ACCUEATE MEASUREMENT OF IONIC VELOCITIES, ETC. 459 



(3) The specific mobility (u or v) or the velocity which the ion would have under 



a potential gradient of 1 volt per centimetre if the salt were completely 

 dissociated. These are related to the actual mobilities as follows : 



U = au and V = av, 



a being the degree of dissociation of the salt. 



(4) The " lonen Beweglichkeit " (l a or l k ) of KOHLRAUSCH, which is given by the 



relation 



p. being the molecular conductivity. 



The use of the term " specific ionic velocity " for l a and l k is confusing, 

 and we suggest instead the name " specific ionic conductivity." 

 (5) The " ionic conductivity " for a given strength of solution can then be 

 conveniently represented by 



L re and L k , where L,, = a/,, and L* = //,. 



The actual mobilities U and V are obtained from the measured velocities In- 

 dividing the latter by the potential gradient in the measuring tube. This potential 

 gradient, II, is easily calculated as follows : According to OHM'S law the current, C, 

 flowing through the apparatus, C = H/>: C is measured, and /, or the resistance of 

 1 centim. of the liquid, is obtained from its specific conductivity, K, and the area of 

 cross-section, A, of the tube, v = 1/*A and hence IT = C/A. 



The actual mobilities of the ions of a number of salts have been measured, and the 

 results at 18 are contained in Columns 6 arid 9 of Table III, and, for comparison with 

 these, two sets of figures are given in Columns 7 and 10 and 8 and 11 respectively. 

 These figures are calculated from KoHLRAUSCH's conductivity data in the following 



&> 

 manner : 



The molecular conductivity 



^ 

 whence 



K = 



t) being the concentration in gramme equivalents per cubic centimetre. 



Now, K is the specific conductivity, or the quantity of electricity carried in one 

 second between two electrodes 1 centim. apart in a tube of 1 sq. ceutim. cross-section 

 under a potential gradient of 1 volt per centimetre. 



This obviously depends on (a x rj), or the number of ions present, on their specific 

 mobility (u and v), and on the quantity of electricity carried by one gramme ion, viz., 

 96540 coulombs. Hence 



K = 7?(-t-v) 96540 = 17 (U + V) 96540, 

 3 y 2 



