HENRY CAVENDISH. 439 



Great difficulty is very often encountered in interpreting the work 

 of former experimenters in terms of modern units, yet Cavendish had 

 such a clear insight and worked so quantitatively that we can readily 

 express his results in terms of modern nomenclature and units. His 

 'inches of electricity/ for instance, can be directly compared with 

 modern measurements, for while his 'inches' express the diameter of a 

 sphere of equal capacit}^, modern measurements express capacity as the 

 radius of the same sphere in centimeters. When we consider the crude- 

 ness of some of Cavendish's apparatus, we are amazed at the accuracy 

 of the results he obtained. The capacity of a circular disc, for example, 

 was determined experimentally by him to be 1/1.57 of that of a sphere 

 of the same radius, while the most modern calculation gives 1/1.571 

 for the same ratio. 



Cavendish also entertained exceedingly clear views on what we now 

 know as 'potential' and 'resistance,' and, besides Coulomb's law of 

 inverse squares, his papers contain anticipations of Faraday's 'specific 

 inductive capacity' and 'electric absorption,' and Ohm's 'law of elec- 

 trical resistance.' In observing that the charges of coated plates were 

 always several times greater than the charges computed from their 

 thickness and the area of the coatings, Cavendish not only anticipated 

 Faraday's discovery of the specific inductive capacity of different sub- 

 stances, but actually measured its numerical value in some cases. He 

 also considered the question, of fundamental importance in the theory 

 of dielectrics, whether the electric induction is strictly proportional to 

 the electromotive force which produces it, or in other words is the 

 capacity of a condenser the same for high as for low potentials. He re- 

 garded his results as not decisive, but, in observing that the apparent 

 capacity of a Florence flask was greater when it continued charged a 

 good while than when it was charged and discharged immediately, 

 Cavendish discovered the phenomenon called by Faraday 'electric ab- 

 sorption,' which was fully studied later for different kinds of glass by 

 Dr. Hopkinson, and connected with the long-known phenomenon of 

 'residual charge.' 



But besides this series of experiments on electric capacity, an- 

 other course of experiments on electric resistance was going on between 

 1773 and 1781, the knowledge of which seems never to have been com- 

 municated to the world till Maxwell edited Cavendish's electrical re- 

 searches in 1879. We learn from the manuscripts thus published that 

 Cavendish was his own galvanometer, comparing the intensity of cur- 

 rents by the intensity of the sensations he felt in his wrist and elbows 

 when they passed through his body. The accuracy of his discrimina- 

 tions of the intensity of shocks is truly marvelous, whether we judge 

 by the consistency of his results among themselves or by comparing 

 them with the latest results obtained with a galvanometer, using all 



