20 Messrs. Stroud and Henderson on the 



Kohlrausch's method of measuring trie conductivity of elec- 

 trolytes by the use of alternating currents and a telephone is 

 superior to any method at present in use in which con- 

 tinuous currents are employed. That there are difficulties in 

 the use of this method, arising from self-induction, capacity, 

 &c, is admitted by Kohlrausch himself. Ever since the intro- 

 duction of this method in 1875 physicists have not ceased to 

 try to improve the continuous current methods, with the 

 desire unquestionably of employing a galvanometer instead of 

 a telephone as the indicating apparatus. In the method used 

 by Fitzpatrick*, where the alternating currents are confined 

 to the four arms of the bridge, we have an example of a suc- 

 cessful attempt to utilize the advantages of alternating currents 

 while dispensing with the telephone in favour of the more 

 satisfactory galvanometer. 



In the face of all the experimental work done in the past 

 it may seem a bold thing to say that if the proper conditions 

 are only satisfied, continuous currents are preferable in every 

 way to alternating currents for the measurement of electro- 

 lytic conductivity, but such we believe to be the fact. 



The idea underlying the method to be described struck one 

 of us some six months ago. This idea was to obviate the 

 detrimental effects of polarization in the electrolytic cell by 

 inserting a second cell with the same size of plates &c, but 

 with a very different length of electrolytic conductor in the 

 corresponding arm of a Wheat-stone bridge circuit. 



The notion of employing such a balancing cell we subse- 

 quently found was suggested in 1877 by Kohlrausch and 

 employed by Tollinger f. 



The method used by Tollinger consisted in having an 

 electrolytic cell in each arm of the bridge, but the method of 

 finding the resistance was to obtain a balance for a particular 

 position of the electrodes in one cell, then to diminish the 

 distance between them by a measured amount and increase 

 the resistance in that arm so as to restore the balance. A 

 conceivable source of error arises in consequence of the 

 motion of the electrode through the liquid producing a possible 

 alteration in its polarization. 



The results of Tollinger's work show that resistances 

 measured by his continuous current method were 0"6 per 

 cent, lower on the average than the same resistances measured 

 by the alternating current method. 



ElsasJ has also employed the balancing cell method. He 



* Brit. Assoc. Eep. 1886, p. 328. 

 t Wied. Ann. vol. i. 1877, p. 510. 

 % TVied. Ann. xliv. p. 666. 



