 
32.16 ft.XiOB»32i.6 ft. 

129. What velocity will a body have at the beginning of the 

tenth second f • 

32.16 ft. X 9=292.44 ft. 

130. How far will a body fall during the tenth second? 

16.08 ft. X 19=305*52 ft. 

131. How far will a body fall during ten seconds f 
16.08 ft.X 100=1,608 ft. 

132. If another force acts jointly zuith gravity ^ how should the 

problem be treated ? 
Add the eflfect of the other force to the effect of gravity. 



MECHANICS. 27 



133. If a body is thrown downward with a force that starts it 

with a velocity of 25 ft, a second, what will be its veloc- 
ity at the end of the tenth second ? 
321 6 ft.+25 ft.=346.6 ft. 

134. How far will it travel in the tenth second? 
305.52 ft.+25 ft.=33o.52 ft. 

135. How far in ten seconds f 
1,608 ft.+25o ft=i,858 ft. 

E— The PKNDUI.DM. 

136. What is a pendulum? 

A weight suspended so as to be able to swing to and fro. 

137. What is the path of the weight called? 
The arc. 

138. What is an oscillation ? 

The motion from one end of the arc to the other. 

139. What does half the arc measure ? 

An angle at the point of suspension, called the amplitude 
of oscillation. 

140. What is the first law of the pendulum ? 

The vibrations of a given pendulum are performed in 
equal times, whether the arc is long or short. 

141. How was this fact ascertained? 

By Galileo's observation of the motion of a chandelier in 
the cathedral of Pisa. 

142. How are physical phenomena generally ascertained? 
By observation or experiment. 

143. What is the second law of the pendulum ? 

The time of oscillation is independent of the weight of 
the pendulum. 

144. How may one show this to be true f 

By the only true scientific method, trial. 

145. What is the third law of the pendulum, ? 

The length of the pendulum varies inversely as the 
sc[uare of the number of oscillations made in a given 
time. 



28 MECHANICS. 



146. What is a seconds pendulum ? 
One that vibrates once a second. 

147. How long is a seconds pendulum at the level of the sea and 

in the latitude of Cleveland f 
About 39.1 inches. 

148. How long at the equator? 
About 39 inches. 

149. How long at the poles? 
About 39.2 inches. 

150. What produces these variations ? 

Corresponding variations in the intensity of the earth's 
gravit5\ 

151. How long is a pendulum that vibrates twice a second? 
One- fourth as long as a seconds pendulum ( — j 

152. How long is a pendulum that vibrates once in ten seconds ? 
One hundred (lo^) times as long as a seconds pendulum. 

153. What gives motion to the machinery of a clock ? 

The force of gravity acting on its weights, or the force 
of the elasticity of the spring. 

154. Does not the pendulum make the clock *^go ? " 
It does not. 

155. What then is the use of the pendulum ? 

To regulate the speed at which the clock goes. 

156. If a clock ^^ gains time ^' what is the remedy? 
Lengthen the pendulum by lowering the " bob " of the 

pendulum. 

157. If a clock *' loses time,^* what is the remedy? 
Shorten the pendulum by raising the bob. 

F— S1MP1.K Machines. 

158. What is a machine ? 

A contrivance for applying energy advantageously for 
the doing of work. 



MECHANICS. ' 29 



159. Can a machine create energy f 

It can not ; on the contrary, it absorbs energy by the 
friction of its moving parts and by other means. 

160. Of what use are machines f 

They enable us to apply our strength in ways that we 
otherwise could not do, and to employ other sources 
of energfy than our own strength. 

161. What goes into a machine? 
Energy. 

162. What comes from a machine? 
Transformed energy. 

163. What name is given to the magnitude of the force that ads 

upon the machine f 
The power. 

164. What name is given to the magnitude of the force with 

which the machine acts upon some external resistance f 
The weight or load. 

165. What is the first general law of machines? 

The product of the numbers representing the power and 
the distance it moves equals the product of the num- 
bers representing the weight and the distance it moves. 

166. What is the second general law of machines? 

The product of the numbers representing the power and 
its velocity equals the product of the numbers repre- 
senting the weight and its velocity. 

167. To what classes of machines do these laws apply ? 
To all machines. 

168. What is the efficiency of a machine ? 

Part of the work done upon a machine is consumed by 
friction and other resistances that correspond to waste. 
The efficiency is the ratio between the work done on 
the machine and the work done by the machine. It is 
always less than unity and is generally expressed as a 
percentage. 

169. What is a lever? 

A rigid bar that may freely move about a fixed point ot 
line, called a fulcrum. 



30 MECHANICS, 



170. What are the three principal points of a lever f 

The fulcrum and the points where the weight and the 
power respectively are applied. 

171. How are they generally designated f 
By the initial letters, F, Wand P 

172. How are levers classified ? 

According to the relative positions of these tliree points. 

173. Explain more fully. 

If -Fis between the other two points, the lever is said to 
be of the first class ; if W^ tne lever is of the second 
class ; if P, the lever is of the third class. 

174. What is the power arm of a lever f 

The perpendicular distance from F to the line in which 
/*acts. 

175. What is the weight arm of a lever ? 

The perpendicular distance from ^to the line in which 
^acts. 

176. What is the static law of the lever? 

A lever will be ia equilibrium when the product of the 
numbers representing the power and the power-arm 
equals the products of the numbers representing the 
weight and the weight-arm. 

177. In what other ivay can you state the law ? 

The weight will be as many times as great as the power 
as the power-arm is times as long as the weight-arm. 

178. What weight at the end of an arm two feet long may he 

supported by a power of ys Pounds y acting at the end of 
an arm four feet long? 
The power-arm is twice as long as the weight-arm ; hence 
the weight may be twice the power, or 150 pounds. 

179. What caution is necessary in applying these statical lawsf 
The theoretical lever is without weight. In practice, 

the lever itself must be balanced before use. 

180. What is a balance ? 

A lever of the first class with equal arms. 



MECHANICS, 31 

181. Name two ways in which dishonest dealers cheat their cus- 

tomers. 
By using light weights, or by using a balance with un- 
equal arms. 

182. How tnay the first cheat be detected f 

By re weighing with standard weights on a true balance. 

183. How may the second cheat be determined f 

By changing goods and weights to opposite sides of the 
balance. 

184. With a false balance^ how may the true weight befoufidt 
Bjr weighing first on one side ; then on the other ; find- 
ing Uie product of the two false weights and extract- 
ing the square root of the product. 

185. In what other way f 

Counterpoise the goods with shot or sand; remove the 
l^oods and add standard weights until the shot or sand 
IS thereby counterpoised in its turn. 

186. What is a compound lever f 

A combination of levers where the weight-arm of one 
acts on the power-arm of the next, as in hay scales. 

187. State the statical law of the compound lever. 

The product of the numbers representing the power and 
the lengths of alternate arms beginning with the 
power-arm equals the product of the numbers repre- 
senting^ the weight and the lengths of alternate arms 
beginning with the weight-arm. 

i88. What is a wheel and axle f 

A wheel so joined to a cylinder as to revolve with it upon 
a common axis. 

189. How is it related to the lever f 

It is a modified lever of the first or second class, the 
radii of the wheel and of the axle corresponding to the 
arms of the lever. 

190. What advantage has it over the lever f 

In raising a body a considerable distance it avoids the 
intermittent action and consequent loss of time due 
to a frequent readjustment of the fulcrum. 



32 MECHANICS. 



191. How are power and weight generally applied to the wheel 

and axle f 
The power is applied to some point on the circumference 
of the wheel and the weight is suspended by a rope 
which acts upon the circumference of the cylinder or 
axle. 

192. State the static law of the wheel and axle. 

The power multiplied by the radius, diameter or circum- 
ference of the wheel equals the weight multiplied by 
the corresponding dimension of the axle. 

193. What is a capstan f 

A modified wheel and axle, in which the wheel is repre- 
sented by one or more radii, at the free end of which 
the power is applied. 

194. What is a train of wheel work f 

A combination of wheels and axles, in which cogs or 
teeth upon the circumference of a wheel engage cogs 
upon the circumference of an axle that carries another 
wheel. 

195. To what does it correspond f 
To tlie compound lever. 

196. How are the power and weight generally applied ? 
The power to the wheel at one end of the train and the 

weight to the axle at the other end. 

197. When so arranged^ what is the static law f 

The continued product of the power and the radii of the 
wheels equals the continued product of the weight and 
the radii of the axles. 

198. What is the effect of such a combination f 

A small power acting through a great distance moves a 
great weight through a small distance. 

199. How can the opposite effect be secured? 

By applying the power to the circumference of the axle 
at one end of the train and the weight to the circum- 
ference of the wheel at the other end. 

200. State in common phrase the effect of the inore comtnon ar- 

rangement. 
Velocity is exchanged for intensity of power. 



MECHANICS, 33 



201. What law of machines applies in either case f 

The product of the power into its velocity (or distancr) 
eq[uals the product of the weight into its velocity (or 
uistance). 

202. May not the diameters or circumferences of the wheels and 

axles or the number of cogs borne by the wheels and 
axles be used instead of the radii in the static law for 
wheel work ? 
They may, for such parts are severally proportional to 
the corresponding radii. 

203. What is a pulley ? 

A wheel turning on an axis carried in a frame called a 
block, the grooved edge of the wheel carrying a cord 
or rope. 

204. What is the advantage gained by the use of a fixed pulley ? 
A change of the direction in which the power acts. 

205. What ts the static relation of power to weight in such a 

case f 
They are equal. 

206. Illustrate, 

Hoisting a flag on a mast or staff! If the flag weighs 5 
lbs., the hand has to exert at least a force of 5 pounds, 
plus enough to overcome the friction of the pulley and 
the rigidity of the rope, but this is more convenient 
than climbing to mast-head and pulling up the flag. 

207. What is the effect of a single movable pulley f 

The power will support a weight twice as great, but it 
must move twice as far. 

208. Illustrate the former statement. 

One end of a rope is tied to a support at the top of a 
well ; thence it reaches down into the well and there 
carries a pulley from which a weight is suspended. A 
man at the top of the well holds the other end of the 
rope. The movable pulley divides the rope into two 
parts, and each part of the rope carries an equal share 
of the load. Thus the fixed support and the man each 
carry half the load. 



34 MECHANICS. 

209. Illustrate the second statement. 

If the man wishes to raise the load a foot, he has to i)ull 
up enough rope to shorten each of the two supporting 
parts of the rope a foot ; that is, he must pull up two 
feet of rope. 

210. Does the man lose or gain by such a transaction f 

That depends upon circumstances. If he can lift the 
load directly, without injury to himself, he will save 
time and eflfort by doing so. But if the load is too 
heavy for him thus to lift, he will profit by the use of 
a pulley, pulling through the double distance for the 
sate of getting a result otherwise beyond his powers. 

211. Is the number of parts of cord supporting the weight 

necessarily limited to two ? 
No. By using several movable and fixed pulleys, the 
number of parts of the cord may be increased at pleas- 
ure. 

212. Give the static law of the pulley. 

With a pulley using a continuous cord, the weight will 
equal the power multiplied by the number of the parts 
of the cord carrying the movable block. 

213. Illustrate, 

If the movable pulley divides the cord into 4 parts, a 
power of 100 pounds will support a weight of 400 
pounds. 

214. What agents are here ignored ? 

Friction and the rigidity of the ropes. They both op- 
pose motion. If the weight is to be supported merely, 
the power theoretically necessary ma^' be diminished 
by anything less than the resistance of the friction 
and rigidity. If the weight is to be lifted, the theoret- 
ical power must be increased by something more than 
that same resistance. 

215. What is an inclined plane ? 

It is a firm plane surface inclined so as to make an 
oblique angle with the force to be overcome. 

216. Against what force is it most commonly used? 
The force of gravity. 



MECHANICS. 35 

217. Illustrate. 

To lift a barrel of flour into a wagon, a plank may be 
placed with one end on the ground and the other end 
on the wagon. The barrel may be rolled up the plank 
by a man who is not strong enough to lift it to that 
height 

218. In such a case^ what does the plank represent? 

The hypothenuse of a right angled triangle, one of the 
other sides of which is horizontal and the third dide 
vertical. 

219. If the man pushes in the direction of the hypothenuse^ 

what static law applies ? 

When the power acts parallel to the inclined plane, it 
will support a weight as many times as great as itself, 
as the length of the plane is times as great as the 
vertical height 

220. Illustrate, 

If the plank is 12 feet long, and the wagon is three feet 
high, the barrel of flour (r96 lbs.) maybe supported on 
the plank by a force of 49 lbs., and rolled up the plank 
by any force greater than that. 

221. State the law in briefer form. 

The weight supported on the plane equals the power 
multiplied by the ratio between the height and the 
hypothenuse of the triangle. 

222. Suppose the man pushes horizontally f 

In that case, the weight supported on the plane equals 
the power multiplied by the ratio between the height 
and the base of the triangle. 

223. What is a wedge f 

A moving inclined plane. 

224. What is a screw f 

A cylinder with a spiral groove or ridge called the 
thread, working in a nut with a corresponding thread. 
The cylinder (or the nut) may be turned by a wheel, 
or by a lever representing a radius of a wheel. 



36 MECHANICS, 



225. What is the action of the screw f 

When the cylinder turns in the nut (or the nut upon the 
cylinder), for each revolution, the distance between 
the nut and the head of the screw is increased or 
lessened by the distance between two adjoining turns 
of the thread. 

226. What is the law of the screw f 

The power will support a weight as many times as great 
as itself as the ci' cumference described by the power is 
times as great as the distance between two adjoining 
turns of the thread. 

227. Illustrate, 

If the screw has "4 threads to the inch" (1.^., if the 
thread runs spirally around the cylinder 4 times in an 
inch measured lengthwise the cylinder), and the power 
describes a circular path 50 inches long, the weight will 
be (4X50=) 200 times as great as the power. 

228. What is a compound machine f 

A combination of two or more simple machines. As a 
lever may act upon another lever to form a compound 
lever, so a capstan may act upon a pulley. 

229. How can you find the effect of a compound machine f 
Starting with the given power, find what weight one of 

the component machines will enable it to support. 
Consider this result as a new power acting on the next 
component machine, and so on. The result given by 
the last component machine will be the one desired. 

230. Give another method. 

Find the ratio between power and weight for each of the 
component machines. Multiply the given power by 
the continued product of these ratios. 

231. Illustrate, 

Suppose that a capstan increasing the intensity of the 
power tenfold acts upon a set of pulleys that increase 
it fourfold, and that this pulls a load up an inclined 
plane that increases it threefold. Then, a force of 50 
lbs., acting on the capstan, will support a weight of 50 
lbs. X loX 4X 3=6,000 lbs. 



MECHANICS, 37 



232. What difference would be found between theory and prac- 

tice? 
Friction and other resistances would be very great, so 
that the given 50 lbs. would support much more 
than the computed 6,000 lbs., and move considerably 
less. 

G— The Mechanics of Liquids. 

233. In what condition is matter the most compressible f 
The aeriform. 

234. /;/ what condition is matter the least compressible f 
The liquid. Liquids are practically incompressible. 

235. What liquid may be taken as a type of its class f 
Water. 

236. What is Pascal's law f 

Pressure upon a confined mass of liquid is transmitted 
undiminished in all directions, and acts with the same 
force upon all equal surfaces. 

^37. Illustrate, 

If a vessel filled with water is provided with two mov- 
able pistons of one square inch and of four square 
inches sectional area, respectively, a pressure of 10 lbs. 
upon the smaller piston will exert a pressure of 10 lbs. 
upon each square inch of the larger piston, which 
must, therefore, be loaded with 40 lbs., to keep it in its 
place. 

238. What important application is made of this principle f 
The hydraulic press, one of the most efficient appliances 

for developing high pressures. 

239. Give a rule for ascertaining the pressure exerted by a . 

hydraulic press. 
Multiply the pressure exerted by the smaller piston by 
the ratio between the sectional areas of the two pis- 
tons. 

240. What is the great condition of the equilibrium of liquids? 
Their free surfaces must be horizontal. " Water seeks 

its level." 



38 MECHANICS, 

241. Give a familiar illustration. 

In the water-pipes 'of a city, the water rises in every 
house to the height of the water in the reservoir, or 
stand-pipe. 

242. Give another illustration. 

Every artesian well illustrates the same principle. 

243. What is Archimedes^ principle ? 

A body*s loss of weight when immersed in a fluid equals 
the weight of the fluid which it displaces, i. e., it equals 
the weight of a like volume of the fluid. 

244. Illustrate, 

The true weight of a cubic inch of iron is its weight in a 
vacuum. Its weight in air is less by exactly the 
weight of a cubic inch of air. Its weight in water is 
less by exactly the weight of a cubic inch of water 

245. A body immersed in a liquid will displace how much of 

the liquid ? 
Its own volume. 

246. How may this fact he utilized f 

To find the volume of an irregularly shaped solid, like a 
fragment of stone. Immerse the solid in a vessel full 
of a lic^uid that will not dissolve or act chemically upon 
the solid ; catch and measure the liquid that runs over. 

247. A body floating on a liquid^ displaces how much of the 

liquid? 
Its own weight of the liquid. 

248. What is specific gravity f 

The ratio between the weight of a body and the weight 
of the same volume of some other substance taken as 
a standard. 

249. What is the relation between specific gravity and density ? 
Specific gravity is equivalent to relative density. The 

later scientific writers generally use the word density 
instead of specific gravity. 

250. What are the common standards ? 

Pure water at the temperature of greatest density (4" 
centigrade, or 39.2° Fahrenheit) is the standard for sol- 



MECHANICS, 39 

ids and liq^uids. Air (or hydrogen), under a pressure 
of 760 millimeters of mercury, and at a temperature of 
freezing water (o** centigrade or 32** Pabreuheit) is the 
standard for aeriform bodies. 

251. Give an important caution regarding specific gravity ^ or 

density . 

All specific gravities are abstract numbers. This is im- 
plied in the definition, for a ratio is an abstract num- 
ber. The fact is sometimes forgotten, and trouble fol- 
lows. 

252. How is the density of a sotid or liquid found f 

By dividing the weight of the given body by the weight 
of the same volume of water. 

253. Name the three elements of every such problem. 
Dividend, divisor and quotient Any two being given, 

the third may be found. 

254. How is the divisor found ? 

Sometimes in one way and sometimes in another. 

255. State the most common way. 

Weigh the solid in air. (This gives the dividend. It 
differs so little from the vacuum weight as to make 
little practical difference). Suspend the solid by a hair 
or thread, and weigh it while immersed in water. The 
diflference between the two weights is the " loss of 
weight in water," and is, therefore, the weight of an 
equal volume of water — the divisor sought 

256. Give an example. 

A body weighs, in air (dividend), ' 50 ounces. 

" " " •* water 45 

Weight of equal volume of water (divisor) 5 " 
Density, (50 oz. -f-5 oz. ==) 10. 

257. Suppose that a body having a density of 10 weighs 30 

ounces in air. How much will it weigh in water f 
The density, 10, was obtained as a quotient, with 
50 oz. as the corresponding dividend. Hence, the di- 
visor must have been 5 oz. This 5 oz. was the weight 
of an e(;^ual weight of water, and, therefore, the loss of 
weight m water. If the body weighs 50 02>. in air and 
loses 5 oz. in water, it must weigh (50 oz. — 5 oz. =) 45 
oz. in water. 



MECHANICS. 



258. Give an easy method for finding the density of liquids. 
WciRh an empty bottle. Fill it with the given liquid. 
The difference lietween the two M-eij^hts is your divi- 
dend. Empty and rinse t'le bottle. Fill it with water 
and weigh it again. Thi-* we^jfht. minns the weight of 
the empty bottle, is your divisor. Having dividend 
and divisor, you know how tsi get the quuLient, which 
is the density desired. 
A'<9/(^.— Concerning hydrometers, and the Jolly balance, in- 
struments much used iu determining the densities of bodies, 
sec Avery's " School Physics," pages 168 and 178. 

259* U ^ ^ole is bored in the side of a vertical dam, what is 
the distance from the center of the hole to the surface oj 
the water on the upper side of the dam called? 
The head of water. 

260. How could you compute the velocity per second with which 
the water would pass through such a holef 
It would be the same as the velocity of a body that had 
freely fallen through a distance equal to the head of 
the water. 

361. How could you compute the quantity of water that would 
be discharged by such a hole in a given time f 
Multiply the area of the orifice (e. g., in square inches) 
\iy the velocity per second (in inches). The product 
will represent the number of cubic inches discharged 
per second. Multiply this by the number of seconds. 

262. What caution might you give in a case like this f 

To use only comparable units. It would not do to 
measure the area of the orifice in inches and the ve- 
locity in feet, or yards, or meters. 

263. What other caution ? 

Not to forget that the friction of the sides of the hole 
will reduce the theoretical discharge somewhat. 

264. What other caution f 

That we have assumed that the head remains constant, 
while, in most cases, it would grow less. 

265. In such a case^ how would you correct your result? 
Measure the head at the beginning of the trial, and again 

at the end. Use the average of these two heads in- 
stead of the initial head. 



MECHANICS, 41 



266. Ho7v would you find the average head ? 

Divide by two the sum of the initial and the final heads. 

267. What is the use of water-wheels f 

To apply the weight and motion of water to the opera- 
tion of machinery. 

268. Name the leading classes of water-wheels. 

The overshot, the undershot, the breast, and the turbine. 

269. What is the action of the overshot wheel? 

The water is caught in buckets at the top of the wheel, 
which is turned upon its horizontal axis by both the 
force of the current and the weight of the water. The 
buckets are emptied as they near the bottom of the 
wheel. 

170. What is the actiot^ of the undershot wheel? 

The water strikes the paddles (carried by the rim of the 
wheel) when they are at or near their lowest position, 
and thus turns the wheel by the force of the current. 
This wheel is less efficient than the overshot wheel. 

V'. What is the action of the breast wheel? 

The construction of the wheel is much like that of the 
undershot wheel, but it is placed so as to receive the 
water at or a little below the level of the axis. It 
therefore derives its working power from the force of 
the current and the weight of the water. It is more 
efficient than the undershot, and less efficient than the 
overshot wheel. 

»72. What is the action of the turbine wheel ? 

It turns about a vertical axis at the bottom of a penstock, 
and under the pressure of a considerable head of 
water. Fixed guides direct the water from all sides, so 
that it strikes the buckets at the angle of greatest 
efficiency. It is the most effective variety of water 
wheel yet constructed. 

H— The Mechanics of Gases. 

^TS. Of what does pneumatics treat ? 

Of aeriform bodies and their mechanical properties and 
uses. 



A2 MECHANICS, 

274. What airi/ornt substance may be taken as a type f 
Air. 

275. To what fnay the air which constitutes the earth's atmos- 

phere be compared? 
To an ocean, at the bottom of which men live, as fishes 
live in the waters of the sea. 

276. Ha^ air weight f 

It has. It is a kind of matter, and all matter has weight 

277. How does it compare with water in this respect f 
Water is 770 times as heavy. 

278. What is atmospheric pressure ? 

The pressure which the air, by virtue of its weight, 
exerts upon bodies at or near the surface of the earth, 
/. ^., at the bottom of the aerial ocean. 

279. What is its measure ? 

About 15 lbs. to the square inch, or one kilogram to the 
square centimeter. This is at the sea level. It grows 
less as we reach higher elevations. 

280. What is a mercury barometer ? 

A tube a little more than 30 inches long, closed at the 
upper end, filled with mercury, and terminating below 
in a cistern of mercury. 

281. What supports the mercury in the tube f 
Atmospheric pressure. 

282. What is in the tube above the mercury f 

Nothing, unless it is a minute quantity of the vapor of 
mercury. The barometric vacuum is considered nearly 
perfect 

283. What is the use of the barometer? 
To measure atmospheric pressures. 

284. Nothing else ? 

Nothing else. All other conclusions based upon baro. 
metric readings are inferencesfrom what the barometer 
tells about the atmospheric pressure. 

285. What is the kinetic theory of gases ? 



MECHANICS. 43 



Th 



?as consists of free molecules in constant motion. 

These motions are in straight lines, and with uniform 
velocities, until the molecules strike other molecules, 
or a restraining body. These molecules strike upon 
the^ walls of the containing vessel so rapidly that 
their effect is that of a continuous pressure. 

286. What is this continuous pressure 0/ C07tfined gas catted f 
Elastic force. 

287. When a bottte is tightty corked, what conception may we 

form? 

That the glass walls are vigorously bombarded by atmos- 
pheric molecules from within and without. The total 
effect of these innumerable impacts on the inner side 
of the wall equals the total effect of the action on the 
other side. 

288. How is this expressed in technicat phrase f 

The elastic force of aeriform bodies supports the press- 
ure exerted upon them, and is equal to it. 

289. How may this be illustrated ? 

By confining a pint of air in an india-rubber quart bag. 
The walls hang flabby. Elastic force and atmospheric 
pressure are equal. Place the bag under the receiver 
of an air-pump and exhaust some of the air, thus re- 
ducing tlie pressure on the outside of the bag. The 
impacts of the confined molecules not being opposed 
by an equal force from without, beat back the walls of 
the bag; the confined gas increases its bulk and re- 
duces its elastic force until an equality is restored. 

290. Give a further illustration of the elastic force of gases. 
By continuing the action of the air-pump, and thus re 

moving the opposing pressure upon the bag, the, 
elastic force of the confined air may burst the bag, the 
imprisoned molecules striking such blows that when 
they are not counterbalanced by external pressure, 
they actually batter down the wall that restrains their 
motions. 

291. Is anything known as to the velocities of these molecular 

motions f 
It is said that hydrogen molecules have a velocity of 
more than a mile a second. The molecules of other 
gases have smaller velocities. 



44 MECHANICS. 



292. Does this mean that a hydrogen molecule actually moves a 

mile in a second ? 

By no means, for there will be innumerable collisions 
within the second ; it simply moves at that rate be- 
tween collisions. 

293. State another result of these molecular motions. 

The phenomenon known as the diffusion of gases, by 
which two gases placed side by side mutually penetrate 
each other and become thoroughly mixed. 

294. What is Boyle's {or Mariotte's) lawf 

At a given temperature, the volume of a given quantity 
of gas is inversely proportional to the pressure upon it. 

295. What is an air-pump ? 

An instrument for removing air from a vessel. 

296. What is a condenser ? 

An instrument for compressing a large amount of gas 
into a vessel. 

297. How does it differ from an air pump ? 

Its valves open toward the vessel, while the valves of the 
air-pump open from the vessel (receiver). 

298. In the use of an ordinary lift-pump^ what causes the as- 

cent of the water f 
When the action of the pump removes the atmospheric 
pressure from the water in the pipe, the atmospheric 
pressure on the surface of the water outside the pipe 
pushes the water up the pipe. 

299. To what height can water be thus raised? 
About 34 feet. 

300. Why is the height so limited? 

Because the weight of a vertical column of water of that 
height equals the atmospheric pressure. 

301. What is a siphon ? 

A bent tube, open at the ends, one of the arms being 
longer than the other. 

302. For what is it used? 

To transfer liquids from a higher to a lower le^*"* 



MECHANICS. 45 



303. How is it used? 

Fill the tube with the liquid to be transferred. Place 
the end of the short arm in the liquid, and brinfr the 
end of th.; long arm below the level of the liquid. 
Water may be thus drawn from a barrel over its side 
without disturbing any sediment at its bottom. 

304. How long will the liquid run through the siphon f 
Until the level of the liquid falls below the end of the 

short arm, or to the level of the liquid into which the 
long arm dips. 

305. Over how high a ridge may water be thus carried ? 
Thirty-four feet, more or less, according to the atmos- 
pheric pressure at the given time and place. 

306. Over how high a ridge may mercury he thus carried? 
One as high as the mercury column of a barometer 

(which measures the atmospheric pressure) at the 
given time and place. 

307. What natural phenomenon is due to such action ? 

The flow of intermittent springs, in which the channel 
by which water escapes from a subterranean reservoir 
forms a siphon. 

308. When will such a spring begin to flow ? 

When the level of the water in the reservoir reaches the 
highest point of the outlet siphon. 

309. When will such a spring cease to flow ? 

When the level of the water in the reservoir falls to the 
end of the short arm of the outlet siphon. 



46 ACOUSTICS. 



CHAPTER IIL 
ACOUSTICS- 



1. What is acoustics f 

The branch of physics that treats of sound. 

2. What is sound f 

The mode of motion that may affect the auditory nerve, 
and thus awaken the sensation of hearing. 

3. What is the nature of this motion f 

Undulatory rather than progressive. The only thing that 
has an onward movement is the pulse or wave, which is 
a change in the relative positions of the particles of the 
undulating substance. The wave may travel to a great 
distance; the journey of the individual particle is very 
limited. 

4. What is a vibration ? 

When a body moves so that it periodically returns to its 
initial position, its motion between two successive pas- 
sages in the same direction through any position is a 
vibration. 

5. How does a vibration differ from an oscillation ? 

A vibration corresponds to a double or ** complete " oscil- 
lation, and its reciprocating motion is more rapid. 

6. What is a wave-period ? 

The time required for a vibration of an individual par- 
ticle of the substance in which the wave exists. 

7. What is a wave length ? 

The distance from any vibrating particle to the next par- 
ticle that is in the same relative position or " phase." 

8. What is the amplitude of a wave ? 

The length of the path of a vibrating particle. 



ACOUSTICS, 47 

9. What is the relation between wave-period and zvave length f 
The wave will advance one length during one period. 

la When these are knozvn, how may the velocity of the wave be 
computed ? 
By multiplying the wave-length by the number of vibra- 
tions made in a second. 

11. What is the cause of sound ? 
Vibrations of some body of matter. 

12. What is the ordinary medium through which sound is 

propagated? 
Air. 

13. Of what does a sound wave consist t 
Of a condensation and a rarefaction. 

14. What is the motion of an air particle in the propagation of 

sound? 
Backward and forward, in the line of propagation. Sound 
waves are longitudinal and not transverse, as are water 
waves in which the particles move up and down across 
the line of propagation. 

15. What is necessary for the propagation of sound? 

Some elastic medium. Sound is motion, and motion 
must be of some form of matter. 

16. upon what does the velocity oj sound depend? 

Upon the elasticity and the density of the medium in 
which it is propagated, varying directly as the square 
root of the elasticity, and inversely as the square root 
of the density. 

17. What is the velocity of sound in air ? 

About 1,090 feet per second at the freezing temperature. 

18. What is the effect of temperature upon the velocity ? 

A rise of temperature increases the elasticity and de- 
creases the density of the atmosphere. There is, there- 
fore, for each added Fahrenheit degree, an added veloc- 
ity of about 1. 12 feet, or about 2 feet for each centigrade 
degree. 

19. What is a phonograph ? 



48 ACOUSTICS. 



An instrument in which sound waves fall upon a disc|)ro- 
vided with a needle or tooth at its back. The vibrating 
disc causes the tooth to make a series of indentations 
in the tin-foil or wax surface of a cylinder moving with 
a rotary and slow longitudinal motion. These indenta- 
tions constitute the record of the words spoken. When 
the cylinder is replaced so that the tooth is at the 
beginning of the record, and slowly moved as origin- 
ally, the tooth drops into the successive indentations 
which it previously made, and thus sets up vibration-sin 
the disc similar to those originally produced by the 
sonorous waves. These duplicated vibrations of the disc 
set up sonorous waves comparable with those originally 
produced by the vocal apparatus of the speaker whose 
words were thus recorded. Recent improvements in 
the phonograph have greatly improved its utility. 

20. What is meant by the reflection of sound ? 

When a sound wave strikes an obstacle it is turned back 
in the same medium in accordance with the law of re- 
flected motion, i. e.^ so that the angle of incidence 
equals the angle of reflection. This process is called 
reflection of sound. 

21. What is meant by the refraction of sound? 

The bending of the lines of propagation by passing them 
obliq^uely from one medium to another of diflFerent 
density. 

22. What is a focus of sound? 

A point at which a number of sonorous rays are made to 
converge by refraction or by reflection. 

23. Illustrate the meaning of conjugate reflectors of sound. 
Two curved reflectors (preferably parabolic) may be set 

facing each other at a considerable distance apart, so 
that the ticking of a watch placed at the focus of one 
reflector may be distinctly heard at the focus of the 
other reflector. Such reflectors and foci are said to be 
conjugate to each other. 

24. Why is the sound heard with the aid of the reflectors when, 

at the same distance ^ it would be inaudible without them f 

The first reflector being near the watch, collects a large 

number of diverging lines of sound, and reflects them 

parallel to the second reflector, which converges them 



ACOUSTICS, 49 



to the conjugate focus. When the ear is placed at this 
conjugate focus, it receives the energy of all the lines 
that fell upon the first reflector, which energy is much 
greater than that of the fewer diverging lines that could 
come direct from the watch to the ear at the same r**" 
tance. 

25. IVhai phenomena do such reflectors explain f 

The phenomena of whispering galleries, like that in the 
dome of the United States Capitol. 

26. What is an echo f 

A sound that is audible after one or more reflections, as 
by a wall or cliff" at a distance. 

27. How do sounds differ ? 

In loudness, pitch and quality. 

28. Upon what does the loudness of a sound primarily depend? 
Upon amplitude of vibration. y 

29. How does distance affect the intensity of sound? / 
The intensity varies inversely with the square of the di^ 

tance from the sounding body. 

30. Why is this? 

The sonorous wave advances as a series of spherical shells. 
When the energy of a wave is concentrated in a small 
shell, it will be more intense at any point in that shell 
than when the same energy is diffused over a greater 
shell. The distance from the sounding body is the 
radius of the shell, and surfaces of spheres are propor- 
tional to the squares of their radii. 

31. What are acoustic tubes? 

Tubes for confining the propagation of sound waves to a 
single direction. As there is no spherical diffusion of 
the energy of the sonorous wave, the sound may be thus 
transmitted to great distances. 

32. How does the size of the vibrating body affect the intensity 

of the sound? 
If the sonorous body has a large surface, its vibrations 
produce well-defined condensations and rarefactions, 
and the sound is correspondingly intense; if it is small 
or thin, the air easily flows around it instead of being 
set in vibration by it, and the sound is correspondingly 
feeble. 



50 ACOUSTICS. 

33. upon what does the pitch of a sound depend f 

Upon the rate of vibration of the sounding body; the 
more rapid the vibration, the higher the pitch. 

34. How may the wave-length of a musical sound be computed? 
By dividing the velocity of the sound by the number of 

vibrations made {i. e.^ by the number of waves sent out) 
in a second. 

35. What is an interval? 

The difference in pitch between two tones. 

36. How is it described? 

By the ratio between the vibration-numbers of tb^ two 
tones. 

37. What is an octave? 

An interval that is represented by the ratio, 2 : i. 

38. What is a musical scale ? 

A standard series of tones lying within the interval of an 
octave. 

39. What is the gamut ? 

A musical scale consisting of 8 familiar tones, commonly 
called, do. re^ mi, fa, soL la, si, do; or, C, £>, E, j^, G, A, 
B.C. 

40. What are the relative vibration-numbers of these t^tnes f 
24. 27, 30, 32, 36, 40, 45, 48. 

41. If the tone produced by 264 vibrations per second is called 

DO, how may the vibration numbers of the k (ucessive 
eight tones of the scale constituting the octavt tibove be 
computed? 
By multipl)dng each of the above numbers by i? 

42. By what other name is the gamut known ? 
The major diatonic scale. 

43. What is the chromatic scale ? 

A series of 13 tones separated by 12 equal inte* /►Js. 

V 44. What is the fundamental tone of a body? 

The lowest tone that it can produce, or the t>;ie that it 
emits when vibrating as a whole 



ACOUSTICS, 51 

45. What are the tones produced by the vibrating segments of 

a sonorous body called? 
Overtones, partial tones, or harmonics. 

46. How may a string be made to vibrate in segments f 

By lightly touching it at exactly a half, a third, or a fourth 
of its length from one end, while it is vibrating. 

47. If touched at the middle^ how will the string divide? 
Into two vibrating segments separated hy a point of no 

vibration called a node. The overtone will be an octave 
above the fundamental. 

48. If touched at a third its length from one end? 

Into three segments with two nodes. And so on for 
smaller segments. 

49. How may these nodes be shown to exist ? 

By placing little paper riders at various points on the 
string. When the string is bowed so as to produce 
seginents, the riders at the nodes will remain in place, 
while the others will be thrown oflF. 

50. Can a string be made to vibrate as a whole^ without vibrat- 

ing in segments at the same time ? 
No ; the fundamental is always accompanied by harmonics. 

51. How may the existence of harmonics in the tones of a 

piano be demonstrated ? 
Hold down the key of middle C, thus liftin|jf the damper 
from the corresponding string and leaving it free to 
vibrate. Now strongly strike the key just an octave 
below, holding that key also down for a few seconds. 
Remove the finger from the lower key, thus letting the 
damper fall upon it, bringing it to rest. When its 
sound has died away, you will hear the sound of middle 
C, which is still free to vibrate. These vibrations of 
higher pitch are sympathetic vibrations caused by the 
vibrations of the two segments of the string originally 
struck. 

52. How may two sound waves blend? 

The condensation of one may coincide with the condensa- 
tion of the other, or condensation may coincide with 
rarefaction, or the* waves may meet in any intermediate 
phase. 



52 ACOUSTICS. 

53. To what may the combination of fundamentals and har- 

monics be compared ? 
To ripples on waves of water. The waves may be of the 
same size, but their forms may be varied by varying the 
ripples. 

54. Of what is the waveform^ so considered, the cause? 

Of the quality or timbre f f the sound, the characteristic 
by which we distinguish, for example, the tone of a 
violin from the tone of a flute, when the pitch and loud- 
ness are the same. In these cases, the fundamentals are 
alike, but the harmonics are dififerent. Hence the total 
effects of the two combinations, i, e., the wave-forms, 
are different. By such differences, we distinguish one 
instrument from another and recognize the voices of 
friends. 

55. What instrument is nearly free from harmonics f 

A tuning-fork. Its tones are almost simple tones, while 
those of a piano are decidedly compound. 

56. Is there any way in which a deaf person may study the 

differences in tones? 
The graphic method and the optical method (See Avery's 
*' School Physics," pages 238 and 241) enable such study. 

57. What effect may be noticed after prolonging a vocal tone 

near an open piano ? 
One of the wires will reproduce the same tone. 

58. What if the pitch of the vocal tone is changed? 
Another wire will reproduce the tone. 

59. In either case^ what about the other strings of the piano f 
They remain mute. 

60. What is the teaching of these experiments ? 

That string is excited to audible action which is able to 
vibrate synchronously with the sonorous waves that set 
it in motion. In other words, each string absorbs ki- 
netic energy only from those waves, the like of which, in 
the matter of wave-length, it is capable of reproducing. 

61.. May one tuning-fork thus set in audible vibration another 
tuning fork by the energy of waves sent through the air? 
Yes, if the forks are tuned to unison, so that they will 
vibrate at the same rate. 



ACOUSTICS. 53 

62. May a like result he produced by one musical string upon 

another? 

Yes, under the same conditions. 

63. What are vibrations thus produced called? 
Sympathetic vibrations. 

64. What is meant by reinforcement of sound? 

An increased loudness of sound due to the blending of 
two or more series of similar sonorous waves in like 
phases, i. e.^ condensation with condensation and rare- 
faction with rarefaction. 

65. What is resonance f 

A variety of reinforcement of sound due to sympathetic 
vibrations, as, for example, the resonance of sounding 
boards. 

66. May the vibrating substance be aeriform as well as solid? 
Yes, as in the organ-pipe and in Helmholtz's resonators. 

67. What are sounding boards ? 

Thin pieces of wood over which are supported musical 
strings. The bridges of the violin, guitar, piano, and 
similar stringed instruments carry the tremors caused 
by the vibration of their strings to their sounding 
boards, and thus set the latter in vibration at the same 
rate as the strings. The sound of these instruments is 
due more to the vibration of these resonant surfaces 
than to the vibrations of the strings themselves. 

68. Upon what does the excellence of a violin^ for example^ 

depend? 
Upon the sonorous character of the wood. Were it other- 
wise, the violinist would pay more attention than he 
does to the selection of his " catgut," and not so often 
dream of owning a real Cremona or Stradivari. 

69. What is interference of sound ? 

The coincidence of two series of sonorous waves in oppo- 
site phases; i. <?., condensation with rarefaction. 

70. What is the effect of thus combining two series of waves of 

equal strength and wave-length ? 
Silence. 



54 ACOUSTICS, 

71. What is the effect of thus comhining two series of waves oj 

unequal length ? 
Beats. 

72. What are beats? 

A peculiar palpitating eflFect caused by a succession of 
sonorous reinforcements and interferences. 

73. How may such beats be easily produced f 

By striking simultaneously, in a quiet room, one of tbe 
white keys at the lower end of the key-board of a piano 
and the adjoining black key. 

74. How many beats will there be per second f 

As many as the vibration number of one of the piano 
strings exceeds that of the other. 

75. How are the laws of musical tones most conveniently 

studied f 
By means of musical strings like those of the violin, 
guitar, piano, and especially the sonometer. 

76. What varieties of such vibrations are there? 
Transverse, torsional, and longitudinal vibrations. 

77. Which of these are the most important? 
Transverse vibrations. 

78. What is the first law of musical strings ? 

Other conditions being the same, the vibration-numbers 
vary inversely as the length of the string. 

79. Illustrate this law. 

If the length of a given string is halved, the string will 
vibrate twice as fast; if its length is doubled, it will 
vibrate only half as rapidly. 

80. What is the second law of musical strings ? 
Other conditions being the same, the vibration-numbers 

vary directly as the square roots of the stretching 
weights or tensions. 

81. Illustrate this law. 

If the tension of a given string is increased fourfold, the 
string will vibrate twice as rapidly. 



\' 



ACOUSTICS, 55 

82. What is the third law of mimical strings f / 
Other conditions being the same, the vibration-numbers V 

vary inversely as the square roots of the weights per 
linear unit. 

83. Illustrate this law. 

If a string is made of the same length and stretched with 
the same tension as a given string, but is only one-fourth 
as heavy, it will vibrate tmce as rapidly. 

84. How are musical instruments classified ? 

As stringed or as wind instruments, as their tones are due 
to the vibrations of solids or of confined columns of air. 

85. Name some wind instruments. 
The fife, flute, cornet, and organ. 

86. Give the laws for the vibrations of air columns, 

{a) The vibration-numbers vary inversely as the lengths / 
of the columns. Kb) The pitch of a closed-pipe is an 
octave below that of au open-pipe of the same length. 

87. What other vibrations may produce musical tones? 
Those of rods, plates, and bells. The study of these is 

very interesting. (See Avery's " School Physics," pages 
264 and 265.) 



56 HEAT, 



CHAPtFR rv 
HEAT, 



1. What « heat f 

A form of molecular motion that produces the sensation 
of warmth. 

2. What is temperature ? 

The state of a body considered with reference to its ability 
to communicate heat to other bodies. 

3. What is a thermometer ? 

An instrument for measuring temperatures. 

4. What are the most common kinds of thermometers? 

The mercury thermometers known as Fahrenheit's ther- 
mometer, and as the centigrade thermometer. 

5. How do they differ ? 

In the centigrade thermometer, the freezing temperature 
of water is marked zero, and the boiling point of water 
100 decrees. In Fahrenheit's thermometer, the freezing 
point is marked 32 degrees, and the boiling point 212 
degrees. Hence, an interval of 100 centigrade degrees 
equals an interval of 180 Fahrenheit degrees. 

6. Ho2v may the reading of a Fahrenheit thermometer be 

changed to the equivalent centigrade reading ? 
Subtract 32 from the number of centigrade degrees, and 
multiply the remainder by \, C=f (F — 32). 

■7. Hoiv may centigrade readings he changed to equivalent 
Fahrenheit readings ? 
Multiply the number of centigrade degrees by \ and add 
32. 7^=|C-f32. 

8. How may heat be produced f 

The sun is our great source of heat, but any other form of 
energy may be converted into heat. 



HEAT, 57 

9. How may heat be transferred? 
By conduction, and by convection. 

10. What is conduction ? 

The method of diffusing heat in which tlie energy of vibra- 
tion is passed from molecule to molecule. The heat is 
said to flow from the hotter to the cooler parts of the 
body. 

1 1. How do substances differ in regard to thermal conductivity ? 
Good electric conductors are good heat conductors. 

Liquids and gases are poor conductors of heat. Alcohol 
may burn on the surface of water without aflFecting a 
good thermometer an inch below. 

12. What is convection ? 

The method of diffusing heat by the motion of heated 
fluid masses. The heated fluids expand, and gravity 
pulls denser masses under them, thus forcing them into 
motion. The draft of stoves, and lamps, and the trade 
winds depend upon convection. 

13. Is there not a third way of transferring heat? 

It is sometimes said that heat is transferred by radiation, 
but the thing thus transferred is not heat at all. What 
is thus erron<iously called "radiant heat" is radiant 
ener^^y, and will be considered in the next chapter. 

14. What is the first effect of heat upon bodies? 
Expansion. The heat increases the vibrations of the 

molecules and thus forces them further apart. 

15. Name one marked exception to this rule. 

Water at the freezing temperature (0° C.) contracts as it is 
heated until it reaches the temperature of 4° C. If 
heated above this point, it expands. This is, therefore, 
the temperature of the maximum density of water. 
Hence, when a pond or river freezes, the water below 
the ice is warmer than the ice. 

16. What is coefficient of linear expansion ? 

The elongation per linear unit for each degree that the 
temperature is raised above the freezing point. 

17. What is the coefficient of cubical expansion ? 
The increase in volume per unit of volume. 



58 HEAT. 

1 8. At what rate do solids expand? 
At different rates for different substances. 

19. At what rate do gases expand? 

About Y^3 of their volume for every centig^rade degree or 
^Jij for every Fahrenheit degree, alwve the freezing 
point of water. In other words, for gases under con- 
stant pressure, the coefficient of expansion is ^fj or 
0.00366 (for a centigrade degree). 

20. What is the absolute zero of temperature? 

The point at which the molecular motions that consti- 
tute heat wholly cease. It has never been reached, but 
it lies about 273° below the centigrade zero. 

21. Hoiv will the volume of a given quantity of gas ^ under con- 

stant pressure^ be affected by heating tt from 0° C to 
273** c? 
It will be doubled. 

22. How may the gas at this temperature be restored to Us 

original volume ? 

By doubling the pressure upon it. 

23. What are the laws of fusion ? 

Under a constant pressure, every solid begins to melt at a 
certain temperature which is invariable for the ^iven 
substance. When cooling, the substance will solidify at 
the temperature of fusion. 

The temperature of the solid, or liquid, remains at the 
melting point from the moment that fusion or solidifica- 
tion begins until it is complete. 

24. What is evaporation ? 

The quiet formation of vapor at the free surface of a liquid. 

25. What is boiling? 

The rapid formation of vapor bubbles in the mass of a 
liquid. 

26. What are the laws of boiling ? 

Under a constant pressure, every liquid begins to boil at a 
certain temperature which is invariable for the given 
substance. When cooling, the substance will liquefy at 
the boiling point. 



HEAT. 59 

The temperature of the liquid, or vapor, remains at the 
boiling point from the moment that it begins to boil or 
liquefy. 

An increase of pressure raises the boiling point; a de- 
crease of pressure lowers the boiling point. 

27. What work is done by heat energy when water is boiled in 

the open air? 
Cohesion is overcome, as also are liquid pressure, and 
atmospheric pressure. 

28. How may the amount of heat required for boiling be re- 

duced f 
By reducing the amount of work to be done by the heat; 
by reducing the atmospheric pressure. Thus, water 
boils at a temperature below 212° F., on a mountain top 
where this pressure is notably less than at the sea level. 
The same principle is utilized in the vacuum pans of 
sugar refineries. 

29. How m>ay the amount of heat required for boiling be in- 

creased? 
By increasing the amount of work to be done by the heat. 
Water confined under a pressure of more than one 
atmosphere will not boil until the temperature is above 
212° F. When the gauge of a steam boiler shows a 
pressure of 30 pounds to the square inch, the tempera- 
ture of the water and steam in the boiler will be about 
121° C, or 250° F. This principle is utilized in Papin*s 
digester. 

30. State three facts concerning steam. 

Steam has the teqjperature of the water with which it is 
confined, is invisible and, under the ordinary atmos- 
pheric pressure, occupies nearly 1.700 times as much 
space as the same quantity of water in liquid form. 

31. What is the dew point ? 

The temperature at which the watery vapor in the atmos- 
phere IS condensed. 

32. What is distillation ? 

A process of separating substances mingled in liquid form 
by utilizing the diflFerences in their boiling points. Thus, 
pure water may be separated from brine by heating the 
brine to 100° C. The water will be vaporized at this 



6o HEAT. 

temperature and, as steam, may be led to a cool receiver 
and there condensed as fresh water. The salt of the 
brine will not be vaporized at that temperature and will, 
therefore, remain behind. In similar manner, a dilution 
of alcohol may be separated into water and alcohol by 
heating the mixture to about 90° C At this tempera- 
ture, the alcohol will be vaporized and may be led off for 
condensation, while the water remains in the retort. 

33. Whatiscalorimetryf 

The measurement of the amount of heat that a body ab- 
sorbs or emits. 

34. What is a heat unit? 

The amount of heat required to warm a unit weight of 
water one degree above the freezing point. 

35. What is a calory f 

The amount of heat required to raise the temperature of a 
gram of water one centigrade degree. 

36. What other name has this water-gram-degree unit? 
A therm. 

37. What is a large calory ? 

The amount of heat required similarly to warm a kilogram 
of water. 

38. What is the latent heat of a body ? 

The amount of heat that it requires for liquefaction or for 
vaporization without increase of temperature. It is 
called latent because it is hidden from thermometric 
measurement. The heat tJiat disap|)ears is consumed in 
the work of overpowering cohesion in the case of fusion 
or of solution, and in overcoming cohesion and pressure 
in the case of vaporization. Wfien the vapor liquefies, 
or when the liquid solidifies, the latent heat reappears 
as ** sensible heat " and produces a rise of temperature. 
Thus, tubs of water freezing in a vegetable cellar may 
give out enough sensible heat to protect the vegetables 
from freezing. This principle is often utilized in this 
way. 

39. Explain the action of a common freezing mixture. 
When salt and ice are mixed, both dissolve. For thi* 

double solution, heat is abstracted from the cream in the 
freezer and becomes latent in the liquid brine. Hence, 
the temperature falls and the cream is frozen. 



HEAT, 



40. Explain the action of a refrigerating machine. 

Some gas (like ammonia) is liquefied by high pressure, 
large quantities of latent heat being transformed into 
sensible heat in the process. When the liquefied gas 
has cooled, it is allowed to expand to the gaseous form 
in tubes that run through vats of water. This vaporiza- 
tion abstracts large q^uantities of heat from the water 
(changes it from sensible to latent heat, from kinetic to 
potential energy), and thus reduces the temperature of 
the water, often to the freezing point. In this latter 
case, the refrigerating machine becomes an ice machine. 

41. What is the latent heat of water? 

The latent heat of one gram of water is 80 calories. This 
means that the amount of heat required to melt a 
quantity of ice without changing its temperature is 
eighty times as great as the heat required to warm the 
same quantity of water one centigrade degree. The 
latent heat of one gram of steam is 537 calories. This 
means that the amount of heat required to evaporate a 
quantity of water without changing its temperature is 
537 times as great as the heat required to warm the 
same quantity of water one centigrade degree. 

42. What is the specific heat of a body? 

The ratio between the quantity of heat required to warm 
that bodv one degree and that required to warm an 
equal weight of water one degree. 

43» What substances have the highest specific heats ? 

Hydrogen is the highest, about 3.4. Water comes next; as 
it is the standard, its specific heat is i. For^his reason, 
the ocean and lakes are cooled or warmed more slowly 
than the land. It requires more heat to raise the tenv 
perature of the water than it does to raise the tempera- 
ture of the solid constituents of the earth. 

44. What is the thermal capacity of a body ? 

1 he number of calories required to raise its temperature 
one degree. 

45. How is it related to specific heat? 

It is the product of the specific heat into the mass of the 
body measured in grams. 



62 HEAT, 

46. What is thermodynamics 9 

The branch of ph^^sics that considers the equivalent of 
heat and mechanical energy. 

47. Whatis Joule's principle f 

The disappearance of a definite amount of mechanical 
energy is accompanied by the production of an equiva-. 
lent amount of heat. 

48. What is the mechanical equivalent of heat? 

The numerical relation between work-units and the equiv- 
alent heat- units. 

49. What is the value of a water-pound-Fahrenheit heat-unit f 
778 foot-pounds. 

5a Of a Tvater- pound-centigrade heat-unit f 
1,400 foot-pounds. 

51. Of a calory ? 

427 gram- meters, or 4.2 X 10^ ergs. 

52. Whence the heat of combustion? 

When coal is burned, the carbon and oxygen particles rush 
together with tremendous violence, energy of position 
being converted Into energy of motion. The molecular 
motions produced by this clashing of particles consti- 
tute heat and have a mechanical as well as thermal value. 
The heat thus derived from the combustion of a pound 
of coal would raise the temperature of 8,080 pounds of 
water one centigrade degree, or about 80 pounds of 
water 100 centigrade degrees. Unfortunately, not all of 
the heat can be utilized for the purpose. Similarly, the 
combustion of one pound of hydrogen would warm 
34462 pounds of water one centigrade degree. In other 
words, the combustion of a gram of hydrogen would 
yield 34 1462 calories. 

53. What is a steam, engine ? 

A machine for transforming the energy of the invisible 
motions of molecules of steam into ordinary forms of 
mechanical energy. 



RADIANT ENERGY. 63 



CHAPTER V. 
RADIANT ENERGY. 



1. What is the luminiferous ether f 

Aa exceedingly attenuated form of matter occupying all 
space, interplanetary as well as intermolecular. It 
carries off the encrgjr of the invisible motions of mole- 
cules, much as the air carries off the energy of a vibrat- 
ing guitar string. " It makes the universe a whole and 
renders possible the communication of light and 
energy between star and star." It passes between the 
particles of ordinary matter as freely as the air passes 
between the trees of the forest. 

2. What is radiant energy? 

Energy that is transferred by periodic disturbances in the 
ether. 

3. What is a ray f 

A line along which energy is propagated. 

4. What is a beam ? 

A collection of parallel rays. 

5. What is a pencil ? 

A collection of converging or diverging rays. 

6. What is the smallest vibration-number of radiant energy 

yet observed f 
About 10 X 10**. 

7. The greatest? 
About 1,622 X 10**. 

8. What is the total range of radiant energy thus measured t 
More than 7 octaves. 

9. How much of this constitutes light? 
Little more than one octave. 



64 RADIANT ENERGY, 

\ lo. Whatis light? 

Radiant energy that is capable of affecting the optic nerve. 

11. How is light propagated ? 

In straight lines, as long as it travels in a homogeneous 
medium. 

12. How do we see bodies ? 

By the light that they radiate or irregularly reflect to our 
eyes. The rays from a visible object cross at the pupil 
of the eye and form upon the retina of each eye an in- 
verted image of the object By some obscure psycholog- 
ical-physiological process this image is converted into 
consciousness. Rays of light that do not enter the eye 
are invisible. It seems as though the energy of the 
waves must be expended upon the retina to excite 
vision. Every point of a directly visible body sends a 
cone of rays to each eye, of which cone the pupil of the 
eye is the base. A point in a visible body always ap- 
pears to be at the apex of such a cone ; i. e., the point 
will appear to be in the direction from which the rays 
enter the eye. 

\ 13. What is a transparent body ? 

One that transmits light so freely that objects may be dis- 
tinctly seen through it. 
\ 
^ 14, What is a translucent body ? 

One that transmits light so imperfectly that objects may 
be seen through it only imperfectly. 

v\ 15. What is an opaque body ? 

One that does not transmit light. 

>^ 16. What is a shadow f 

The darkened space from which an opaque body cuts ofi 
light 

\ 17. What is an image f 

The optical counterpart of an object. 

18. How may inverted images be easily produced f 

By allowing light to pass through a small hole in a shutter 
of a darkened room and receiving it upon a white 
screen. 



RADIANT ENERGY, 65 

19. What is the velocity 0/ light f 
About 186,000 miles per second. 

20. How does the intensity 0/ incident radiation vary ? 
Inversely as the square of the distance; it also varies with 

the angle at which the radiation strikes the surface 
upon which it is incident. 

21. What is photometry f 

The measurement of the light emitted by a body. 

22. What is the reflection of radiant energy ? 

The turning back of ether waves by the surface on which 
they fall into the medium from which they came. 

23. In what ways may radiant energy be reflected ? 
Irregularly when the waves fall upon a rough surface, and 

regularly when they fall upon a polished surface. 

24. What is the effect of the irregular reflection of light ? 

To make visible the bodies that thus diflFuse the light that 
falls upon them. 

25. What is the effect of the regular reflection of light ? 
To form an image. 

96. What ts the law of reflected energy f 

The same as that of all reflected motion ; the angles ol 
incidence and of reflection are equal. 

«7. What is the effect of a plane mirror ? 

To change the direction of the cones of rays that a body 
sends to the eye. But the eye seems to see the object 
in the direction from which the rays enter the eye and 
hence in the direction of the mirror, and behind it. In 
fact, the eye sees not the object, but a virtual image of 
the object. This image appears as far behind the mirror 
as the corresponding object is in front of the mirror. 

28. What is the effect of a concave mirror ? 

To increase the convergence or to decrease the divergence 
of incident rays. If the incident rays are not too diver- 
gent, they will be reflected to a focus. 

29. What is the effect of a convex mirror? 

To increase the divergence or to decrease the convergence 
of incident rays. In the case of either kind of mirror, 
the ray obeys the law of reflected motion. 



66 RADIANT ENERGY, 

30. What is refraction of radiant energy ? 

A change in the direction of the radiation when it passes 
obliquely from one medium to another of different 
density. 

31. What are the laws of refraction ? 

When radiant energy passes perpendicularly from one 
medium to another, it is not refracted. When radiant 
energy passes obliquely from a rarer to a denser medium 
it is refracted toward the perpendicular erected at the 
point of incidence. When radiant energy passes ob- 
liquely from a denser to a rarer medium, it is refracted 
from the perpendicular. 

32. Mention some familiar effects of the refraction of light. 

A stick partly in air and partly in water (like the oar of a 
boat) seems bent at the surface of the water. Light 
from the sun comes to us after the sun has actually 
gone down below the horizon, the solar rays being re- 
fracted to us by the earth's atmosphere. 

33. What is total reflection ? 

When a ray passes from one medium into one denser, it is 
bent so that the angle of refraction is less than the 
angle of incidence. Conversely, when a ray passes from 
the denser medium, the angle of refraction ought to be 
greater than the angle of incidence. But in thus at- 
tempting to pass from the denser medium, the ray may 
have an angle of incidence at the refracting surface so 
great that the angle of refraction would be greater than 
a right angle. Under such circumstances the ray can- 
not emerge, and instead of being refracted at such sur- 
face, it will be reflected back into the denser medium. 

34. What is the angle at or beyond which total reflection takes 

place called ? 
The critical angle. For rays seeking to pass from water 
to air, it is 48° 35^; from glass to air, 40° 49^; from 
diamond to air, 23° 43^. Hence a glass prism, the sec- 
tion of which gives an isosceles, right angled triangle, 
ma^ be used for total reflection. A ray from the air 
striking perpendicularly upon one of the faces about 
the right angle enters the glass without refraction. It 
strikes the opposite face (between the two acute angles) 
at an angle of 45°. This exceeds the critical angle for 



RADIANT ENERGY, 67 

glass and air. Consequently the ra^ cannot be refracted 
at that point. Instead of passing into the air it will be 
reflected back into the glass and pass out of the third 
face of the prism at right angles to its original direc- 
tion. Such a prism is called a cathetal prism. 

35. Why is the reflection said to he ''total? " 

Because all of the light incident upon the reflecting sur- 
face is reflected, which is not the case in ordinary reflec- 
tion. Hence, such a surface constitutes a perfect 
mirror, the only one known. 

36. What is a prism ? 

A transparent body, the refracting plane surfaces of which 
are not parallel. 

37. What are some of the effects of a prism ? 

The refracted rays are bent toward the thickest part of 
the prism. An object seen through a prism seems to 
be moved in the direction of the thinnest part of the 
prism ; i. e., toward the edge that separates the refract- 
ing surfaces. Rays of differing wave-lengths are re- 
fracted in different degrees and, therefore, separated, 
producing the phenomenon known as dispersion of 

38. What is a lens f 

A transparent body with two refracting surfaces, at least 
one of which is curved. 

39. Lenses are of what two classes? 

Conve:!t ; thicker at the middle than at the edge. Concave ; 
thicker at the edge than at the middle. Convex lenses 
are comparable to two prisms with their backs in con- 
tact ; concave lenses to two prisms with their edges in 
contact. 

40. What is the principal focus of a convex lens ? 

The point at which incident parallel rays are made by re- 
fraction to converge. The focus may be found by hold- 
ing the lens facing the sun, using it as a burning glass. 

41. What are the conjugate foci of a lens ? 

Rays from a luminous point on one side of a convex lens 
(and not too near) will be brought to a focus on the 
other side of the lens. Rays sent from the latter point 



4 



( 



68 RADIANT ENERGY, 

will converge at the former. Two points so related are 
called the conjugate foci of the lens. 

42. Describe the images formed by a convex lens. 

If the object viewed is at more than twice the distance of 
the principal focus, the image will be smaller than the 
object and inverted. If the object is at a distance greater 
than that of the principal focus (focal distance), the 
image will be magnified and inverted. If the object is 
at a distance less than that of the principal focus, the 
image will be magnified and erect. This last is the 
eflFect of the simple magnifying glass, or reading lens. 

43. Describe the images formed by a concave lens. 
They are smaller than the object and erect. 

44. What is meant by dispersion of light f 

The separation of differently colored rays by a prism. A 
sunbeam may be separated by a prism into parts that 
exhibit the seven colors of the rainbow, violet, indigo, 
blue, green, yellow, orange, and red. These differently 
colored rays have different wave-lengths, violet waves 
being the shortest and red waves the longest. They are 
separated by the prism because the rays are refracted in 
varying degrees, the waves of the shortest wave-length 
(violet) being refracted the most and those of greater 
wave-lengths less. 

45. What is the colored band thus produced called f 
A solar spectrum. 

46. May these differently colored rays be recombinedf 

Yes ; by bringing them to a focus with a convex lens or a 
concave mirror. When recombined, they again produce 
white light. 

47. What is color ? 

A property of light that is dependent upon wave-length. 

48. To what is it analogous ? 

Color is to light what pitch is to sound. 

49. What determines the color of a body ? 

The wavelength of the light that it reflects or transmits 
to the eye. 



RADIANT ENERGY. 69 

50. What is tJie frequency of the ether waves that can excite 

visioft f 
About 5x10^* per second. If the frequency is much 
higher or lower than this, the retina of the eye cannot 
respond with vibrations of similar frequency. 

51. What are complementary colors f 

Any two colors which when ,blended form white light 

52. What is a rainbow ? 

Solar spectra formed by rain drops. 

53. What is the cause of a rainbow ? 

The refraction, reflection and dispersion of sunlight by 
rain- drops. 

54. What are the conditions of a rainbow f 

A shower during sunshine, the observer standing with 
his back toward the sun and his face toward the falling 
water-drops. 

55. What is spectrum analysis f 

The process of analyzing composite radiation or of iden- 
tifying substances by the spectra of their incandescent 
vapors. 

56. What are Fraunhofer lines f 

Dark lines across the spectrum of the sun, by means of 
which it is established that nearly two score of the ele- 
ments known to us are present in the sun. 

57. What is the relation between radiation ^ reflection^ and 

absorption of radiant energy? 
Good radiators are good absorbers and poor reflectors. 

58. Name some effects of radiant energy other than vision. 
Chemical (or actinic) and thermal. 

59. State an important fact concerning the chemical effects of 

radiant energy. 
Certain chemical changes (^.^., those on which photog- 
raphy depends) may be brought about by rays of widely 
differing wave-lengths, but they are most stimulated by 
tlie short-wave energy of the violet and the ultra-violet 
rays. 



70 RADIANT ENERGY. 

60. IVhai about the transparency of glass to this short-wave 

energy? 
It absorbs most of it; prisms and lenses of c^uartz are, 
therefore, desirable for the study of the chemical effects 
of radiant energy. 

61. Name a source of artificial light that is particularly rich 

in short-wave energy. 
The electric light. This is so rich in actinic rays that it is 
now extensively Used in taking photographs at night 
and on dark days. The magnesium light is used for 
similar purposes. 

62. State an important fact concerning the thermal effects oj 

radiant energy. 
Heating effects are produced by rays of widely diflfering 
wave-lengths, but they are more marked in the long- 
wave energy of the red and the infra-red rays. 

63. What about the transparency of glass to this long-wave 

energy f 
It absorbs most of it ; prisms and lenses of rock-salt are, 
therefore, desirable for the study of the thermal effects 
of radiant energy. 

64. Mention a familiar example of non-luminous thermal 

energy. 
The energy radiated by hot water or by a stove heated 
below the temperature of redness. Similar rays of 
great wav«-length form part of every sunbeam. 

65. How may the luminous and non-luminous rays of a beam 

of light be sifted from each other? 
By passing the beam through an aqueous solution of alum 
contained in a spherical flask, the " obscure heat " rays 
win be absorbea. The remaining rays will be refracted 
to a focus of ^reat illumination, but of little heating 
power. B^ using a similar flask filled with a solution 
of iodine m carbon disulphide, the luminous rays will 
be absorbed and those of greater wave-length brought 
to a focus — invisible, but hot enough to explode gun- 
cotton. 

66. What is fluorescence ? 

The property that some substances have of lowering the 
vibration-frequency of ether waves to the range of vision. 



RADIANT ENERGY, ^\ 

67. What is phosphorescence? 

The property that some substances have of shining in the 
dark after exposure to light. 

68. What is calorescence t 

The property that some substances have of raising the 
vibration-frequency of ether waves to the range of vision. 

69. What is interference of light f 

The combination of two series of luminous waves in oppo- 
site phases so that they destroy each other, and produce 
darkness or colors complementary to those destroyed. 
Colors thus produced by the destruction of some of tiie 
rays of the sunbeam are often seen in soap-bubbles and 
thin layers of oil on water. 

70. What is polarization of light ? ^ 
A process of limiting the vibrations of luminous waves 

to a single direction. The phenomena are very beauti- 
ful, but the explanation is so complex that the pupil is 
referred to a larger work. 

71. How may light be polarized f y 
In three ways ; by absorption, by reflection, and by double^/^ 

refraction. 

72. How is light polarized by absorption f 

By passing it through certain substances, like plates cut 
from a crystal of tourmaline. These plates will trans- 
mit rays the vibrations of which lie in a certain plane, 
and will absorb the others. A tourmaline plate thus 
used is called a polarizer. A similar plate used to show 
that the light has been polarized would be called an 
analyzer. 

73. How would a tourmaline plate be used as an analyzer ? 
By turning it about the direction of the polarized rays as 

an axis. In a certain position, it will freely transmit 
the polarized li^ht When turned a quarter of the 
way around, it will absorb the rays transmitted by the 
polarizer. 

74. How is light polarized by reflection t 

By allowing it to fall upon a non-metallic mirror at a cer- 
tain angle, which depends upon the nature of the reflect- 
ing surface. For glass, the beam must fall upon the 



72 RADIANT ENERGY, 

reflector so that the angle of incidence shall be 54® 35'. 
The ordinary mirror made of glass, with amalgam at 
the back, will not answer; it is a metallic mirror. A 
tourmaline analyzer may be used to show that the light 
has been polarized by the reflection. 

75. How is light polarized by double refraction ? 

Light moves through glass more slowly than it does in 
air. This fact underlies the phenomena of refraction 
by glass. A crystal of Iceland spar also retards the 
vibrations of light passing through it. But the retarda- 
tion of vibrations whose plane is parallel to the axis of 
the crystal is different from the retardation of the vibra- 
tions whose plane is perpendicular to that axis. Hence 
the two sets of rays will be refracted in different degrees, 
and thus separated. Either one of them is polarized. 

76. What is a Nicol prism f 

A crystal of Iceland spar cut in a plane passing through its 
two obtuse angles. The two halves are then cemented 
with Canada balsam. When a beam of light enters the 
crystal, it is doubly refracted. One set of rays strikes 
the layer of balsam at such an angle that it is reflected 
out at the side of the prism and there quenched. The 
other rays strike the balsam at such an angle that they 
pass through it and the other half of the prism, and 
emerge alone as polarized light. One Nicol may be 
used as a polarizer and another as an analyzer. 

77. Name one use of polarized light in the arts. 

It is used largely in determining the qualities of sugars 
in process of refining. 

78. What is the most wonderful of all optical instruments t 
The eye, which is an instrument for projecting a real 

image upon the retina, a screen of nerve filaments. 

79. What is necessary to distinct vision f 

The image formed on the retina must be clearly defined, 
well illuminated, and of sufficient size. 

80. What is near-sightedness f 

A defect in the eye arising from too great refracting power 
(convexity). The remedy lies in concave-lens glasses. 



RADIANT ENERGY. 73 

81. What is far-sightedness f 

A defect in the eye arising from too feeble refracting 
power. The remedy lies in convex-lens glasses. 

82. What is the visual angle ? 

The angle included between two rays of light coming from 
the t-xtremities of an objec to the centre of the eye. 

8^. What is a stereoscopic picture ? 

Two slightly different pictures of the same object placed 
side by side upon a card. The right hand picture rep- 
resents the object as it would be seen by the right eye 
alone, and the other picture as it would be seen by the 
left eye. 

84. What is a stereoscope ? 

Two half-lenses (convex) held in a frame with their edges 
toward each other, and at a distance apart equal to that 
between the eyes of an observer. When held before 
the eyes, a diaphragm prevents either eye from seeing 
both pictures. The instrument blends the pictures and 
gives the appearance of solidity, /. e.^ " makes the pic- 
tures stand out." 

35. What is a simple microscope t 

A magnifying glass, or convex lens, held so that the object 
to be viewed lies between the lens and its principal 
focus on the side opposite the eye of the observer. 

86. What is a compound microscope ? 

Two or more convex lenses placed in a tube so that the 
magnified image formed by the objective is further 
magnified to the eye of the observer by the eye-piece. 
The eye-piece is larger than the objective. The image 
is inverted. 

87. What is a Galilean telescope f 

One in which the objective is a double-convex lens and 
the eye-piece a double-concave lens. The image is 
magnified, erect, and near. 

88. What is an opera-glass ? 

Two Galilean telescopes carried side by side in a frame. 



74 RADIANT ENERGY, 

89. What is an astronomical refractor? 

A telescope consisting of two or more convex lenses 
placed in a tube somewhat after the manner of the com- 
pound microscope, the objective being larger than the 
eye-piece. The image is magnified and inverted. 

9a What is the largest refracting telescope in the zvorldf 

That of the Lick Observatory in California, It is 60 feet 
long. The objective is 38X inches in diameter. It was 
ground by Alvin Clark's Sons, of Cambridge, Mass. 
The mountings were designed and made by Messrs. 
Warner & Swasey, of Cleveland. 

91. What is an astronomical reflector ? 

A telescope consisting of a concave mirror placed at the 
end of a tube, and having an eye-piece of convex lenses 
for magnifying the image formed by the mirror. 

92. Name the most notable reflecting telescope in the world. 
That built by the Earl of Rosse, the mirror of which is 6 

feet in diameter and has a focal distance of 54 feet 

93. What is a terrestrial telescope ? 

A refracting telescope in which two convex lenses have 
been interposed between the objective and the eye-piece 
for the purpose of rendering the image erect instead of 
inverted. 

94. What is an optical lantern ? 

An optical arrangement in which the rays of the lamp or 
other source of light are concentrated by a convex lens 
upon a photographic transparency called a " slide." The 
rays transmiltea and colored by the slide are brought, 
by a series of convex lenses in front of the slide and 
called the objective, to a focus upon a screen. The 
image upon the screen is magnified and inveited. That 
the picture seen by the observers may be erect, the slide 
is put into the lantern upside-down. The picture on 
the screen will be colored as the slide is colored. Such 
a lantern is often called a magic lantern. 



ELECTRICITY AND MAGNETISM, 75 



CHAPTER VI. 
ELECTRICITY AND MAGNETISM. 



A. — Gbnkrai« View. 

1. If a glass rod is rubbed briskly with silk and quickly held 

near a few shreds of cotton or scraps of paper ^ what will 
take place f 
The cotton or paper will fly up to the glass rod. 

2. By what other simple apparatus may a similar effect be pro- 

duced? 
By a stick of sealing wax recently rubbed with flannel. 

3. What is an electric pendulum f 

A well-rounded pith-ball suspended by a fine silk thread. 

4. When the rubbed rod of glass or sealing wax is presented to 

an electric pendulum^ what may be noticed f 
The ball will fly to the rod, and, after brief contact, will 
be driven from it. It may then be driven about by the 
rod without touching it. 

5. What are the causes of these motions ? 
Blectric attraction and repulsion. 

6. Of what are these attractions and repulsions manifestations ? 
Of a state or condition called electrification. Bodies in 

this state are said to be electrified. 

7. In what two ways may electrification appear? 

As a charge residing on a body or as a current flowing 
along it. 

8. How does frictional or static electricity appear? 
As a chare's. 



76 ELECTRICITY AXD MAGNETISM. 

9. W liai is a conductor of clectricily ? 

A substance that easily permits a transference of electrifi- 
cation along it. 

la What is am imsulaiorf 

A substance that oflFers great resistance to such trans- 
ference. 

11. Nam€ some good conductors. 
Metals, acids, and water. 

12. Name some good insulators. 

Sealing wax, glass, porcelain, india rubber, silk, and dry 
air. 

15. What is positive electrification ? 

Such as that produced by rubbing glass with silk. 

14. What is negative electrification f 

Such as that produced by rubbing sealing wax with flannet^ 

15. Are these two kinds of electricity typical? 

They are. All electrified bodies act like either the glas& 
or the sealing wax. 

16. Can one of these electrifications be developed without devet 

oping the other f 
No. The rubbed g*ass rod is positively electrified; the 
silk rubber is electrified negatively. In every case, the 
rubbing and rubbed bodies are electrified equally, oppo- 
sitely, and simultaueousl}'. 

17. What is the process of electrification ? 

When two dissimilar substances are brought into contact 
and then separated, they are equally and oppositely 
electrified. If one of these substances is a good non- 
conductor or insulator, it will remain electrified or 
charged. 

18. Why may a glass rod or a stick of sealing 7vax held in the 

hand he electrified^ while an iron rod thus held may not be? 
Because in either of the f rmer cases, the rubbed parts of 
the rod are insulated by the material of the rod itself 
and the surrounding air, so that the electrification 
developed by friction cannot escape. 



ELECTRICITY AND MAGNETISM. 77 

19. May an iron rod^ then, be electrified? 
Yes, after insulation. 

20. When is a body said to be charged? 

When it has been insulated and then electrified by fric- 
tion, as were the glass and sealing wax, or by contact 
with a charged body, as was the electric pendulum, or 
by induction. 

21. How may a charged body be discharged? 

By touching it with the hand or other body in electrical 
connection with the earth. 

22. What is the law of electric attraction ? 

Bodies that are oppositely electrified attract each other ; 
bodies that are similarly electrified repel each other. 

23. What is electrification by conduction ? 

The process of electrifying or charging a body by putting 
it in contact with an electrified body. 

24. What is an electroscope ? 

An instrument for the detection of the presence, or the 
determination of the sign of an electric charge. 

25. How is it used to detect the presence of a charge ? 

Bring the body to be tested near the uncharged pith-ball. 
If the pith-ball is attracted, the body tested is electrified. 
But the charge may be -|- gr — . 

26. How is it used to determine the sign of the charge ? 
Allow the pith-ball to touch the body now known to be 

charged until itself is similarly charged and repelled. 
If the charged pith- ball is repelled by an electrified 
glass rod, the original charge was -[-. If, on the other 
hand, it is repelled by the electrified sealing wax, the 
original charge was — . 

27. What is the location of an electric charge ? 

The outer surface of an insulated, electrified body. 

28. State one effect of this fact. 

It enables us to make the metallic conductors used in 
dealing with electric charges hollow without loss of 
efficiency. A glass ball covered with gold leaf will hold 
as great a charge as a gold ball of the same size. 



78 ELECTRICITY AND MAGNETISM. 

29. Does the shape of a conductor affect the distribution of an 
electric charge carried by it ? 
It does. The electrification tends to accumulate at the 
more pointed parts. On a sphere, it is evenly dis- 
tributed. On an egg-shaped conductor, it is most dense 
at the small end. If the conductor has a sharp point, 
the electricity rapidly and quietly escapes therefrom, 
the air particles carrying oflF the charge by convection. 

50. What care does this necessitate f 

That conductors for electric charges be smooth, with 
rounded edges, and kept free from dust 

31. Does this action of pointed conductors enable any uses thai 

compensate for this care and trouble ? 
It is of prime importance in the action of electric machines 
and lightning rods. 

32. What is an electrical field of force f 

The space around an electrified body and through which 
the electric force acts. 

33. What is electric potential ? 
Degree of electrification, 

34. What is an equipotential surface f 

A surface that has the same potential at every point. 

35. What is electromotive force? 

The agency that tends to produce a transfer of electrifi- 
cation from a point of higher to a point of lower po- 
tential. 

36. What is electrification by induction ? 

The process of electrifying a body without contact with 
another body. 

37. When is a body said to be electrically polarized? 

When an electrified body is brought near an insulated 
conductor, the opposite electrifications of the latter are 
separated, one of them being attracted to the nearest 
end, and the other repelled to the further end of the 
conductor. In this condition, the conductor is said to 
be polarized. If the disturbing charge is removed, the 
separated electrifications of the conductor will again 
mingle by reason of their mutual attraction. 



ELECTRICITY AND MAGNETISM. 79 

38. How may a conductor be charged by conduction t 

Suppose that a bod^^ with + electricity is brought near 
the east end of an insulated conductor. The conductor 
is immediately polarized, — electrification being drawn 
to the east end and + electrification being repelled to the 
west end of the conductor. If, now, the charged body 
is brought into contact with the conductor, the attracted 
— electricitjr passes to the charged body, and the con- 
ductor, having only + electricity, is left positively 
charged by conduction. 

39. What about the potentials of the two conductors after con- 

tact? 
They will be equal, and less than the origrinal potential 
of the charging body. 

40. Why less? 

Because the potential of the charg^ing body was lowered 
by the neutralizing opposite electrification attracted 
from the polarized conductor. 

41. How may a body be charged by induction t 

When the conductor is polarized, as before, touch it with 
the hand, and the repelled -f- electrification escapes to 
the earth. Remove first the hand and then the charg- 
ing body, and the conductor will be found to be nega- 
tively electrified. 

42. How does this ajffect the potential of the charging bodyf 
It has no effect upon it, and the operation may be re- 
peated with equal effect with other conductors. 

43. What caution is necessary? 

To remove the touching hand from the conductor before 
you remove the polarizing charge (the body originally 
electrified). 

44. Why is this necessary? 

Because, if the charged and polarizing body is first re^ 
moved, the + electrification of the polarized conductor 
will (being no longer repelled by the charge of like 
sign) return through the hand and miuf^le with the — r 
electrification of the conductor, neutralizing it and leav- 
ing the conductor in its ordinary, unelectrified condi- 
tion. 



8o ELECTRICITY AND MAGNETISM. 

45. How may the condition of the conductor, or of either of its 

endSy be tested at any stage of this process f 
By using the pith-ball (or other) electroscope. 

46. What is the capacity of a conductor ? 
The ratio of its charge to its potential. 

47. What is the unit of capacity f 

The farad, which is the capacity of a conductor tbat re- 
quires unit quantity to produce unit difference of po- 
tential. 

48. What is meant by the condensation of electricity ? 

If pieces of tin-foil are pasted on opposite sides of a larger 
pane of glass, one piece connected to an electric ma- 
chine and the other piece to the earth, it will be found 
that the tin- foil coats may be more intensely charged 
than either one alone. This process is called the con- 
densation of electricity. 

49. What is an electric condenser ? 

A device for accumulating a large charge with a small 
electromotive force, substantially as just described. It 
consists of two- conductors, slightly separated by a die- 
lectric. 

50. What is a dielectric f 

The medium between two electrified bodies through or 
across which electric force is acting ; it is generally a 
non-conductor or insulator. 

51. What is the most common form of condenser? 
The Leydenjar. 

52. What is a Leydenjar? 

An electric condenser, consisting of a glass jar, coate«», 
within and without to within a few inches ot the mouth 
with tin-foil, and with a metal rod passing through the 
cover, terminating in a knob or disk above and connect- 
ing below with the inner coat of the jar. 

53. How is it charged ? 

Holding the jar by its outer coat, present the knob to the 
electrified prime conductor of an electric machine or 
the charged cover of an electropliorus. The inner coat 
is charged by conduction, and the electrification there- 
by induced on the outer coat " binds " that of the inner 
coat and enables it to receive an increased charge. 



ELECTRICITY AND MAGNETISM, 8i 

54. How is it discharged? 

By bringing the inner and outer coats into electrical con- 
nection. This may be done by touching the knob, but 
such an experiment is likely to be painful. Abetter 
way is to cover a stout piece of wire with rubber tubin^jj 
and place a knob on each end of the wire. Hold this 
discharger by the insulating rubber, bring one knob 
into contact with the outer coat of the jar, and the other 
into contact with the knob of the jar. The separated 
electrifications of the two coats will reunite with a brill- 
iant flash. 

55. What is a Leyden battery ? 

A number of Leyden jars, with all of the knobs con- 
nected, and with all of the outer coats connected. 
Great care is needed in experimenting with such a bat- 
tery. 

56. May such a battery be overcharged ? 

Yes. Then the separated electrifications rush together 
through the glass with destructive effects. 

57. When will this take place f 

When the attraction between the charges of the two coats 
exceeds the resistance offered by the intervening glass 
(dielectric). 

58. To what is the discharge of such a battery analogous ? 
To a stroke of lightning. 

59. What is electricity ? 

That which is transferred from one body to another in 
the process of electrifying them. This is not a very 
definite statement, but it is as definite as it is safe to 
make in the present state of knowledge on the subject. 
Electricity is probably a form of matter, and may be 
identical with the ether. 

60. Is electrification also a form of matter? 
No ; it is a form of potential energy. 

61. The nature of electricity being unknown^ is a theory of 

electricity of any use ? 
•Yes. It enables us to classify phenomena, aod may lead 
to important discoveries. 



82 ELECTRICITY AND MAGNETISM. 

62. What caution is necessary in the use of such a theory f 

To remember that it is a mere theory, and not let it blind 
us to the importance of any fact that may seem to con- 
flict with it. It should be used with a willingness to 
discard it for anything better that may be oflFered. 

63. What may a theory thus held be called f 
A provisional theory. 

64. State one provisional theory of electrification. 

When two electrified bodies are separated against their 
mutual attraction, the intervening medium is thrown 
into a state of strain. This state of strain in the die- 
lectric constitutes electrification. 

65. State some other way of developing electrification. 

By the chemical action of one or more liquids on two dis- 
similar substances, like zinc and copper. 

66. What is the common name for electrification thus developed f 
Current, voltaic or galvanic electricity. 

67. Describe a simple apparatus for developing voltau: elec- 

tricity. 
Slowly pour sulphuric acid into about ten times its vol- 
ume of water. In a tumbler half filled with this diluted 
acid, place a strip of sheet zinc about one inch by four. 
While the zinc is still wet, rub it with a few drops of 
mercury until it has a silver-like appearance. Place the 
zinc and a similar piece of copper in the liquid in the 
tumbler, being careful that the two metals do not touch. 
Above the liquid, ioin the metal plates by a copjjerwire 
a foot or two in length. A current of electricity will 
flow through the wire. 

68. What have we thus formed ? 

An electric circuit, composed of amalgamated zinc plate, 
wire, copper plate, and acid. 

69. How are the plates of a voltaic cell designated f 

The one on which the acid acts the more vigorously is 
called the positive plate ; the other is called the nega- 
tive plate. 

70. What is the assumed direction of the current t 



ELECTRICITY AND MAGNETISM. 83 

From the positive plate, through the liquid to the nega- 
tive plate, and from the negative plate through the wire 
to the positive plate. 

71. If the connecting wire is cut, what are its free ends called? 
Electrodes. 

72. How are the electrodes designated f 

The positive plate carries the negative electrode, and the 
negative plate carries the positive electrode. 

73. Can you give a simple rule for designating both plates and 

poles ^ a voltaic cell or battery ? 
The plate, or electrode, from which the current flows is 
+ ; the plate, or electrode, toward which it flows is — . 

74. Whai is an electric circuit ? 

The path through which the current flows. 

75. When zvill electrification be thus developed ? 
When the circuit is *' closed," or complete. 

76. How may the electric flow or current be stopped at every 
point? 

By breaking the circuit at any point. 

77. How may this be done? 

In several ways, such as cutting the wire or lifting either 
plate from the acid. 

73. What is this apparatus called? 
A voltaic or galvanic cell. 

79. What is a voltaic battery? 

A number of joined voltaic cells. 

80. What is the best familiar analogue of current electricity ? 
Water flowing through pipes that make a circuit 

81. Do such pipes offer a resistance to the passage of the water ? 
Yes. 

82. What is the cause of such resistance ? 
Friction. 

83. Does an electric circuit offer a resistance to the passage of 

the electric current ? 
Yes. 



84 ELECTRICITY AND MAGNETISM, 

84. What is electric resistance f 

The property of a conductor, on account of whicb the 
electric current passing through it is diminished, part 
of the electric energy being transformed into heat. 

85. What is the cause of such resistance ? 
We do not know. 

86. What is the unit of resistance f 

The ohm. It is nearly the resistance of 1,000 feet of No, 
10 copper wire, or 40 feet of No. 24 copper wire. 

87. What is a tnegohm ? 
A million ohms. 

88. What is a microhm ? 

A millionth of an ohm. 

89. What are the laws of resistance f 

Resistance varies directly as the length of the conductor ; 
inversely as the area of cross-section of the conductor ; 
and with the material of which the conductor is 
made. 

90. What causes water to flow through pipes ? 

Pressure enough to overcome the resistance (friction). 

91. What causes electrification to flow through a wire f 
Electromotive force, the nature of which is not known. 

92. How is this term abbreviated ? 
E. M. F. 

93. What ts the unit of electromotive force f 

The volt. It is nearly the same as the E. M. P. of a voltaic 
cell consisting of a copper and a zinc plate immersed 
in a solution of zinc sulphate, and a little less than the 
E. M. P. of a Daniell cell (which measures 1.079 volts.) 

94. What is a microvolt? 
A millionth of a volt 

95. Upon what does the strength of the current or rate offlozv 

depend? 
Upon the resistance of the circuit and the E. M. P., varying 
directly with the latter and inversely with the former. 



ELECTRICITY AND MAGNETISM, 85 

96. What is the unit of current strength or rate of flow ? 

The ampere, or the current that an E. M. F. of i volt sends 
through a conductor that has a resistance of i ohm. 

97. What is Ohm's law f 

An expression of the relation of these three functions of 
current, thus : volts -=- ohms = amperes, or 
E 
C=- 

Any two of these being known, the other may be found. 

98. What is a milliampere f 

A thousandth of an ampere. 

99. What is the strength of the ordinary telegraphic current f 
From 5 to 15 milliamperes. 

100. What is the strength of the ordinary electric arc light cur- 
rent? 
From 5 to 10 amperes. 

loi. If a voltaic battery has an E. M. F. of 20 volts ^ and a total 
resistance of 10 ohms^ what is the strength of the cur- 
rent? 
2o-r-io=2, the number of amperes. 

102. The E. M. F. of a battery is 18 volts. The current meas- 

ures 15 amperes The external resistance of the cir- 
cuit is 4 volts. What is the internal resistance of the 
battery ? 
E 
-^=— . . • . 18-7-1.5=12. 
C 
The total resistance is 12 ohms. From this deduct 4 
ohms for the external circuit, and we have left 8 ohms 
for the internal resistance. 

103. What is a coulomb ? 

The quantity given per second by a one-ampere cur- 
rent. 

104. What quantity will be given by a lo-ampere current in a 

minute ? 
600 coulombs. 



86 ELECTRICITY AND MAGNETISM, 

105. What is a joule f 

It is the unit of work. Joules = volts X coulombs. 

106. What is its heat equivalent ? 
a 24 of a calory. 

107. What is a watt? 

The unit of electrical activity. Watts = volts X am- 
peres-; 746 watts = I horse-power. 

108. What is Joule's law? 

The work done by an electric current measured in watts^ 
equals the product of the fall of potential measured in 
volts, multiplied by the number of amperes multiplied 
by the number of seconds : 
Jr= ECt, 

109. What statement may be made concerning the distribution 

ofheai in an electric circuit ? 

The heat developed by the current in any part of the cir- 
cuit is proportional to the resistance of that part of 
the circuit. 

iia How long will a current raise the temperature of its con- 
ducting wire f 

Until it melts it and thus breaks the circuit, or until the 
loss of heat by conduction and radiation equals the 
heat supplied oy the current. 

111. What rule applies to the heating of wires by electric cur- 

rents f 

The rise of temperature in different parts of a wire of 
uniform material, but varpng diameter (the current 
remaining the same), will be inversely proportional to 
the fourth power of the diameters. 

112. What uses are made of wires thus heated f 

The^ are used in exploding mines, as a cauterizing sub- 
stitute for the knife in surgery, and for other pur- 
poses. 

113. What is a shunt f 

When a circuit contains two branches, each branch is s 
shunt to the other. 



ELECTRICITY AND MAGNETISM, 87 

114. Describe a simple experiment with a voltaic cell. 

Wind 15 or 20 turns of stout cotton-covered copper 
wire around an iron nail and join the ends of the wire 
to the zinc and copper plates, thus completing the 
circuit so that an electric current will flow through 
the wire around the nail. To either end of the nail, 
bring a few iron filings. They will cling to the nail. 

115. What is the cause of the clinging ? 
Magnetic attraction. 

116. What docs the nail now constitute ? 
A temporary magnet. 

117. How long Tvill the nail retain its newly acquired attract- 

ive or magnetic powers ? 
As long as the electric current flows through the wire 
coiled around it 

118. What will happen if the circuit is suddenly broken ? 
The current will stop, and the iron will cease to he a 

magnet and will promptly drop the filings. 

1 19. What is a magnet ? 

A body that attracts iron and steel, and that when freely 
suspended points north and south. 

12a How are magnets classified f 
As natural or artificial. 

121. What is a natural magnet f 

A variety of iron ore known as loadstone (Peg O^. 

122. How are artificial magnets classified ? 
As permanent or temporary, 

123. Of what is a permanent magnet made t 
Of tempered steel. 

124. Of what is a temporary magnet fnade t 
Of soft iron. 

125. How is this attractive power {magnetism) distributed t 
It is greatest at or near the ends of the magnets. 

126. What are these points of greatest attraction called ? 
The poles of the magnet 



88 ELECTRICITY AND MAGNETISM. 

127. Can a magnet have only one pole ? 
Every magnet has two poles. 

128. How are substances classified with reference to their mag- 

netic qualities f 
Iron and other substances that are attracted by a mag- 
net are called paramagnetic ; bismuth and other sub- 
stances that are repelled by a magnet are called dia- 
magnetic. 

129. What is a permanent magnet ? 

A bar of hardened steel that has been magnetized by an 
electric current (as the nail was magnetized) or in 
some other way. 

130. In what other way may a permanent magnet be easily 

made f 
By slowly drawing a bar of steel from end to end across 
the end of a good magnet, repeating the motion sev- 
eral times, and always in the same direction. 

131. Name some common articles that may be thus magnet- 

ized. 
Sewing needles and knitting needles, steel pens and 
knife blades. 

132. How may a needle thus magnetized be instructively used? 
By floating it by a thin slice of cork upon the surface of 

water. It will come to rest pointing north and south. 

133. What other fact may be noticed ? 

The same end of the needle always points to the north. 

134. What is a magnetic needle? 

A small bar magnet freely suspended. 

135. How are its ends designated ? 

The north-seeking end of the needle is called the 
marked or + pole ; the other end is called the un- 
marked or — pole. 

136. What is a compass needle ? 

A magnetic needle that turns in a horizontal plane. 

137. What is a dipping needle f 

A magnetic needle that turns in a vertical plane. 



ELECTRICITY AND MAGNETISM. 89 

138. How may the bar magnet be suspended for a horizontal 

needle ? 

By balancing it after magnetization upon a vertical pivot, 
by suspending it by a hair or silk fiber, or by resting 
it upon a slice of cork floating upon water, as already 
indicated. 

139. How may a bar magnet be suspended for a dipping 

needle f 
By supporting it before magnetization by a horizontal 
axis passing through or just above its center of mass, 
like the scale-beam of a balance. 

140. Will the bar hang horizontal after magnetization ? 

It will at the earth's magnetic equator, but in the tem- 
perate zones one end will be markedly depressed be- 
low a horizontal line drawn through the point of sus- 
pension. 

141. What is an electromagnet f 

A bar of soft iron around which an electric current is 
passing, as in the case of the nail-magnet previously 
described. It is a temporary magnet. 

142. Hc:v d? electromagnets compare ^ as to strength, with per- 

manent magnets? 
They may be made much more powerful. They are, 
therefore, preferable for making permanent magnets. 

143. Describe still another process of magnetizing a steel bar. 
Pass the bar through a coil of wire (helix) carrying 

a powerful current of electricity. The bar should pass 
into the helix at one end and come out at the other 
end. 

144. What is a magnetic field ? 

The space surrounding a magnet and through which the 
magnetic force is acting. 

145. What are magnetic lines of force ? 

Imaginary lines in a magnetic field, indicating the direc- 
tion in which a marked pole would move. 

146. How are such lines drawn ? 

One line of force for each dyne of magnetic force exist- 
ing in the field. 



90 ELECTRICITY AND MAGNETISM. 

147. How may the direction of the lines of magnetic force at 

any point be easily shown f 
By suspending a small magnetic needle at that point 
The length of the needle will indicate the direction of 
the line. 

148. What is a collection of lines of force called? 
A flow or flux of force. 

149. What is the unit of magnetic flux called t 
A weber. 

150. How is it represented / 
By one line of force. 

151. What is the first law of magnetic poles f 

Unlike poles attract each other ; like poles repel each 
other. 

1 52. What is the second law of magnetic poles ? 

Magnetic force varies inversely as the square of the 
distance. 

153. How will the attraction of a magnetic pole for a piece of 

iron be affected by increasing the distance between thetn 
threefold? 
It will be only one-ninth as great. 

154. If a small bar of soft iron is brought near a good magnet, 

'\efid on end'' but not quite touching it, how will the 
iron be affected? 
The iron bar will be a magnet while it and the magnet 
remain in these relative positions. 

155. How may its condition be described? 
It is magnetically polarized. 

156. To what is its condition analogous ? 

To that of the electrically polarized conductor previously 
mentioned. 

157. What is this called ? 
Magnetic induction. 

158. How will the poles of the induced [temporary) magnet be 

related to those of the inducing magnet ? 
The + of one will face the — of the other. 



ELECTRICITY AND MAGNETISM, 91 

159. When will this polarizing power of the inducing magnet 

reach its maximum f 
When the distance between magnet and bar is reduced 
to zero. 

160. How is the fact that a succession of nails may be suspended 

from a tnagnet explained f 
By magnetic induction, the magnet making the nail in 
contact with it a temporary magnet, this temporary 
magnet converting the nail below it into another tem- 
porary magnet, and so on. 

161. State one effect of breaking a magnet. 

Each piece becomes a magnet with two poles of its own. 

162. How does magnetization affect the size of a steel barf 
It increases its length and diminishes its thickness. 

163. What other phenomenon takes place? 

A faint click is sometimes heard at the moment of mag- 
netization or demagnetization, as though the metal 
particles were rubbing against each other. 

164. When a piece of iron is strongly magnetized and demag- 

netized in rapid succession (e. g., the armature core of a 
dynamo) y what happens to the iron f 
It becomes hot, as though the magnetic changes were 
accompanied by internal friction. 

165. State one theory of magnetization. 

The molecules of a magnetic substance are magnets. 
When the magnetic axes of these molecular magnets 
are turned in the same direction, the body is thereby 
magnetized. 

166. What simple experiment tends to confirm this theory? 

If a glass tube full of iron filings is magnetized, the fil- 
ings may be seen to set themselves end on. When 
thus set, the contents of the tube act as a magnet un- 
til this arrangement is destroyed by shaking the filings 
from their orderly arrangement. 

167. How is a magnet related to energy? 

It is a store of potential energy, due to the fact that, at 
some time, a definite amount of energy was expended in 
its magnetization. By reason of the energjr thus stored, 
the magnet can do a definite amount of work by at- 
tracting a limited amount of iron. 



92 ELECTRICITY AND MAGNETISM, 

i68. What about this energy after the magnet is fully loaded? 

The magnet has done its full work and can do no more. 
It has expended all the energy with which it was origi- 
nally endowed, and, of course, can spend no more un- 
til it receives more. 

169. How may the magnet receive afresh supply of energy f 

When the load is torn from the magnet, energjr is ex- 
pended upon the magnet which, in its turn, is again 
stored as potential energy in the magnet, to be ex- 
pended at some future time as magnetic attraction. 

170. State a property of the electric current not yet mentioned. 

It has a magnetic character, as is shown in every electro- 
magnet, and in the deflection of the magnetic needle 
by a current passing through a wire held near the 
needle. 

171. What is the effect of an electric current upon the magnetic 

needle f 
It tends to set the needle at right angles to the direction 
of the current, so that if the conducting wire passes 
north and south near the needle, it will tend to set the 
latter east and west. 

1 72. How is this principle utilized ? 

In the galvanoscope and galvanometer. 

173. What is a galvanoscope f 

An instrument in which an electric current is carried by 
a coil of insulated copper wire many times around a 
magnetic needle, so that the deflection of the needle 
from its normal position (north and south) may declare 
the passage of a current through the circuit of which 
the coil forms a part. 

174. What is a galvanometer f 

A galvanoscope of the kind just described, so modified 
that the amount of deflection may declare the strength 
of the deflecting current. 

175. What is a long coil galvanometer? 

One with many turns of fine wire around the needle; /. e^ 
a coil of high resistance. 



ELECTRICITY AND MAGNETISM, 93 

1 76. When should such a galvanometer h^ used ? 

When it is necessary to measure a current in a circuit of 
otherwise great resistance. 

177. What is a short- coil galvanometer? 

One with fewer turns of coarser wire, i. e.^ a coil of low 
resistance. 

178. When should such a galvanometer be used f 

When it is necessary to measure a current in a circuit of 
low resistance. 

179. What is a magnetomotive force ? 

The force to which is due the existence of magnetic 
lines of force. 

180. What is the pertneability of a substance ? 

Its magnetic conductivity ; i. e,^ the ratio between the 
number of magnetic lines of force that pass through a 
given area of the substance and the number of hues 
that pass through a like area of the magnetic field 
alone. 

181. IVhai is reluctance ? 
Magnetic resistance. 

182. What is its unit called? 
The oersted. 

I S3. What is reluctivity ? 
Specific reluctance. 
184. What is terrestrial magnetism ? 

The magnetism of the earth taken as a whole, which 
acts as if a bar magnet about 5,000 miles long extended 
nearly north and south through the earth's center. 

18 s. Does the north pole of the earth tend to draw the compass 
needle northward? 
No. The north pole of the earth attracts one end of 
the needle ana repels the other end of the needle. 
These two forces act as a couple to turn the needle 
into a north and south position, but as to producing 
a motion of translation, one negatives the other. 

i86. What is magnetic inclination or dip ? 

The angle that the axis of a dipping needle maKes with 
the horizon. 



94 ELECTRICITY AND MAGNETISM, 

187. What is the inclination at the earth^s magnetic equator f 
Zero, 

188. What at the earth's magnetic poles f 

Ninety degrees ; the needle ther6 stands vertical. 

189. What is the dip at intervening places? 

Nearly the same as the latitude of the g^iyen place. 

19a What name is given to lines drawn through plcuxs of equal 
inclination f 

Isoclinic lines. 

191. Is the dip of the needle constant f 
It is not ; it changes continually. 

192. Why is this? 

Because of a corresponding change of the magnetic poles 
of the earth. 

193. Does the magnetic needlepoint to the true north? 

At a comparatively few places it does. At most places 
its axis varies from a true north and south line. 

194. What is magnetic inclination or variation ? 

The angle between the axis of a compass needle and a 
geographical meridian. 

195. What name is given to lines drawn through pUues of equal 

declination f 
Isogonic lines. 

196. What is the line drawn through places where the needle 

points to the true north called ? 
The line of no variation, or an agonic line. 

197. What is the magnitude of the .declination of an agone or 

agonic line. 
Zero. 

198. How does the American agone cross the United States? 
It passes near Charleston, through the mountains of 

West Virginia, and near Columbus, Toledo and Ann 
Arbor. 

199. Is this line stationary ? 

No. It is now moving slowly westward. 



ELECTRICITY AND MAGNETISM, 95 

•^ ■ 

Soa How does the needle at places not in the line of no varia- 
tion point ? 
At places east of that line, the variation is westerly, it 
being now nearly 20 degrees in Maine and increasing 
year after ye«r. At |>Iaces west of the line, the varia- 
tion is easterly, it being now about 20 degrees in Ore- 
gon and Washington and decreasing. 

201. What necessity does this impose in land-surveying ? 

To note the date with other records of the survey so 
that the true direction of the lines may be subsequent- 
ly traced, by correcting the " bearings " for changes 
in declination. 

202. Why have these questions on electricity and magnetism 

been so mingled ? 
Because electricity and magnetism are so closely related 
that one cannot learn much about either until one has 
learned something about the other. 

B—Elfxtric Gbnerators, etc. 

J03. What is the most common class of electric generators t 
Voltaic ctUs and batteries. 

204. Into what tivo classes are voltaic cells divided? 
Those using one liquid and those using two liquids. 

205. Name some of the principal single liquid cells. 
The Smee, Leclanch^ and potassium dichromate. 

206. Name some of the principal two liquid cells. 
The Daniell, Grove, and Bunsen. 

207. Are there other fortns ? 

Yes, in aim )st bewildering variety. 

Note — For descriptions of the various forms of voltaic 
cells, the student is necessarily referred to some 
arger work, like Avery's ** School Physics." 

208. What substance is generally used for the positive plate t 
Zinc. 

209. How should the zinc be prepared for voltaic use f 
By amalgamating it. 



96 ELECTRICITY AND MAGNETISM, 

210. How is this done ? 

By cleaning the plate in dilute acid and rubbing it well 
with mercury (quicksilver). 

21 1. What is a voltaic battery ? 

Two or more voltaic cells so connected that the current 
flows through all in the same direction. 

212. What is the most common form of connecting such cells ? 
In series. 

213. How are cells connected in series ? 

The 4- plate of one cell is connected to the — plate of 
the next. 

214. By what other term is this method of grouping known ? 
Coupling cells tandem or " for intensity." 

215. In what other way may cells be coupled? 
In parallel. 

216. How are cells coupled in parallel? 

By joining all of the + plates to one wire» and all of the 
-- plates to another wire. 

217. By what other terms is this method know/i ? 

Coupling cells in multiple arc, abreaU or ** for quantity." 

218. What is a battery of high internal resistance ? 
One the cells of which are joined in series. 

219. What is a battery of low internal resistance? 
One the cells of which are joined in parallel. 

220. Why is a battery with cells in series one of high internal 

resistance ? 
Because the length of the liquid conductor through 
which the current has to pass is that of a single cell 
multiplied by the number of cells. 

221. Why is a battery with cells in parallel one of low internal 

resistance ? 
Because, while the length of the liquid conductor 
through which the current has to pass is only that of 
a single cell, its sectional area is that of a single cell 
multiplied bv the number of cells. 
212. How is a series battery related to E, M. F, and to internal 
resistance? 



ELECTRICITY AND MAGNETISM, 97 

Its E. M. F. and internal resistance are those of a single 
cell multiplied by the number of cells. 

223. How is a multiple arc battery related to E. M. F. and to 

internal resistance ? 
Its E. M. F. is that of a single cell, and its internal re- 
sistance is that of a single cell divided by the number 
of cells. 

224. What is the best form of battery ? 

There is no *' best form '* of battery. One form is better 
for one purpose and another form for another purpose. 

225. When is a series battery preferable ? 

When the current is to work through a high external 
resistance. 

226. When is a multiple arc battery preferable ? 

When the current is to work through a low external re- 
sistance. 

227. In any given case^ what method of joining the cells will be 

the most effective? 
The one which, under the given conditions, will yield the 
great ^st.current strength (measured in amperes). 

228. What practical rule governs such cases? 

The maximum eflfect will be secured by making the in- 
ternal resistance of the battery equal to the external 
resistance. 

229. How may this 'rule be best illustrated ? 

By a few practical examples, joining a given number of 
given cells in different ways. 

230. Assume 4 cells, each luith an E. M, F. of i volt and an 

internal resistance of 4 ohms. This battery is to work 
through an external resistance of 4 ohms What will 
be the current strength when they are coupled in series ? 
The E. M. F. will be four times that of one cell, or 
4 volts. The internal resistance will be four times 
that of one cell, or 16 ohms. The total resistance of 
the circuit will be (164-4=) 20 ohms. Substituting 
these values in the formula for Ohm's law, we have 

E 4 
C= — = — = 0.2, the number of amperes. 
R 20 



98 ELECTRICITY AND MAGNETISM. 

231. What will be the current strength when the same cells are 

coupled in multiple arc ? 
The E. M. F. will be that of a single cell, or i volt. 
The internal resistance will be one-fourth that of one 
cell, or I ohm. The total resistance of the circuit 

E I 
will be 5 ohms. C = — = - = 0.2, the number of 
amperes. R 5 

232. Which way is the more effective ? 

In this case, the two ways would be equally eflFective. 

233. Can you suggest a better way than either of these f 

Join two cells in series and the other two cells in series, 
thus forming two rows of cells. For each row the E. 
M. F. will be 2 volts and the internal resistance will be 
8 ohms. Now consider each row as a cell and couple 
them in multiple arc. The battery of two rows wiil 
have an E. M. F. of 2 volts and an internal resistance of 
4 ohms. We now have an equality between the internal 
and external resistances. The total resistance of the 
circuit will be 8 ohms. 

E 2 
C^ — = - ^ 0.25, the number of amperes. This is the 

R 8 
greatest current possible under the given condition**. 

234. For the sake of additional practice upon this very practical 

point, assume a battery of 8 cells, each with an E. M. F. 
of 2 volts and an internal resistance of 8 ohms These 
cells are to be joined and ivork through an external resist 
ance of 16 ohms. What will be the current strength when 
they are coupled in series ? 
The E. M. F. will be 16 volts. The internal resistance 
will be 64 ohms. The total resistance will be 80 ohms. 

E 16 
C = — = — = 0.2, the number of amperes. 

R 80 

235. What will be the current strength when the same cells are 

coupled in multiple arc? 
The E. M. F. will be 2 volts. The internal resistance will 
be I volt. The total resistance will be 17 volts. 

E 2 
C^ — = — = 0.1 176+, the number of amperes. 

R 17 



ELECTRICITY AND MAGNETISM. 99 

236. Which way is the more effective t 
Coupled in series. 

237. Can you suggest a still better way ? 

Join four cells in series and the other four cells in series, 
forming two rows as before. For each row, the E. M. F. 
will be 8 volts and the internal resistance 32 ohms. 
Join these two rows in multiple arc, thus forming a 
battery with an E. M. F. of 8 volts and an internal resist- 
ance of 16 ohms. We now have an equality between 
the internal and the external resistances. The total 
resistance of the circuit is 32 ohms. 

E 8 
C'=' —-=- -^025, the number of amperes. This is 

R 32 
the greatest current possible under the given con- 
ditions. 

238. In general^ when should the cells be joined in series f 
When the external resistance is large. 

239. In general^ when should the cells be joined in parallel t 
When the external resistance is small. 

240. Suppose 3 cells joined in series. If one end of a stout 

copper wire is twisted about the wire connecting the first 
and second cells, and the other end of the same wire is 
twisted about the wire connecting the second and third 
cells f what would such a zvire be called f 
A shunt. 

241. How would the condition of the second cell be described ? 
" Shunted out of the circuit." 

242. What about the passage of the current from the first to the 

third cell f 
It would have two open paths, one through the second 
cell and one through the shunt. 

243. By which path would the current go? 

By both, the current flowing in each path being inversely 
proportional to the resistance of that path. 

244. How would such a shunt affect the total resistance between 

the first and third cell ? 
It would make it very much smaller. 



loo ELECTRICITY AND MAGNETISM, 

245. If the resistance through the second cell is 9.9 ohms and the 

resistance through the shunt is 1 ohtn^ what part of the 
current will go through the cell? 
Only one per cent. 

246. How may half of the current be forced through the cell ? 
By increasing the resistance of the shunt until it equals 

the resistance of the cell. 

247. Why tuas the cell said to be shunted •* out of the circuit f " 
Because so little of the current would go through it that 

practically it is ** out of the circuit.'* 

248. What common use is made of shunts ? 

To shunt out an electric lamp or other piece of apparatus 
for exchange or repair without interrupting the current. 

249. What are induced currents ? 

Momentary currents produced in a conductor that forms 
a closed circuit by varying the number of lines of mag- 
netic force that pass through the coil formed by the 
conductor. 

250. Describe a simple form of apparatus for producing an in- 

duced current. 
Close the circuit of a voltaic battery by a stout insulated 
copper wire, part of it being wound into a coil or helix. 
Provide another helix with a larger number of turns of 
finer wire, the ends of this being joined to the ends of 
a long-coil galvanometer, thus closing the second cir- 
cuit. The first circuit is called the primary ; the fine 
wire circuit is called the secondary. 

251. How may this apparatus be used ? 

Bring the primary near the secondary. As it approaches, 
the needle of the galvanometer shows that a current is 
flowing in the secondary. No current from the voltaic 
battery (i. e., from the primary circuit,) can pass through 
the galvanometer, which is in a totally distinct circuit. 
The approaching primary current induces a current in 
the secondary circuit. 

252. What happens when the two coils are at rest ? 

The induced current ceases, although the primary current 
continues to flow. The needle of the galvanometer 
comes to rest. 



ELECTRICITY AND MAGNETISM, loi 

253. How may the induced current he again produced ? 

By separating the two coils. The galvanometer shows 
that an induced current now flows through it in the op- 
posite direction. 

254. What do these results indicate ? 

That the induced current is due to a change in the induc- 
tive influence of the primary current. 

255. How may this variation be brought to a maximum ? 
By breaking and closing the primary circuit 

256. In what other way may a current be induced in the second- 

ary coil ? 
By using a good magnet instead of the primary current. 

257. Describe the effects of thus using a bar magnet. 

When the magnet is thrust into the interior of the coil, 
an induced current flows while the motion of the magnet 
continues. When the magnet becomes stationary, the 
current ceases to flow and the needle of the galvanome- 
ter gradually comes to rest. When the magnet is with- 
drawn, an induced current flows in the opposite 
direction. 

258. What will determine the E. M, F. of the induced currents ? 
The rapidity of the change of distance between the 

magnet and the coil ; i. e., the rapidity with which the 
number of magnetic lines of force that pass through 
the coil is changed. 

259. What is a magneto f 

A device for inducing electric currents in closed coils of 
wire by varying the relative positions of the coils and 
permanent magnets. 

260. What is a dynamo f 

A device for inducing electric currents in closed coils of 
wire by varying the relative positions of the coils and 
electromagnets. 

261. What are the essential parts of a dynamo f 

The field magnets, the armature, and the commutator or 
collecting rings. 

262. What are the field magnets f 



I02 ELECTRICITY AND MAGNETISM. 

Powerful electromagnets that supply the magnetic field 
in which the armature is tumeo. They are wound in 
diflFerent^ ways, according to the uses for which the 
dynamo is designed. 

263. What is the armature f 

A ring or cylinder, generally made of soft iron, that car- 
ries coils of insulated copper wire, and that is arranged 
for rapid rotation in the field of the field magnets. 

264. Where are the induced currents generated f 

In the coils of the armature, as they successively approach 
and recede from the poles of the field magnets. 

265. For what are such machines used? 

For developing currents for electric lighting, electro- 
plating, operating machinery and telegraphic apparatus 
and almost countless uses. 

266. What is a commutator? 

A special device for successively reversing the connec- 
tions of the armature coils so that the alternating cur- 
rents induced therein may all How in the same direction. 

267. What is an alternator f 

A d^amo without a commutator. It yields a current the 
direction of which alternates with great rapidity. 

268. What is an electric motor? 

A dynamo worked backwards. 

269. Illustrate your meaning. 

Mechanical energy is used to turn the armature of a 
dynamo and thus to develop a current of electricity. 
This current maybe carried by wires from the binding 
posts of the dynamo to the binding posts of a second 
dynamo. The passage of the current through the sec- 
ond dynamo will turn its armature backward, and the 
revolution of its shaft may be made to operate a street 
car, turning lathe, or other machinery. 

27a How may a small wire be made to transmit a great electric 
energy to a great distance ? 
By using a current of great E. M. F. and small size, i. e., 
a current high in volts and low in amperes. 



ELECTRICITY AND MAGNETISM, 103 

^i. What is the objection to such a current f 

It is difficult and dangerous to handle, requiring intelli- 
gent care and thorough insulation. 

272. How may such a current be produced? 

By dynamos constructed for that purpose. 

273. May such currents be exchanged for the safer currents 

of low potential ? 
Dynamos are now made for alternating currents of high 
potential. When these currents have been transmitted 
to the place of use by conductors of small size (and, 
therefore, moderate cost), thev are made to induce cur- 
rents of the E. M. F. desired, by the use of coils wound 
for that purpose. 

274* What is a transformer? 

A combination of a primary and a secondary coil so wound 
that a current at a given voltage may be received by the 
primary, and an induced current at a voltage higher or 
lower as desired delivered by the secondary. 

275. What is an induction coil ? 

A contrivance for inducing high potential currents in a 
secondary coil by rapidly making and breaking the cir- 
cuit of a primary current. It is a "step up" transformer. 

276. By what other name is the induction coil knozvn. 
RuhmkorfF coil. 

277. How are the two coils arranged ? 

The primary is generally placed in the hollow of the sec- 
ondary^. A bundle of soft iron wires acting as a core to 
the primary, increases the efficiency of the apparatus. 

278. How is the efficiency further increased ? 

By placing in the base of the apparatus a tin-foil con- 
denser, connected with the primary circuit. 

279. How is the primary or inducing current interrupted? 

By an automatic vibrator, like that of the familiar electric 
bell. 

280. What characterizes this induced current ? 

Very high E. M. F. or great diflference of potential. The 
induction coil may be made to produce sparks several 
feet in length. 



I04 ELECTRICITY AND MAGNETISM, 

281. What is an electrophorus ? 

A shallow tinned pan filled with resin, and a movable 
metallic cover with an insulating handle. The metallic 
and resinous surfaces touch at only a few points ; they 
are practically separated by a thin layer of air. 

282. For what is it used ? 

For developing electric charges. 

283. How is it used? 

When the resin is rubbed with flannel or cat-sk.in it be- 
comes negatively electrified. When the cover is placed 
on the resin, it is polarized by induction. When the 
cover is touched with the finger, the repelled — electri- 
acation of the cover escapes to the earth, the -{- elec- 
tricity of the cover remaining * 'bound*' by the attraction 
of the — electrification of the resin. Remove the cover 
by its insulating handle. It is positively charged and 
will give a spark to the knuckle or other unelectrified 
body presented to it. 

284. What is an electric machine f 

A device for developing larger supplies of static electricity 
than would be easily practicable with the rubbed rods 
or the electrophorus. 

285. Name some of the common forms. 

The plate machine, the dielectric machine, the Holtz 
machine, and the Wimshurst machine. 

Note,—^OT descriptions of some of these machines and 
explanations of their operation, the student is neces- 
sarily referred to some larger work, like Avery's 
** School Physics." 

286. What is the difference between static and current electricity f 
They are essentially identical, differing chiefly in the 

matter of voltage or E. M. F. 

287. What is the nature of the electric discharge f 
Oscillatory. 

288. What is meant by atmospheric electricity f 

The electrification diffused through the atmosphere. 

289. How is this diffused electrification concentrated f 
Chiefly by the clouds. 



ELECTRICITY AND MAGNETISM. 105 

190. What is lightning ? 

A disruptive electric discharge between two clouds or 
between a cloud and Ihe earth. 

291. What is thunder ? 

The sound that is caused by a flash of lightning. 

292. State the analogy between the discharge of a Ley den jar 

and a stroke 0/ lightning. 

The electrified cloud corresponds to the inner coat of 
the jar, the earth to the outer coat, and the intervening 
and insulating air (dielectric) to the glass walls of the 
jar. 

293. What about the electric tension just before a lightning 

stroke ? 
It must be very great to perforate an insulating layer of 
air sometimes a mile in thickness. 

294. How may a Leyden battery be quietly discharged f 

By bringing near it the pointed end of a sewing needle 
held in the hand. The opposite electrification induced 
by the attraction of the charge quietly streams in 
convective discharge from the point of the needle and 
gradually neutralizes the charge by combining with it. 

295. To what is such a quiet discharge comparable ? 

To the most frequent and eflfective use of a lightning 
rod, from which electrification is drawn in convective 
discharge from the earth to neutralize the opposite 
charge of the overhanging cloud. 

296. What is the most important work of a good lightning rod? 
The prevention of a lightning stroke in the way just de- 
scribed. 

297. Does the rod get fair credit for this work f 

Seldom. The operation is so unobtrusive that it is not 
noticed. 

298. What other work may a good lightning rod perform f 
When the charge of atmospheric electricity is so great 

that a stroke actually takes place, the rod oflFers an 
easy route for its passage to the earth and thus lessens 
the danger and damage. 



io6 ELECTRICITY AND MAGNETISM. 

299. What constitutes a good lightning rod? 

A continuous metal rod or strip, terminating below in 
earth that is always moist and above in a point that is 
always sharp. 

300. How may the sharp tip he best secured ? 

By the use of some metal that corrodes and fuses with 
great difficulty. 

301. Name such a metal. 

An alloy of iridium and platinum. 

302. How may a proper earth connection be secured ? 

By personal attention on the part of the house owner 
when the rod is put up. Agents have been known to 
run a rod into the ground three feet and charge for 
ten feet. Subsequent inspection is difficult and such 
a rod is dangerous. 

303. What is the use oj the glass insulators that cotnmonly sup- 

port lightning rods f 
To facilitate the sale of the rod. For any other purpose, 
they are undesirable. 

304. Who discovered the identity oJ lightning and electricity f 
Benjamin Franklin, in his famous kite experiment. 

305. Who invented lightning rods f 
Benjamin Franklin. 

306. What is a thermo-electric pair ? 

Two bars of dissimilar metals (generally antimony and 
bismuth) soldered together and with their free ends 
joined by a wire to make an electric circuit 

307. What is its electric use ? 

When the junction of the two bars is heated or cooled, a 
current of electricity is sent through the circuit. 

308. What is the direction of the current ? 

When the junction is heated, the current flows across it 
from bismuth to antimony ; when it is cooled, the cur- 
rent flows in the opposite direction. 

309. What is a thermo-electric pile ? 

A combination of thermo-electric pairs in series, so 
arranged that the alternate junctions constitute one 
face of the pile. 



ELECTRICITY AND MAGNETISM, 107 

310. For what is the thermo-pile chiefly used ? 

For detecting changes of temperature at the exposed 
face of the |)ile. This change develops an electric cur- 
rent which is detected by a galvanoscope or measured 
by a galvanometer. The apparatus may be made very 
sensitive, much mofe so than the mercury thermom- 
eter. 

C — Ei*ECTRiCAu Measurements. 

311. To what is the wonderful recent advance in electrical 

science and art largely due f 
To the adoption and general use of electrical measure- 
ments. 

312. Name the practical electrical units most used. 
Ohm, volt, ampere, and watt. 

313. How are resistances measured ? 

By comparison with the resistances of standard coils of 
wire made for that purpose. 

3 14. What are such coils called ? 

Resistance coils. A series of such coils constitutes a 
resistance box. 

315. What method of measuring resistance is most used f 
That known as the VVheatstone bridge, for the description* 

of which the pupil is referred to some larger work. 

316. What is fneant by the internal resistance of a voltaic cell f 
The resistance offered by the liquids of the cell and by 

the hydrogen that sometimes adheres to the negative 
plate. 

317. How may this internal resistance be reduced ? 
By making the plates larger. 

318. Why? 

Because this increases the sectional area of the liquid 
conductor that lies in the circuit. 

319. In what other way may it be reduced? 
By bringing the plates nearer together. 



io8 ELECTRICITY AND MAGNETISM, 

320. Why? 

Because this diminishes the length of the liquid con- 
ductor that lies in the circuit 

321. Upon what does the E. M. F. 0/ a voltaic cell depend ? 
Upon the nature of the liquids and plates, and not upon 

their size. 

322. Can E. M. F. be measured f 
Yes. 

323. In what way ? 

By comparison with the known E. M. F. of the current 
of a standard cell or by the use of an instrument called 
a volt-meter. 

324. How is current strength or amperage measured? 
Generally by the use of a galvanometer. Galvanometers 

used for such a purpose are called ammeters. 

D— ApPI^ICATIONS of E1.ECTRICITY. 

325. How is an electric current used for illuminating pur- 

poses ? 

By the introduction into the circuit of a resistance that 
transforms the energy of electricity into the energy of 
heat. The heat raises part of the conductor to such a 
high incandescence that it becomes luminous. 

326. What are the two classes of electric lamps ? 
Incandescence and arc. 

327. What is an incandescence lamp ? 

One in which a continuous conductor of high resistance 
is used for developing heat. Thus, a piece of fine 
platinum wire may be introduced into the circuit of a 
dynamo; this wire would become intensely hot and 
emit a beautiful light. 

328. Is platinum thus used ? 

Not now. Its use marked one of the early stages in the 
evolution of the modern electric lamp, but it was 
found practically impossible to prevent overheating 
and consequent fusion of the wire. Hence platinum 
was abandoned and a carbon (charcoal) filament 
adopted in its place. 



ELECTRICITY AND MAGNETISM, 109 

Will not the carbon melt f 

Carbon has never been melted. But at the high temper- 
ature employed, it will burn. 

How is combustion of the carbon avoided? 
By enclosing it in an air-tight glass bulb from which the 
air has been pumped. 

How are incandescence lamps generally placed in the cir- 
cuit f 
In parallel. 4 

What is an arc lamp f 

One in which two carbon rods are placed end to end in 
the circuit of a dynamo. The passage of the current 
operates a mechanism by which the carbon tips are 
separated about an eighth of an inch, thus forming a 
eap across which the current has to flow. This gap 
introduces such a high resistance that the carbon tips 
become highly incandescent. 

What is the space between the incandescent carbon tips 

called? 
The voltaic arc. 

By who7n was the voltaic arc first produced ? 
By Sir Humi)hre^ Davy, early in the 19th century, at the 
Royal Institution in I^ondon. 

In what consist the chief improvements in the arc electric 
light since his day ? 

In better means for producing the electric current (Davy 
used an enormous voltaic battery) and in better devices 
for regulating the distance between the carbon tips. 

How are arc lamps generally placed in the circuit ? 
In series. 

What is electrolysis ? 

The chemical decomposition of a compound, called an 
electrolyte, by means of an electric current 

In what condition must an electrolyte be that it may thus 

be decomposed ? 
In the liquid condition. 



no ELECTRICITY AND MAGNETISM. 

^ 1_ 

339. What are the products of electrolysis called? 
Ions. 

34D. What is an anion ? 

The ion that goes to the + electrode. 

341. What is a cation ? 

The ion that goes to the — electrode. 

342. What is the + electrode sometimes called? 
I An anode. 

343. What is the — electrode sometimes called? 
A cathode. 

344. Nafne som^ important applications of electrolysis, 
Electrotyping and electroplating. 

345. In these processes, how is the metal carried ? 

With the current through the liquid bath. It is, there- 
fore, deposited on the -• electrode. 

346. Substances that thus accumulate at the negative electrode 

in electrolysis are how designated f 
As electro-positive. 

347. Substances that similarly accumulate at the positive 

electrode are how designated ? 
As electro-negative. 

348. What are the electro-chemical relations of anions and 

cations ? 
Anions are electro-negative and cations are electro- 
positive. 

349. What ions are yielded by the eleftrolysis of zinc sulphate 

{white vitriol) ? 
Zinc and sulphuric acid. 

350. Do not these substances form the essentials of a voltaic 

battery? 

They do. 

351. Do they not tend to reunite by virtue of their chetni^ai 

affinity ? 
They do. 



ELECTRICITY AND MAGNETISM, in 

352. What is the effect of this tendency in the electrolysis of zinc 

sulphate ? 
To produce a current acting in opposition to the electro- 
lyzing current 

353. What is the E. M. F, of this counter current called f 
The ^. M. F. of polarization. 

354. What is the universal tendency of the E. M, F, of polariza- 

tion ? 

To counteract the E. M. F. of the original current 
Sometimes it overpowers it and reverses the current 

355. Is this tendency noticed with ions other than zinc and 

sulphuric acidf 
The electrolysis of any electrolyte develops an E. M. F. 
of polarization. 

356. In the electrolysis of water^ what is the measure of the E, 

M. F, of polarization ? 
About 1.45 volts. 

557. Can water then he decomposed by a single cell having an 
E, M, F, of one volt ? 
No. The E. M. F. of the electrolyzing current must ex- 
ceed 1.45 volts to overcome the E. M. F. of polariza- 
tion. 

358. In what case is the E, M. F. of polarization zero ? 
When the anode and the kathion are of the same metal. 

359. In the electrolysis of copper sulphate {blue vitriol)^ the 

cation is copper. Wilt a single cell with an E, M. F. 
of one volt serve for the decomposition of copper sulphate 
when the anode is a copper plate ? 

Yes, for the E. M. F. of the cell is greater than the E. 
M. F. of polarization. 

360. What sources of electric currents are chiefly used in elec- 

trolysis ? 
Voltaic batteries on the small scale, and specially con- 
structed dynamos on the large scale. 

361. What is a secondary battery f 

A device for using the E. M. F. of polarization for the 
producing of an electric current 



112 ELECTRICITY AND MAGNETISM, 

362. By what other name is it also knovm f 
A storage battery, or accumulator. 

363. Why a storage battery ? 

As the ions do not reunite when the circuit is open, the 
energy of the chemical affinity of the separated ions 
may be held subject to demand, producing an electric 
current whenever the circuit is closed. 

364. What is the process of operating a storage battery f 
Send a current of electricity through it for the separa- 
tion of the ions. Disconnect it from the electric gen- 
erator and leave the circuit open until a current is 
wanted. When the circuit is closed, the recombination 
of the ions develops an electric current in some degree 
the equivalent to the original electrolyzing current. 

365. What is the real difference between a voltaic battery {i. e.^ 

a primary battery) and a secondary battery ? 
In the primary battery, the product of the chemical 
action is not easily restored to its original form by 
electrolysis, while in the secondary batter}^ it may be 
so restored. 

366. What metal is generally used in a primary battery f 
Zinc 

367. What metal is generally used in a secondary battery ? 
Lead. 

368. What are the chief defects of the storage battery f 

Its great weight and cost, and the fact that by repeated 
oxidation and deoxidation, as the battery is successively 
charged and discharged, the lead plates disintegrate. 

369. Mention some of the familiar uses of electromagnets. 
The electric- telegraph, hotel annunciators, fire and burg- 
lar alarms, electric bells, clocks, etc. They are almost 
innumerable. 

370. State the principle of the etectric-telegraph. 

When the circuit is closed, au electric current flows 
throup^h an electromagnet, energizing it and thus 
enabling it to attract an iron bar called the Armature. 
When the circuit is broken, the current ceases to flow 



ELECTRICITY AND MAGNETISM, 113 

and the magnet loses its power and thus drops its 
armature. The circuit may extend a great distance 
from the magnet to an operator who opens and closes 
the circuit at will and thus works the armature that 
may be miles away. This enables a series of signals 
constituting the telegraphic alphabet 

371. In long distance telegraphy ^ what constitutes the circuit ? 

A carefully insulated wire with the telegraphic instru- 
ments constitutes the direct line. The ends of this 
line are connected with the earth which constitutes 
the return. 

372. How are the signals interpreted ? 

Sometimes they record on a moving slip of paper a series 
of dots and dashes which constitute the letters of the 
alphabet, and may be read like any other style of let- 
ters (script, Roman or italic,) by those who are familiar 
with it. 

373. What is such a recording instrument called ? 
A register. Registers are now seldom used. 

374. What is the instrument used by the sending operator for 

m^aking and breaking the instrument called f 

A key. 
375- ^^ what other way are the telegraphic signals interpreted f 
By the clicking of the armature. An operator who so 
interprets is said to " read by sound." 

376. What is his receiving instrument called f 

A sounder. It is much more simple in construction than 
a register. 

377. What is a relay ? 

A delicate electromagnet in the circuit of the main line, 
the vibrating armature of which operates as a key to 
make and break the circuit of a local battery, the local 
circuit carrying the sounder for the operator at that 
station. -^ 

37a. What is a repeater f 

A relay with its local circuit extended to a great distance, 
and with corresponding increase of the local battery. 



TI4 ELECTRICITY AND MAGNETISM. 

379. Illustrate the use of these instruments. 

Amain circuit might extend from New York to Cleveland. 
As the resistance of the line would be great, the cur- 
rent might be too weak to operate a series of sounders, 
one at every station on the line. Therefore, each sta- 
tion has its local battery and local circuit, the sounder 
being in the local circuit These local circuits are 
made and broken, (and their sounders therefore oper-, 
ated) by the relays, each of which is part of the main 
circuit. If the local circuit at Cleveland is magnified 
until it extends to Chicago, then the relay at Cleveland 
becomes a repeater and the Cleveland local circuit 
becomes, in fact, another main circuit which, in turn, 
may have its local batteries, circuits and sounders at 
every station between Cleveland and Chicago. Thus 
the message sent by the operator at New York may be 
read by the operator at each station on the line between 
New York and Chicago. 

380. What is duplex telegraphy ? 

A system by which a message may be sent each way at 
the same time on a single wire. 

381. What is quadruplex telegraphy f 

A system by which two distinct messages may be sent 
each way at the same time on a single wire. 

382. What are the speed limits of simple telegraphy ? 

A good operator may send or receive 40 or 50 words a 
minute. 

383. What about the higher achievements in the matter of speed? 
It is said that by the use of improved Sj^stems and appa- 
ratus, more than 1,500 words per minute have been 
sent over a single wire. 

384.. State one difficulty in constructing an ocean cable. 

The insulation of the wire must be perfect at every point, 
for the surrounding water is/a good conductor and a 
leak would be fatal to its suc^cess. On the other hand, 
a land line needs insulators only at the points of sup- 
port, for the surrounding air is of itself an admirable 
insulator. 

385. State another obstacle to trans-oceanic telegraphy^ 



ELECTRICITY AND MAGNETISM, 115 

Relays, repeaters, local circuits, etc., nre out of the 
question. Communication between termiui must be 
direct This implies feeble currents and extremely 
sensitive apparatus, like the mirror galvanometer in • 
stead of tho rcgibler or souuder. 

3S6. State another hindrance to trans oceanic telegraphy ? 

A submarine cable constitutes a huge electric condenser 
with the water as the outer coat, the conducting wire 
as the inner coat, and the insulating gutta-percha as 
the dielectric. The necessity of charging or discharg- 
ing tliis condenser for each signal sent interferes 
sadly with rapidiiy of operation. 

387. What is a microphone ? 

An instrument for augmenting small sounds. 

388. What is a telephone? 

An instrument for the transmission of articulate sound 
to a distant po ut by means of electric currents. 

389. Describe the action of the Dell telephone. 

Sonorous air waves create vibrations in a thin iron disk 
opposite the end of a b ir magnet. The vibrations of 
the iron disk create ^ariations in the intensity of the 
magnetism of the bar and thus induce electric cur- 
rents in a helix surrounding the end of the bar. These 
induced currents are carried by the wire continuation 
of the helix to a similar instrument at a distance. 
There the currents vary the magnetism of the steel bar 
and thereby set up vibrations in the iron disk which in 
turn sets up air waves comparable to the original waves 
at the first station. 

390. Is a voltaic battery needed with the apparatus as described ? 
No. Th ♦ currents are wholly induced currents developed 

by the action of the iron disk upon the magnetism of 
the steel bar magnet. The arrangement is the same 
at both stations and the apparatus works in either 
direction. 

391. How is this arrangement modified in practice f 

The sound waves set up by the voice of the speaker are 
directed to the opening of an instrument called a 
transmitter. They produce a varying pressure upon a 



xi6 ELECTRICITY AND MAGNETISM, 

carbon button placed in the circuii of a local voltaic 
battery. This varying pressure produces a varying 
resistance in the button and thus a varying current in 
the local circuit. This varying current flows through 
the primary of a small induction coil in the transmit- 
ter, and thus produces in the secondary the induced 
currents that are transmitted through the main circuit 
to the other station, where they produce sonorous 
waves, as previously described. 

392. Do sound waves traverse the wire between the two stations ? 
No. Sound/ waves at one station produce electric cur- 
rents. These currents pass along the wire to the other 
station and there produce a set of sonorous waves 
similar to the original ones, but wholly distinct from 
them. 

393. What is a telephone exchange ? 

A central office, with wires running to the houses of the 
several patrons of the exchange. Any patron can, on 
call, be connected at the exchange with the line of any 
other patron, to the great convenience of all. 

394. State one effect of the oscillatory nature of the electric dis- 

charge. 
The production of electric waves in some medium, prob- 
ably the ether. 

395. What is known pf these waves? 

They have the transmittive properties of radiant energy, 
and probably are radiant energy of great wave length. 
The recent experiments of Hertz and Tesla show that 
they may be reflected, refracted and polarized the same 
as the short-wave energy that is generally recognized 
as radiant energy, of which light may be taken as a 
type, and that they have the same velocity as light, and 
that the indices of refraction are the same for these 
waves as they are for light. 

396. To what conclusion do these facts tend? 

To the support of what is known as the electromagnetic 
theory of light, from which it follows that the produc- 
tion of electric oscillations of sufficiently minute wave- 
length should yield light; i. e., that light is an elec- 
trical phenomenon, and that optics is a department 0/ 
electricity. 



ELECTRICITY AND MAGNETISM, 117 

397. What is the source of most of the energy available by man f 
The sun. Even the energy of animals is due to their 

food and this in turn to solar energy. *• It has been 
estimated that the heat received by the earth from the 
sun each year would melt a layer of ice over the entire 
globe a hundred feet in thickness. This represents 
energy equal to one horse-power for each fifty square 
feet of surface." 

398. What is meant by the dissipation of energy f 

The tendency of energy to run down to the low level of 
heat uniformly diffused throughout the universe ; and 
equality of temperature; a uniformity of molecular 
motion. 

399. What is meant by the transformation of energy? 

The changes from one variety to another, as from kinetic to 
potential, from mechanical to molecular, vice versa^ etc 

40a Give an illustration of such transformation. 

Molecular energy of position is transformed by com- 
bustion into neat in a furnace. Heat is transformed 
into mechanical energy in the engine. This is trans- 
formed into electric energy by the dynamo. This is 
transformed into heat and light by the electric lamp. 
Here we have the same energy appearing in various 
forms. The potential energy with which we started 
was due to solar energy. Hence the electric light is 
oft transmuted sunshine. 

401. What is meant by the conservcUion of energy f . 

The constancy of the total amount of energies of all 
kinds in the universe. There may be more heat energy 
to-day ; more electric energy to-morrow ; more water- 
power the next day. There may be more energy in 
the sun to-day ; more in the earth to-morrow (or yes- 
terday). But taking energy and the universe as. enti- 
ties, the quantity of the former is constant 



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By JULIA M. DEWEY 

Author of "How to Teach Manners" and "Ethia for Homo 
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Adapted to Grammar Schools, High Schools 
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LIST OF CONTENTS 



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Lesson n — Duties to the 
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Lesson V^- Exercise, Re- 
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Lesson VI — Industry 

Lesocn VII— Economy 

Lesson VIE— Honesty 

Lesson DC — Truthfulness 

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Lesson XI— Order 

Lesson XII— Courage 

Lesson Xm— Love 



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Lesson XXn— The Scate 

Lesson XXm — Self Cul- 
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Lesson XXIV— Nature 

Lesson XXV— Art 

Lesson XXVI— Reading 



Lessons on Manners 

By JUUA M. DEWEY 

Author cf "How to Teach Manners" and "Ethics for Home 
and School " 

Adapted to GrofHmar Schools, High Schools 
and Academies 



LIST OF CONTENTS 
Lesson I — ^Manners in General 
Lesson II — ^Manners at Home 
Lesson in — ^Manners at School 
Lesson IV— Manners on the Street 
Lesson V— Manners at the Table 
Lesson VI — Manners in Society 
Lesson VII— Manners at Church 
Lesson VIII — ^Manners Toward the Aged 
Lesson DC — ^Manners at Places of Amusement 
Lesson X — ^Manners in Traveling 
Lesson XI— Manners in Places of Business 
Lesson XQ — Manners in Making and Receiving Gifts 
Lesson XIII— Manners in Borrowing 
Lesson XTV— Manners in Correspondence 



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Style, Soul of Words, Early English, Language-Change, 
Poetry, Syntax, Variations inWord-Meanings, Distinctions, 
Origin of Language, Old Celtic Friends, English Ortho- 
graphy, Words Changed Since Shakespeare, Commonplace 
Poetry, Aborigines. Reads wittitke fascination of romance. 

The Religion of Beauty 

Second Edition, Revised and Enlarged, 

This is the autobiography of a Soul glad of life — one 
who finds riches in the possessions of others and, above 
all, a golden wealth ia man's Impersonal Estate—in sky 
and STAR, SUN and city, the sea and the open world— 
one who finds the Religion of Beauty in all things, and 
reveals the secret whereby all who will may dig up 
"real wealth" while having a good time. 

Taormina 

Illustrated. New Historic Matter. 
History is told here with Maeterlinck's charm of style; 
scenes are painted with the power and beauty of Heam ; 
philosophy is unconsciously brought forth from events. 
Greek l^end weaves a necklace of imagery which holds 
STNA in its clasp. Martial echoes mingle with the voices 
of ancient poets, the murmur of the Ionian Sea and of 
olive leaves in sunny Sicily, 



To avoid fine, this book should be returned on 
or before the date last stamped below 



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hi. 

550-1 
05S