38 Mr. J. A. McClelland on the Conductivity of 



when B is to earth, and when it is charged to potential V; 

 t is equal to the length of the terminal B divided by the 

 velocity of the current past it which we measure. The 

 velocity v of the carrier under an electric force of 1 volt per 

 centimetre is therefore given by the above equation. 



In practice Y is made such as to diminish the rate of leak 

 from C by about one half. We can check the value of v 

 thus calculated by finding what value of V is just sufficient to 

 give no conductivity at C. 



This method gave the velocity of the carrier, 



•2 cm. per second, 



under a potential gradient of one volt per centimetre. A 

 large number of experiments were made, and the results of 

 different determinations agreed to less than 10 per cent. The 

 velocity of the stream past the terminal B was varied between 

 15 and 30 cm. per second in the different determinations, and 

 the potential of B varied from 1 to 4 volts. 



Another set of determinations of the velocity of the carrier 

 was made with a large cylindrical tube of 4 cm. radius, in 

 which were placed two terminals, B and C (as in the previous 

 case), the lower one (B) being 43 cm. long and "4 cm. radius. 

 This tube was placed above the flame, and the velocity of the 

 stream of gas up it was measured with a wind-gauge which 

 fitted into the tube. A determination with the apparatus 

 gave the following numbers : — 



Leakage from C : 

 200 divisions in 11 seconds when B is at zero potential, 

 200 „ „ 18 ,, when B is at 40 volts potential. 



The velocity of the stream past the terminal B was 122 cm. 

 per second. 



This gives the velocity of the carrier to be 



•22 cm. per second. 



The determinations with the two sets of apparatus agreed 

 as closely as could be expected from the nature of the 

 experiments. 



All the above determinations of velocity of the carrier 

 were made in the gas soon after leaving the flame and when 

 its temperature was about 200° C. This is important, as 

 appears from some work later on. 



This velocity is much less than that found for the carrier of 

 electricity in the case of a gas which has been exposed to 

 Rontgen rays. (See Rutherford, Phil. Mag. Nov. 1897.) 



If we regard the charge on the carrier as unalterable, we 



