ELECTRICAL MEASUREMENT. 241 



conductor the electric static force upon a small insulated and 

 electrified body is exactly zero, then the law of variation of the electric 

 force must be according to the inverse square of the distance. On the 

 other hand, if a certain attraction of a small positively electrified body 

 towards the sides of the supposed hollow electrified conductor is 

 observed, then the force varies according to a law of greater variation 

 of the distance than the inverse square of the distance ; and vice versa 

 if a small body electrified in the opposite way to the electrification of 

 the conductor seems to be repelled from the sides, then the law of 

 diminution of force with the distance will be something less than would 

 be calculated according to the inverse square of the distance. The 

 case supposed is an insulated electrified body an infinitely small 

 body charged with electricity opposite to that of the electrified body. 

 If this small body, then, put into the interior as a test, exhibits attraction 

 towards the sides, the law of variation of the force will shew a greater 

 diminution according to the distance than according to the inverse 

 square of the distance, and vice versa. It was left for Faraday to make 

 with accuracy the concluding experiment which crowned Cavendish's 

 theory. Faraday found by the most thoroughly searching investiga- 

 tion that the electrical force in the circumstances supposed was zero, 

 and supplied the minor proposition of Cavendish's syllogism. There- 

 fore the taw of force varies with the inverse square of the distance. 

 This result was obtained with far less searching accuracy by Coulomb 

 and Robinson, because their method did not admit of the same search- 

 ing accuracy. On this law is founded the whole system of electrostatic 

 measurement in absolute measure. Mathematical theory lays down 

 the proper static unit that quantity, which if a quantity equal to it is 

 possessed by two bodies, those two bodies react upon one another with 

 unit force at unit distance. On this is founded the system of absolute 

 measurement in electrostatics. 



Cavendish's other experiments, and series of experiments because 

 I believe Professor Clerk-Maxwell is to edit a whole series of experi- 

 ments measuring electrostatic quantities led to the general system 

 of electrostatic measurement in absolute measure. 



But now there is another great branch of electrical measurement, 

 and that is the measurement of electro-magnetic phenomena. Our 

 elementary knowledge of electrostatics was complete, with the 



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