Ionization of Platinum by Cathode Rays. 291 



which was afterwards traced to another cause ; it is not 

 necessary, and will not be used in future experiments. But 

 since it is a long and tedious business to change the action 

 of the platinum in the way which will be described, since this 

 operation had been performed before it was discovered that 

 the tube was not necessary, and since it appeared from a 

 comparison of results with and without the tube that its 

 presence does not make any difference to measurements of 

 the current received by P when A is positive, it was not 

 thought worth while to destroy and re-erect the apparatus. 



5. The measurements ma.de were those of the relation 

 between V! — V 2 , the speed of the rays striking P, and I, the 

 current flowing to P. If i x is the current carried by the 

 rays striking P ; i 2 that carried by the rays leaving it, then 

 1 = ^ — ? 2 . The " reflexion coefficient," R, in terms of which 

 most, observers have stated their results, is (i\ — i%)/ii- The 

 absolute value of R could not be estimated accurately, since 

 for the reason given i l could not be measured; but it will be 

 observed that, if v r e may assume that i l is constant^ the value 

 of I will always be proportional to I, and may be taken as a 

 measure of it. The justification for this assumption is that it 

 was certainly true (to any degree of accuracy that matters) 

 when T was not present, while the curve relating I and 

 (Vi— V 2 ) was the same whether T was or was not present. 

 Some estimate of the absolute value of R was made by taking 

 observations with A connected to P for such a value of 

 V]— V 2 that the reflexion is a minimum ; the current then 

 flowing to the electrometer is then ii~i 3 , where i 3 is the 

 current flowing from P to T. The value R'^I/^ — i 3 will 

 be greater than R ; but a comparison with the values obtained 

 by other investigators and with the apparatus without T, 

 shows that the difference is not important. For the minimum 

 value of R corresponding to incident rays of a speed just less 

 than 11 volts falling on " ordinary " metals, Gehrts * finds 

 values from 0*51 to 0*62; with the apparatus without T 

 I found from 0*1-5 to 0*55, and O50 in the apparatus with T. 

 My values doubtless tend to be less than those of Gehrts 

 because his metals were quite flat and highly polished, while 

 it was impossible for me to attain such a condition in a very 

 thin strip of metal supported by leads through which it is to 

 be heated. 



6. It has been said that V l — V 2 is the speed of the rays. 

 This statement is not quite accurate : it would be accurate if 

 it were certain that the only potential differences in the 



* Gehrts, Ann. d. Phys. xxxvi. p. 1003 (1911). 

 U2 



