740 Mr. A. E. Garrett on Electrical 



with the total distance between two parallel plates, the 

 current is given by the following formnla : 



•TT2 32 7T .„ 



where V = difference of potential between the plates in 

 electrostatic units, 

 R = velocity of ion in cms. per second for unit 



difference of potential per cm. 

 i = the current obtained due to the ions per unit of 



surface measured in electrostatic units, 

 I — distance between the plates, measured in cms. 

 In deducing this equation it is assumed 



(a) That the layer in which ionization occurs is very small 

 compared with I. 



(b) That the current i is only a very small fraction of that 

 obtained with voltages large enough to produce a saturation- 

 current ; i. e., so that for small changes of voltage, the current 

 varies as the square of the voltage. 



In my investigations of the velocities of the ions from the 

 iodides which I tested, I arranged my apparatus so as to be 

 able to make use of the above formula. The apparatus was 

 that shown in fig c 2. The glass tube E was about 50 cms. in 

 length and was heated in the centre. The whole was made 

 air-tight. 



The ions were driven from the lower electrode C, which 

 was connected to one pole of a battery of small storage-cells, 

 the other pole of which was earthed, on to the insulated 

 electrode B, which was connected with one pair of the 

 quadrants of a Dolezalek electrometer, the other pair of 

 quadrants being earthed. 



The capacity of the whole system was measured so that 

 the value of the current could be obtained in absolute units. 



It will be seen from the equation that the value of the 

 current i does not depend upon the number of ions present 

 but only upon their velocity, so long as the conditions specified 

 above are fulfilled. In order to do this a very thin layer of 

 the salt to be tested was placed on C, and the distance between 

 B and C was made equal to 2 cms. The variation of the 

 current with the potential was also tested from time to time, 

 in order to see whether the voltage applied was of the right 

 order of magnitude. 



The following results were obtained. They refer to the 

 positive ions obtained from the various materials named ; the 

 salts in each case being dry. 



Bismuth iodide, lead iodide, barium iodide, and calcium 



