20 Introduction 



thereby detect a deviation from the law of the inverse square not exceeding 

 one in 72,000. See Note 19. 



The second experiment, Art. 135, is a repetition of the first with 

 bodies of different shape. 



The third experiment, Art. 265, shows that in comparing the charges 

 of bodies, the place where the connecting wire touches the body, and the 

 form of the connecting wire itself, are matters of indifference. 



The fourth experiment, Art. 269, shows that the charges of bodies of 

 the same shape and size, but of different substances, are equal. 



The fifth, Art. 273, compares the charge of a large circle with that of 

 two of half the diameter. According to the theory the charge of the large 

 circle should be equal to that of the two small ones if they are at a great 

 distance from each other, and equal to twice that of the small ones if they 

 are close together. Cavendish tried them at three different distances and 

 compared the results with his calculations. 



The sixth experiment, Art. 279, compares one long wire with two of 

 half the length and half the diameter, placed at different distances. 



The seventh, Art. 281, compares the charges of a globe, a circle, a 

 square, an oblong and three different cylinders, and the eighth, Art. 288, 

 shows that the charge of the middle plate of three parallel plates is small 

 compared with that of the two outer ones. 



Cavendish next describes his experiments for comparison of the charges 

 of coated plates of glass and other substances, but begins by examining 

 the sources of error in measurements of this kind. 



The first of these which he investigates is the spreading of electricity 

 on the surface of the plates beyond the coatings of tinfoil. He distinguishes 

 two kinds of this spreading, one a gradual creeping of the electricity over 

 the surface of the glass, Art. 300, and the other instantaneous, Art. 307. 



He attempted to check the first kind by varnishing the glass plates 

 and by enclosing their edges in a thick frame of cement, but he found very 

 little advantage in this method, and finally adopted the plan of performing 

 all the operations of the experiment as quickly as possible, so as to allow 

 very little time for the gradual spreading of the electricity. 



He next investigated the instantaneous spreading of electricity on the 

 glass near the edge of the coating. He noticed that at the instant of 

 charging the plate in the dark, a faint light could be seen all round the 

 edges. He also observed that after charging and discharging a coated 

 plate of glass many times without cleaning it, a narrow fringed ring of 

 dirt could be traced all round the coating, the space between this ring 

 and the coating being clean, and in general about -fa inch broad. 



He also observed that the flash of light was stronger the first or second 

 times of charging a plate than afterwards. 



To determine how much the capacity of a coated plate was increased 

 by this spreading of the electricity, he compared the capacity of a plate 



