Experiments on Electrical Capacity and Resistance 23 



Cavendish tried whether this was the case with a charged plate, of air, 

 by lifting one of the electrodes and changing the air between them and 

 then replacing the electrode. He found that the charge was not altered 

 during these operations, and concluded that the charge resides, not in the 

 air, but in the metal plates. 



In Arts. 336 to 339 we find a most ingenious method of determining 

 by experiment the effect of the floor, walls and ceiling of a room, and of 

 other surrounding objects, in increasing the apparent capacity of a con- 

 ductor placed in a given position in the room. The method consists in 

 measuring the capacities of two conductors of the same shape but of 

 different dimensions, the centre of each being at the given point in the 

 room. If the experiment had been made with the conductors at an infinite 

 distance from all other bodies their capacities would have been in the 

 ratio of their corresponding dimensions, but the effect of surrounding 

 objects is to make their capacities vary in a higher ratio than that of their 

 dimensions, and from the measured ratio of the two capacities, the cor- 

 rection for the effect of surrounding objects on the capacity of any small 

 body may be calculated. 



Cavendish also verified by experiment what he had already proved 

 theoretically, that the capacity of two condensers is not sensibly altered 

 when they are placed near to each other or far apart. 



But besides this series of experiments on electric capacity, another 

 course of experiments on electric resistance was going on between 1773 

 and 1781, the knowledge of which seems never to have been communicated 

 to the world. 



In his paper on the Torpedo in the Philosophical Transactions for 1776 

 (Art. 398) he alludes to "some experiments of which I propose shortly 

 to lay an account before this Society," but he never followed up this 

 proposal by divulging the method by which he obtained the results which 

 he proceeds to state "that iron wire conducts about 400 million times 

 better than rain or distilled water*," and that "sea water, or a solution 

 of one part of salt in 30 of water conducts 100 times, and a saturated 

 solution of sea-salt about 720 times better than rain water." 



Such was the reputation of Cavendish for scientific accuracy, that these 

 bare statements seem to have been accepted at once, and soon found their 

 way into the general stock of scientific information, although no one, as 

 far as I can make out, has ever conjectured by what method Cavendish 

 actually obtained them, more than forty years before the invention of 



* This is equivalent to saying that iron wire conducts 555,555 times better than 

 saturated solution of sea salt. A comparison of the experiments of Matthiessen on 

 iron with those of Kohlrausch on solutions of sodium chloride at 18 C., would make 

 the ratio 451,390. The resistance of iron increases and that of the solution diminishes 

 as the temperature rises, and at a temperature of about 11 C. the ratio of the re- 

 sistances would agree with that given by Cavendish. 



