1877.] electrical experiments of the Hon. Henry Cavendish. 87 



manuscript entitled "Thoughts concerning electricity" seems 

 to form a kind of introduction to this treatise, for it contains 

 several important definitions and hypotheses which are not after- 

 wards repeated. 



Next comes the fundamental experiment, in which it is proved 

 that a conducting sphere insulated within a hollow conducting 

 sphere does not become charged when the hollow sphere is 

 charged and the inner sphere is made to communicate with it. 



Cavendish proves that if this is the case, the law of force 

 must be that of the inverse square, and also that if the index 

 instead of being 2 had been 2 + gL, his method would have 

 detected the charge on the inner sphere. 



The experiment has been repeated this summer by Mr Mac 

 Alister of St John's College with a delicate quadrant electro- 

 meter capable of detecting a charge- many thousand times smaller 

 than Cavendish could detect by his straw electrometer, so that 

 we may now assert that the index cannot exceed or fall short of 

 2 by the millionth of a unit. 



The second experiment is a repetition of this, using one paral- 

 lelepiped Avithin another instead of the two spheres. 



He then describes his apparatus for comj)aring the charges 

 of different bodies, or, as we should say, their capacities. 



He first shows (Exp. 8) that the charge, communicated to 

 a body connected to another body at a great distance by a fine 

 wire, does not depend on the form of the wire, or on the point 

 where it touches the body. 



Exp. 4 is on the capacities of bodies of the same shape and 

 size but of different substances. 



Exp. 5 compares the capacity of two circles with that of 

 another of twice the diameter. 



Exp. 6 compares the capacity of two short wires with that 

 of a long one. 



Exp. 7 compares the capacities of bodies of different forms, 

 the most important of which are a disk and a sphere. 



Exp. 8 compares the charge of the middle of three parallel 

 plates with that of the outer plates. 



In the next part of his researches he investigates the capacities 

 of condensers formed of plates of different kinds of glass, rosin, 

 wax, shellac, &c. coated with disks of tinfoil, and also of plates 

 of air between two flat conductors. He finds that the electricity 

 spreads on the surface of the plate beyond the tinfoil coatings, 

 and he investigates most carefully the extent of this spreading, 

 and how it depends on the strength of the electrification. 



After correcting for the spreading, he finds that for coated 

 plates of the same substance the observed capacity is proportional 

 to the computed capacity, but it is always several times greater 



