Page:Popular Science Monthly Volume 59.djvu/449

Rh 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 capacity, modern measurements express capacity as the radius of the same sphere in centimeters. When we consider the crudeness 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 electrical 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 substances, 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 regarded 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 absorption,' 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, 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 till Maxwell edited Cavendish's electrical researches in 1879. We learn from the manuscripts thus published that Cavendish was his own galvanometer, comparing the intensity of currents by the intensity of the sensations he felt in his wrist and elbows when they passed through his body. The accuracy of his discriminations 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