Discovery of Laws of Electric Resistances of Solutions 25 



ment, always recording his impression of the result, till he found that one 

 adjustment made the shock of the second tube sensibly greater than that 

 of the first, and that another adjustment made it sensibly less. 



From the result of the whole series of experiments he judged what 

 adjustment would make the two shocks exactly equal. 



Instead of using six jars only, he seems latterly to have used the whole 

 battery, electrifying one row to a given degree and then communicating 

 this charge to the whole battery, and taking the discharge of one row at 

 a time through the tubes alternately. He seems to have found some 

 advantage in thus using a discharge of greater quantity and smaller 

 electromotive force. 



The accuracy which Cavendish attained in the discrimination of the 

 intensity of shocks is truly marvellous, whether we judge -by the con- 

 sistency of his results with each other, or whether we compare them with 

 the latest results obtained with the aid of the galvanometer, and with all 

 the precautions which experience has shown to be necessary in measuring 

 the resistance of electrolytes. 



One of the most important investigations which Cavendish made in 

 this way was to find, as he expressed it, "what power of the velocity the 

 resistance is proportional to*." 



Cavendish means by "resistance" the whole force which resists the 

 current, and by "velocity" the strength of the current through unit of 

 area of the section of the conductor. 



(In modern language the word resistance is used in a different sense, 

 and is measured by the force which resists a current of unit strength.) 



By four different series of experiments on the same solution in wide 

 and in narrow tubes, Cavendish found that the resistance (in his sense) 



varied as the 



1-08, 1-03, 0-976, and i-oo 



power of the velocity. 



This is the same as saying that the resistance (in the modern sense) 



varies as the 



0-08, 0-03, 0-024 



power of the strength of the current in the first three sets of experiments, 

 and in the fourth set that it does not vary at all. 



This result, obtained by Cavendish in January, 1781, is an anticipation 

 of the law of electric resistance discovered independently by Ohm and 

 published by him in 1827. It was not till long after the latter date that 

 the importance of Ohm's law was fully appreciated, and that the measure- 

 ment of electric resistance became a recognised branch of research. The 

 exactness of the proportionality between the electromotive force and the 

 current in the same conductor seems, however, to have been admitted, 

 rather because nothing else could account for the consistency of the 



* Arts. 574, 575, 629, 686. 





