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Full text of "On the Discharge of Electricity Produced by the Rontgen Rays, and the Effects Produced by These Rays on Dielectrics through Which They Pass"

274 Prof. J. J. Thomson. On the Discharge of [Feb. 13, 



III. " On the Discharge of Electricity produced by the Rontgen 
Rays, and the Effects produced by these Rays on Dielectrics 
through which they pass." By J. J. Thomson, M.A., F.R.S., 
Professor of Experimental Physics, Cambridge. Received 
February 7, 1896. 

The Rontgen rays, when they fall upon electrified bodies, rapidly 
discharge the electrification, whether this be positive or negative. 
The arrangement} I have used to investigate this effect is as follows : — 
The Ruhrnkorff coil and the exhausted bulb, used to produce the rays, 
are placed inside a large packing case covered with tin plate ; this is 
done to screen off from the electrometer any electrostatic disturbance 
due to the action of the coil. The needle of the electrometer is sus- 
pended by a quartz fibre ; thus, as there is no magnetic control, 
the needle of the electrometer is not affected by changes in the 
magnetisation of the core of the coil. 

The exhausted bulb is placed so that the phosphorescent part of it 
is about 1^ inches from the top of the box, and a hole about an inch in 
diameter is cut in the lid of the box just over the bulb,, so as to allow 
the rays to emerge from the box ; a thin plate of either aluminium or 
tin-foil is used to cover up the hole. The electrified plate, which 
is a little larger than the hole, is placed outside the box about 
2 inches above the hole in the lid", so that the Rontgen rays which 
passed throogh the hole fall upon the plate. This plate is kept 
permanently connected with one of the quadrants of a quadrant 
electrometer; the greatest care is taken with the insulation of this 
plate and of the quadrants of the electrometer. The insulation was 
so good that there was no appreciable leak when the coil was not in 
action. The following is the method of making the experiments : 
The two pairs of quadrants are connected together and the plate 
charged to a high potential by an electrophorus or by temporary 
connection with a large battery of small storage cells. All the quadrants 
of the electrometer are now at the same potential. The two pairs of 
quadrants are now disconnected; if the' insulation is good the 
potentials will remain the same, and there will be no deflection of the 
electrometer; in our experiments the leak is so small that under 
these circumstances the movement of the spot of light is hardly per- 
ceptible. If, now, the Rontgen rays are directed on to the plate a 
violent leakage of electricity from the plate occurs, the potential of 
the quadrants connected with the plate changes, and in a few seconds 
the spot of light reflected from the mirror of the electrometer is 
driven off the scale. This leakage of electricity occurs whether the 
plate is positively or negatively electrified ; if the plate is uncharged 



189 6. J Electricity produced by the Rontgen Rays, 8fc. 275 

to begin with, I have not been able to detect that any charge is ac- 
quired by the plate by exposure to these rays. When the potential 
to which the plate is raised is high, the leakage from the plate is a 
most delicate means of detecting these rays, more so than any photo- 
graphic plate known to me. I have found these rays produce dis- 
tinctly perceptible effects on a charged plate after passing through 
a zinc plate a quarter of an inch thick. The charged plate and 
electrometer are much more expeditious than the photographic plate 
and more easily adapted to quantitative measurements. 

To determine how the radiation of the Rontgen rays depended upon 
the degree of exhaustion of the bulb, the bulb was kept in connection 
with the pump and the leakage was observed at different degrees of 
exhaustion ; no leakage could be detected until the pressure was so 
low that phosphorescent patches appeared on the bulb, and, even after 
the phosphorescence appeared, the leakage was small as long as there 
was any considerable luminosity in the positive column; it was not 
until this had almost disappeared that the leakage from the charged 
plate became rapid. 

If the greatest sensitiveness is required, it is, of course, advisable 
to charge the plate as highly as possible. The leakage due to the 
rays, however, occurs when the potential of the plate does not exceed 
that of the tin-plate cover by more than 3 or 4 volts, and I have not 
yet met with any phenomena which suggest that there is a lower 
Jimit of potential difference below which leakage does not take 
place. 

This leakage differs from that produced by ultra-violet light, the 
laws of which have been unravelled by Elster and Geitel, in several 
essential features, in the first place ultra-violet light only discharges 
a negative charge, while the Rontgen rays discharge both positive 
and negative. Again, the effect of ultra-violet light is only con- 
siderable when the electrified body is a strongly electro-positive 
metal with a clean surface. The effects of the Rontgen rays are, on 
the other hand, very marked whatever the metal, and take place 
when the electrified plate is surrounded by solid or liquid insulators 
as well as when surrounded by air. I have embedded the plate in 
solid paraffin wax, in solid sulphur, placed it inside a lump of ebonite, 
wedged it in between pieces of mica, and immersed it in a bath of 
paraffin oil ; in each of these cases, though the insulation was practi- 
cally perfect when the insulator was not traversed by the Rontgen 
rays, and the potential of the plate differed from that of the metal 
covering of the box by from 10 to 15 volts, yet, as soon as the 
Rontgen rays passed through the insulator, the charge of the metal 
plate leaked away. I have found that the electricity leaks from the 
plate even when the space between it and the nearest conductors con- 
nected to earth is entirely filled with solid paraffin ; hence we conclude 



276 Prof. A. Gamgee. Absorption of the extreme [Feb. 13, 

that when the Rontgen rays pass through a dielectric they make it 
during the time of their passage a conductor of electricity, or that 
all substances when transmitting these rays are conductors of electricity . 
The passage of these rays through a substance seems thus to be ac- 
companied by a splitting up of its molecules, which enables electricity 
to pass through it by a process resembling that by which a current 
passes through an electrolyte. By using a block of solid paraffin in 
which two pairs of electrodes are embedded, the line joining one pair 
being parallel, that joining the other pair perpendicular, to the 
Rontgen rays, which were kept passing through the block, I found 
that there is but little difference between the rate of leakage along 
and perpendicular to the rays. 

I have much pleasure in thanking Mr. J. A. McClelland, of Trinity 
College, Cambridge, and Mr. E. Everitt for the assistance they have 
given me in carrying out these experiments. 



A telegram from Professors Borgman and Gerchun, of St. Peters- 
burg, forwarded by the editor of the 4 Electrician,' to the effect that 
Rontgen rays discharged electricity, and a letter from Professor 
Lodge to the effect that he had definitely ascertained that the phos- 
phorescent glass was the source of the radiation of Rontgen rays, and 
that the radiation starts in all directions, and not normally only from 
the glass, were read. 



IV. " On the Absorption of the extreme Violet and ultra-Violet 
Rays of the Solar Spectrum by Haemoglobin, its Com- 
pounds, and certain of its Derivatives." By Arthur 
Gamgee, M.D. ; F.R.S., Emeritus Professor of Physiology 
in the Owens College, Victoria University. Received 
February 11, 1896. 

In the year 1878 the late Professor J. L. Soret, of Geneva, in his 
first memoir on the absorption of the ultra-violet rays of the spec- 
tram by diverse organic substances,* announced the fact that diluted 
blood, when examined with the aid of a spectroscope provided with 
a fluorescent eye-piece, presented in the extreme violet, between 
Fraunhofer's lines G and H, an absorption band which appeared to 
him to be slightly shifted towards the less refrangible end of the 
spectrum when the blood solution was saturated with carbonic 
oxide. Soret subsequently f confirmed the accuracy of the above 

*' J. L. Soret, " Recherches sur 1' Absorption des Kayons ultra -violets par diverses 
Substances," ' Archives des Sc. Phys. et Nat.,' vol. 61 (Greneva, 1878), pp. 322—359. 
f Soret, 'Archives des Sc. Phys. et Nat.,' vol. 66 (1883), pp. 194, 195, and 204.