24 Introduction 



the galvanometer, the only instrument by which any one else has ever 

 been able to compare electric resistances. 



We learn from the manuscripts now first published, that Cavendish 

 was his own galvanometer. In order to compare the intensity of currents 

 he caused them to pass through his own body, and by comparing the 

 intensity of the sensations he felt in his wrist and elbows, he estimated 

 which of the two shocks was the more powerful. 



As Cavendish does not appear to have prepared an account of these 

 experiments in the manner in which he usually wrote out what he intended 

 to publish, it may be well to describe them here, as we collect them from 

 different parts of his Journals. 



The conductors to be compared were for the most part solutions of 

 common salt of known strength or of other substances. These solutions 

 were placed in glass tubes, more than a yard long, bent near one end. 

 The tubes had been previously calibrated by means of mercury. 



Two wires were run into the tube, probably through holes in corks at 

 each end, to serve as electrodes. The length of the effective column of 

 the liquid could be altered by sliding the wire in the straight part of the 

 tube. 



In order to send electric discharges of equal quantity and equal 

 electromotive force through two different tubes Cavendish chose six 

 jars of nearly equal capacity from "Nairne's last battery." The two 

 tubes to be compared were placed so that the wires run into their bent 

 ends communicated with the outside of this battery of six jars. The 

 wires run into the straight ends of the tubes were fastened to two 

 separately insulated pieces of tinfoil. The six jars were then all charged 

 at once by the same conductor till the gauge electrometer indicated the 

 proper degree of electrification. The conductor was then removed , so 

 that the six jars remained with their inside coatings insulated from each 

 other and equally charged. 



Cavendish then taking two pieces of metal, one in each hand, touched 

 with one the tinfoil belonging to one of the tubes to be compared, and 

 then with the other touched the knob of jar No. i, so as to receive a 

 shock, the charge passing through his body and the first tube. 



He next laid one of the metals on the tinfoil of the second tube, and 

 then touching with the other the knob of jar No. 2, he received a second 

 shock, the discharge passing through his body and the second tube. 



In this way he took six shocks, making them pass alternately through 

 the first and the second tube, and proceeded to record his impression 

 whether the intensity of the shock through the second tube was greater 

 or less than that of the shock through the first, and concluded that the 

 tube which gave the greater shock had the smaller resistance. 



He then adjusted the wire in one of the tubes so as to make the re- 

 sistance more nearly equal to that of the other, and repeated the experi- 



