90 On the Measuring of Electrical Conductivities. 



the result is 



or 



abk 

 where 



>=-^{*-!M\/l^»1)}. 



If 



it follows that 



e=0 if 1=0 and wi = 0, 

 e=J if Z=0 or m = 0, 



e= —1 if I and m are uneven. 



€= + 1 in all other cases. 



b = a, 

 o?k. p = c-a. 0*7272. 



In deducing this result it is sufficient to calculate twice the 

 sum of four terms. 



The derivation of the given expression for p. rests upon the 



c 



assumption that the ratios - and r can be regarded as infi- 

 nite ; in fact, however, very moderate values of these ratios 

 suffice to make that expression very nearly correct. It is so 

 even in the case that 



-L C 



a=zb= 2 



In this case the value of p is particularly easily ascertained. 

 According to one of the equations set up, p is then 



i r 00 



where the modulus t is the same for all three 6 functions; 

 iirt 



Id 



vrrt 

 namely -t-7 2 . Now we have, as is well known, 



1 (2i/+1)2tt2* 



and hence follows 



