124 Prof. J. Zeleny on the Ratio of the Velocities of the 



forces must be used in the two eases respectively, in order to 

 give the same charge to the plate, against the same blast, in 

 the same time. The two ionic velocities then bear the same 

 relation to that of the gas-stream, and to find the ratio of the 

 two velocities for the same electric force it is only necessary 

 to take the inverse ratio of the electric forces used in this 

 case. Thus, if U is the potential-difference between the plates 

 required to force a certain number of positive ions to the 

 plate A in a given time against the gas-current, and if V 

 must be used to drive the same number of negative ones under 

 the same conditions against the same blast, then if U' and V 

 are the experimentally determined potential gradients effec- 

 tive in bringing the ions to the plate A, in the two cases 

 respectively, 



uW = K, and vV' = K, 



where K is a constant depending upon the velocity and nature of 

 the blast used, and upon the rate at which the ions are brought 

 to the plate. 

 Hence 



u ~V ' 



U' 

 The ratio -^ as found experimentally (§11) for the appa- 

 ratus used (§ 2) is nearly equal to ^, the ratio of the potentials 



used, the deviation being, for the most part, due to the differ- 

 ences in the distribution of the free charges in the two cases, 

 caused by the difference between the velocity of the two ions. 



§ 2. The Apparatus. 



The apparatus used for carrying out this method was arranged 

 as is shown in fig. 2. 



P and Q are two 9 centim. square brass plates. To 

 openings in their centres are attached the brass tubes R and 

 S of 3*6 centim. diameter, which are fastened to the board N 

 so that the two plates are parallel and 4 centim. apart. The 

 space between the plates is made to form a closed box by 

 covering the top and sides by glass plates which are blackened 

 uniformly on the inside surfaces with lamp-black for preventing 

 the accumulation of static charges. 



K is a wire gauze covering completely the opening in Q, 

 and T is an insulated circular wire gauze with 7 meshes per 

 centimetre, which is placed in, but does not quite fill, the 

 opening in the plate P. The wire that supports it leads 

 through the ebonite plug M to a pair of quadrants of the 



