Plan of Experiments on Electrostatics 19 



we may express his result in modern language by saying that its electric 

 capacity is \n inches. 



In his early experiments he seems to have endeavoured to obtain a 

 number of conductors as different as possible in form, of which the 

 capacities should be nearly equal. Thus we find him comparing a paste- 

 board circle of 19-4 inches in diameter with his globe of 12-1 inches in 

 diameter, but finding the charge of the circle greater than that of the 

 globe, he ever after uses a circle of tin plate, 18-5 inches in diameter, the 

 capacity of which he found more nearly equal to that of the globe. 



In like manner the first wire that he used was 96 inches long and 

 0-185 diameter, but afterwards he always used a wire of the same diameter, 

 but 72 inches long, the capacity of which was more nearly equal to that 

 of the globe. 



He also provided himself with a set of glass plates coated with circles 

 of tin-foil on both sides. These plates formed three sets of three of equal 

 capacity, the capacities of the three sets being as i, 3 and 9, with a tenth 

 coated plate whose capacity was as 27. 



Besides these he had "double" plates of very small capacity made of 

 two plates of glass stuck together, and also other plates of wax and rosin, 

 the inductive capacity of these substances being, as he had already found, 

 less than that of glass; and jars of larger capacity, ranging up to his great 

 battery of 49 jars, whose capacity was 321,000 "inches of electricity." In 

 estimating the capacity of his battery, he used the method of repeated 

 touching with a body of small capacity. (Arts. 412, 441, 582.) This 

 method is the same as that used by MM. Weber and Kohlrausch in their 

 classical investigation of the ratio of the electric units*. 



Thus the method of experimental research which Cavendish adhered 

 to was the comparison of capacities, and the formation of a graduated 

 series of condensers, such as is now recognised as the most important 

 apparatus in electrostatic measurements. 



We have next to consider the steps by which he established the 

 accuracy of his theory, and the discoveries he made respecting the 

 electrical properties of different substances. 



Cavendish himself, in his description of his experiments, has shown us 

 the order in which he wishes us to consider them. The first experiment f 

 is that of the globe within two hemispheres, from which he proves that 

 the electric force varies inversely as the square of the distance, or at least 

 cannot differ from that ratio by more than a fiftieth part. The degree of 

 accuracy of all the experiments was limited by the sensitiveness of the 

 pith ball electrometer which he used. Bennett's gold leaf electrometer, 

 which is much more sensitive, was not introduced till 1787, but in repeating 

 the experiment we can now use Thomson's Quadrant electrometer, and 



* Elektrodynamische Maasbestimmungen, Abb. iv. p. 235. 

 | Arts. 217 to 235. 



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