Historical and Personal 3 



at Arts. 265 and 273. The framework of the machine is not represented 

 in these figures. 



We learn, however, from Dr Davy*, that 



Cavendish seemed to have in view, in construction, efficiency merely, without 

 attention to appearance. Hard woods were never used, excepting when required. 

 Fir-wood (common deal) was that commonly employed. 



The bodies to be "tried" and the wires and vials for trying them were 

 either supported on glass rods as shown in the sketch at Art. 239, or else 

 hung by silk strings from a horizontal bar 7 feet 3! inches from the floor 

 as mentioned in Art. 466. The electrical connexions were made and broken 

 at the proper times by means of silk strings passing over pullies attached 

 to the horizontal bar. 



One of the bodies, the charges of which Cavendish compared by means 

 of this apparatus, was a globe 12-1 inches in diameter covered with tinfoil. 

 This globe has historical interest as it was not only the standard of capacity 

 with which Cavendish compared that of all other bodies, but it formed 

 part of the apparatus by which he established that the electric repulsion 

 varies inversely as the square of the distance. 



There was also a set of circles of tin plate, one of 36 inches diameter, 

 one of 18-5 and two of 9-3; and also square and oblong tin plates, and 

 squared pieces of stone and slate, and a collection of cylinders and wires 

 of different sizes. 



There was another "machine," represented, with its framework, in 

 Fig. 20, Art. 295, "for trying Leyden vials." 



The "Leyden vials " were most of them flat plates of glass with circular 

 coatings of tinfoil, one on each side. They were made in sets of three, 

 any one of each set being nearly equal in capacity to the three of the 

 former set taken together. Cavendish had thus a complete set of con- 

 densers of known capacity by means of which he measured the capacity 

 of every piece of his apparatus, from the little wire which he used to 

 connect his coated plates, and which he found to contain -28 "inches of 

 electricity," up to his battery of 49 jars, which contained 321,000 "inches 

 of electricity f." 



These "inches of electricity " can be directly compared with our modern 

 measurements of electrostatic capacity. Indeed the only difference is that 

 Cavendish's "inches of electricity" express the diameter of the sphere of 

 equivalent capacity, while the modern measurements express the capacity 

 by stating the radius of the same sphere in centimetres. 



Of each of these plates of glass Cavendish has given a most minute 

 description, so that each, if it were found, could be identified. Mr Cottrell, 

 of the Royal Institution, has been kind enough to examine the catalogue 

 of apparatus there, which contains Cavendish's Eudiometer and Registering 



* Wilson's Life, p. 178. f About half a microfarad. 



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