18 Electrostatics Physical Principles [OH. i 



surface uppermost on a horizontal table. A metal disc is held by an in- 

 sulating handle parallel to the cake of resin and at a slight distance above it. 

 The operator then touches the upper surface of the disc with his finger. 

 When the process has reached this stage, the metal disc, the body of the 

 operator and the earth itself form one conductor. The negative electricity on 

 the resin induces a positive charge on the nearer parts of this conductor 

 primarily on the metal disc and a negative charge on the more remote parts 

 of the conductor the further region of the earth. When the operator 

 removes his finger, the disc is left insulated and in possession of a positive 

 charge. As already explained, this charge may be used and the process 

 repeated indefinitely. 



In all its essentials, the principle utilised in the generation of electricity 

 by the " influence machines " of Voss, Holtz, Wimshurst and others is 

 identical with that of the electrophorus. The machines are arranged so that 

 by the turning of a handle, the various stages of the process are repeated 

 cyclically time after time. 



23. Electric Equilibrium. Returning to the apparatus illustrated in 

 fig. 3, p. 16, it is found that if we remove the inducing charge without 

 allowing the conducting rod to come into contact with other conductors, 

 the charge on the rod disappears gradually as the inducing charge recedes, 

 positive and negative electricity combining in equal quantities and neutral- 

 ising one another. This shews that the inducing charge must be supposed 

 to act upon the electricity of the induced charge, rather than upon the 

 matter of the conductor. Upon the same principle, the various parts of 

 the induced charge must be supposed to act directly upon one another. 

 Moreover, in a conductor charged with electricity at rest, there is no reac- 

 tion between matter and electricity tending to prevent the passage of 

 electricity through the conductor. For if there were, it would be possible 

 for parts of the induced charge to be retained, after the inducing charge 

 had been removed, the parts of the induced charge being retained in position 

 by their reaction with the matter of the conductor. Nothing of this kind is 

 observed to occur. We conclude then that the elements of electrical charge 

 on a conductor are each in equilibrium under the influence solely of the forces 

 exerted by the remaining elements of charge. 



24. An exception occurs when the electricity is actually at the surface 

 of the conductor. Here there is an obvious reaction between matter and 

 electricity the reaction which prevents the electricity from leaving the 

 surface of the conductor. Clearly this reaction will be normal to the 

 surface, so that the forces acting upon the electricity in directions which 

 lie in the tangent plane to the surface must be entirely forces from other 

 charges of electricity, and these must be in equilibrium. To balance the 



