CONDUCTION THROUGH GASES 145 



the cathode ray particles themselves could not be 

 investigated in this way, there seems no reason 

 to suppose that they are exceptions to a rule other- 

 wise universal. If, then, e is the same both for 

 gases and for liquids, m must be different ; the 

 cathode ray particle must have a mass which is 

 only the one eighteen-hundredth part of that of 

 the hydrogen atom. 



Similar values have been obtained for the mass 

 of the negative particles when produced in other 

 ways. In one case, that of the ions due to the 

 incidence at a low pressure of ultra-violet light 

 on metals, both e and ejni have been measured for 

 the same particles. A zinc plate is illuminated 

 with ultra-violet light, and placed opposite to and 

 parallel with a second metallic plate connected 

 with an electrometer, the gas surrounding the 

 apparatus being exhausted to a very low pressure. 

 An electric force is established between the two 

 plates, and the negative ions, produced at the zinc 

 plate, are by this force urged towards the second 

 plate. If no other agency were at work, all the 

 negative ions would reach the second plate, and 

 transfer their charges to the electrometer. Now 

 let us imagine that a magnetic force is applied at 

 right angles to the electric force and parallel to 

 the planes of the plates. The magnetic force will 

 deflect the negative particles from their original 

 straight course, and their path becomes a cycloid. 

 They travel out from the zinc plate, curve round, 

 and approach it again. If the second plate is 

 placed near enough to the first to intercept this 

 curved orbit, all the ions will still reach the plate 

 connected with the electrometer, and the rate at 

 which it gains negative electricity will not be 



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