CONTRIBUTIONS TO SCIENCE. 557 



tected by the electrometer, Cavendish deduced from the 

 results of the experiment that the law of variation of 

 electrical action with the distance must lie between the 

 inverse (2 -^)th and (2 + ^)th power of the distance, 

 and inferred that it is the inverse square. 



Maxwell placed a copper sphere about 10 inches in 

 diameter within a hollow copper sphere, made of two hemi- 

 spheres, about 12 inches in diameter, supporting it upon a 

 ring of ebonite, so that the spheres might be concentric. 

 The outer sphere was carefully insulated, and a hole cut in 

 its surface, which could be accurately filled by a little trap 

 door. To this trap door was attached a wire which rested 

 on the inner sphere when the door was shut down. The 

 door was carried on v a metal arm which turned about a hinge 

 attached to the outer sphere, and could be raised or lowered 

 by a silk thread which passed over a pulley and hung in 

 front of the operator. When the trap door was raised a 

 wire connection could be allowed to fall by relaxing a second 

 silk cord, so as to make contact with the inner sphere through 

 the hole in the outer. The trap door was first closed and 

 the outer sphere charged by means of a Leyden jar which 

 was immediately removed (the electric machine being placed 

 in a distant apartment). The trap door was then raised, the 

 outer sphere discharged by connecting it to earth, and the 

 inner sphere then put in communication with a quadrant 

 electrometer by means of the wire above mentioned. Not a 

 trace of electricity could be detected on the sphere. To test 

 the accuracy of the method, a small brass sphere was insu- 

 lated, and supported at a distance of about 60 centimetres 

 from the outer sphere, and the experiment was repeated. 

 When the copper sphere was charged the brass sphere was 

 connected to earth, and thus a small negative charge was 

 induced upon it by the positive electricity on the copper 

 sphere, and the ratio of this charge to that on the copper 

 could be easily calculated. The brass sphere was then insu- 

 lated, the outer copper sphere discharged as before, and the 

 inner sphere examined with the same result as previously. 

 The brass ball was then discharged, the outer sphere being 



