222 LECTURES TO SCIENCE TEACHERS. 



the torsion balance and placed in position, and the amount 

 of electricity that it possesses is determined. The theory 

 of the proof -plane shows 'that when the proof -plane is applied 

 to the surface of the body to be tested, it, as it were, 

 carries away a small portion of the surface of the body 

 with its electricity upon it. While the proof -plane is in 

 contact with the body it forms a part of the surface ; and 

 when it is lifted normally from the surface of the body with 

 which it is in contact, it is practically a small portion 

 of the surface of the body itself that is carried away. 

 Coulomb, by an error, supposed that the proof -plane carried 

 away as much electricity on each side as corresponds to the 

 space it touched upon. To correct his results for this error 

 is, however, easy. 



Taking, then, the case of the two spheres in contact 

 worked out by Poisson, and the case of an ellipsoid, for 

 which Coulomb himself worked out the distribution mathe- 

 matically, he found, on determining the distribution by 

 experiment, as close an agreement with the theoretical 

 distribution as could possibly be expected. 



By far the best proof that we have of the exactness of 

 the laws of electric attraction which I have just stated is an 

 indirect proof. It was first pointed out by Cavendish that 

 if the law of the inverse square of the distance be true it 

 follows as a consequence that the whole of the electricity 

 upon an electrified closed conductor must reside at the surface 

 of the conductor. The exactness of the law will then be 

 tested by finding whether or not the whole of the electricity 

 does reside at the surface of such a conductor. Cavendish 

 himself undertook some admirable experiments to test this 

 question, and some of the most important of Faraday's 

 electrostatic researches were devoted to it. 



One of the remarkable experiments of Faraday was to 

 place himself within a chamber a twelve-foot cube 

 constructed of light material and covered with tinfoil, 

 taking with him electroscopes of the most delicate con- 

 struction to examine whether any electrification could be 

 found in the interior while the chamber, which was 

 insulated, was electrified ; but while the whole power of the 

 electric machines of the Royal Institution was turned on, so 

 that from every part of the outside of the chamber flashes 

 and brushes of electricity were rushing off, he could find 



