26 Introduction 



measurements of resistance obtained by different methods, than on the 

 evidence of any direct experiments. 



Some doubts, however, having been suggested with respect to the 

 mathematical accuracy of Ohm's law, the subject was taken up by the 

 British Association in 1874, and the experiments of Professor Chrystal, 

 by which the exactness of the law, as it relates to metallic conductors, was 

 tested by currents of every degree of intensity, are contained in the Report 

 of the British Association for 1876. 



The laws of the strength of currents in multiple and divided circuits 

 are accurately stated by Cavendish in Arts. 417, 597, 598. 



Cavendish applied the same method of experiment to compare the 

 resistance of the same liquid at different temperatures*, and he found 

 that "salt in 69 [of water] conducts 1-97 times better in heat of 105 than 

 in that of 58^." He also found that "the proportion of the resistance of 

 saturated solution and salt in 999 to each other seems not much altered 

 by varying heat from 50 to 95." 



Kohlrausch, who has made a most extensive series of experiments on 

 the resistance of electrolytes, gives results from which it appears that the 

 ratio of the resistances of salt in 69 at 105 F. and at 58! F. would be 

 1-59. He also finds that the temperature coefficient for solutions of salt 

 alters very little with the strength. See Note 33. 



Cavendish also tested the resistance of solutions of salt of strengths 

 varying from saturation to one in 20,000 of distilled water, and arrived at 

 the result, which Kohlrausch has shown to be nearly accurate, that for 

 weak solutions the product of the resistance into the percentage of salt 

 is nearly constant. 



Of all substances, that for which different observers have given the 

 most different measures of resistance is pure water. 



It has been found indeed that the presence of the minutest trace of 

 impurity in water diminishes its resistance enormously. Thus Kohlrausch 

 found that it was necessary to use water quite freshly distilled in platinum 

 vessels, for if placed in a glass vessel it rapidly diminished in resistance 

 by dissolving a minute quantity of the glass, and a few minutes exposure 

 to the air of the laboratory, by impregnating the water with a trace of 

 tobacco smoke, was found sufficient to spoil it for a determination of re- 

 sistance. Kohlrausch indeed estimates that the electric conductivity which 

 he observed in the purest water he could obtain might be accounted for 

 by the presence of no more than one ten millionth part of hydrochloric acid, 

 a quantity which no chemical analysis could detect. Hence the hypothesis 

 that water is a non-conductor of electricity, if not true, cannot be dis- 

 proved. 



Some of these remarkable properties of water were detected by 

 Cavendish. He found that the resistance of pump water was 4^ times less 



* Art. 091. 



