The lonisatioii of Dilute Solutions at tlie Freezing Point. 195 



The electrical resistance measurements were made by the method 

 of alternating currents, but the usual telephone indicator was replaced 

 by a D'Arsonval galvanometer. This was done by using a revolving 

 commutator, which, timied by a hand-wheel and cord, alternated the 

 connections of the bridge ^ith the battery and with the galvanometer 

 simultaneously. The usual AATieatstone-bridge method could then be 

 used, and measiuements obtained in the same cell of resistances 

 varying from 10 to 50,000 ohms, the acciu-acy throughout being at 



Fig. 3. 



least 1 in 1000. The method eliminates several troublesome periodic 

 disturbances, and, in this form, seems entirely satisfactory. The 

 surface of the electrodes was platinised in the usual manner, but was 

 afterwards heated to redness. This process gives a roughened plati- 

 niun surface of large area, which is less liable to absorb matter from 

 the solution than is the tmheated platinum black. 



The water used was thrice distilled, twice with alkaline perman- 

 ganate and once in a platinum stUl with a trace of acid potassium 

 sulphate. It had an average conductivity at 18' of about 0-9 x 10~i-^ in 

 C.G.S. units. 



