1915] on Experiments on Slow Cathode Rays 409 



potential difference of 1000 volts. The intensity of the photogTaphs 

 was not diminished when all the radiation which fell on the plate had 

 passed through this strong electric field which would have stopped 

 any charged particles. With regard to the penetrating power of this 

 radiation, I have been able just to detect the photographic effect 

 behind a film of collodion thin enough to show the colours of their 

 plate, when the cathode rays had energy corresponding to 40 volts. 

 ^yhen the energy corresponds to 80 volts the effect behind the 

 collodion, and also behind a film of paraffin wax 1 /x thick, is quite ap- 

 preciable, while after 200 volts there is very considerable penetration 

 of the paralfin and collodion films. 



This photographic method did not, however, lend itself easily to 

 quantitative measurements. For this purpose I made use of the 

 ionisation of gases produced by these soft Eontgen rays. The rays in- 

 stead of falling on a photographic plate passed through a thin window 

 into a tube leading into a chamber containing SO2 or some other gas 

 giving a large number of ions. The quantity of gas was sufficient to 

 absorb all the rays, so that the number of ions produced is a measure 

 of the energy which has passed into the ionisation vessel. Consider- 

 able difficulty was found in finding a suitable window, as the discharge 

 tube itself was a very good vacuum, while it was necessary to have a 

 finite pressure of gas in the ionisation chamber. The film had to be 

 stout enough to stand a difference in pressure of a few millimetres of 

 mercury on the two sides, and yet be thin enough to transmit the 

 very easily absorbed radiation. The difficulty was solved by a device 

 due to my assistant, Mr. E. Everett. Yery thin films of celluloid were 

 prepared by dissolving celluloid in amyl-acetate and pouring a thin 

 layer of the solution on liquid mercury ; this leaves the edges of the films 

 free, if they are held tight the film cracks as it dries. Films prepared 

 in this way were so thin that they showed the colours of thin plates. 

 The other method is to measure the energy of the radiation given off 

 from a target struck by the cathode rays. It is known that when 

 ultra-violet light or Rontgen rays fall on a plate of metal, the plate 

 emits negative electricity, and the quantity of this emitted is pro- 

 portional to the energy as long as the type of radiation is constant. 

 To apply this method the tube T, instead of leading to a camera, 

 lead to a spherical bulb, silvered on the inside, and at the centre of 

 the bulb there was an insulated plate of metal connected with a 

 Dolezalik electrometer ; the silvered bulb was maintained at a positive 

 potential of several hundred volts positive to earth, while the plate 

 of metal at the centre was, before the radiation fell upon it, at the 

 same potential radiation as the earth. When the radiation fell upon it, 

 it lost negative electricity, the amount of which could be determined 

 from the deflection of the Dolezalik electrometer. In order to stop 

 any charged particles passing from the discharge tube into the 

 silvered vessel, parallel plates of metal were placed in the tube T,. 

 and a potential difference of 600 volts was maintained between them. 



