4:62 On some Experiments with Rontgen 9 s Rays. 



was made the cell had by no means recovered four hours 

 afterwards. 



The resistance of the sensitive cell employed was reduced 

 from 1209 ohms to 1185 ohms in 20 seconds by the radiation 

 under the conditions mentioned. The testing battery consisted 

 of two "Obach" cells, and the bridge was made up of two equal 

 arms of 1000 ohms each — the selenium and the variable arm. 

 We mention this in case it may ever turn out that the effect 

 depends on the testing current. 



With regard to the electromotive force which it was supposed 

 might be set up. The cell was kept at rest and undisturbed 

 for three days before the final trial ; it was placed three 

 centimetres from the active tube, which was, as before, in a 

 metallic box, together with the coil. The tube was shut off 

 from the cell by an aluminium plate '7 mm. thick. 



A very sensitive high resistance galvanometer in our 

 possession, which has been described in a paper read before 

 the Royal Society but as yet unpublished, was employed to 

 test for any electromotive force which might be set up. An 

 exposure to the radiation was made while the cell was in 

 series with the galvanometer, and it was found that the cell 

 always exhibited a small electromotive force whether it was 

 exposed to the rays or not. This prevented the test from 

 being very sensitive, but in no case was any electromotive 

 force attributable to the radiation discovered, though if a 

 a voltage of 10~ 7 volts had come into operation its effect could 

 probably not have escaped observation. 



At the time these experiments were made we were unin- 

 formed as to the discharging action of the rays, which has 

 since been so copiously studied. As soon as we saw an 

 account of some of this work we felt that the change of 

 resistance of the selenium cell was no longer an isolated 

 phenomenon to be worked out by itself, but must be studied 

 in conjunction with the similar phenomena observable in other 

 substances, and it is for this reason that the experimental 

 work was not extended so as to include other cells. 



These notes may, perhaps, be summed up as follows : — 



(1) It is easy to make a Rontgen tube of great activity by 

 the most elementary glass-blowing. 



(2) The Rontgen radiation does not consist in the pro- 

 jection of gaseous matter, or if it does the amount of such 

 matter involved is extraordinarily small. 



(3) The Rontgen radiation does not consist in the projec- 

 tion of aether streams having a velocity above a couple of 

 hundred metres per second : this is true whether the radiation 

 takes place in air or in benzene. 



(4) The properties of aether regarded as determining the 



