THE PHENOMENA OF ELECTROSTATICS. 199 



the following articles, and in order to obtain a clear understanding 

 of the factional electric machine, of the Toepler-Holtz electrical 

 machine, and of the Wimshurst electrical machine, it is important 

 to keep in mind the fact that every one of these machines involves 

 the principle of the electrical doubler, inasmuch as the carrier or 

 carriers pass between two conducting bodies to both of which they 

 give up their charges, so that these two conducting bodies take 

 the place of the hollow vessel BB in Figs. 1 1 8 to 121. 



109. Charging by contact and separation. The production of 

 electric charge by the rubbing together of certain substances is 

 one of the most familiar of the phenomena of electricity. When 

 a cat is stroked with the hand in a dry room, the cat's fur and 

 the hand become oppositely charged, and the crackling sound 

 which is produced is due to the production of minute electrical 

 sparks which may be seen if the room is dark. A hard rubber 

 comb becomes strongly charged when it is passed through very 

 dry hair, and the comb will attract small bits of paper or pith. 

 When 'pencil marks are erased from a very dry piece of paper by 

 means of a rubber eraser, the paper becomes charged and it 

 clings to the drawing board or table. 



Two substances when brought into contact always tend to 

 settle to a state of equilibrium in which electric lines of force 

 pass from one substance to the other across the very thin air 

 space between them. Thus, two flat plates of zinc and copper 

 settle to equilibrium with an electromotive force of about 0.9 

 volt between them, so that the intensity of the electric field in the 

 very narrow space between the plates may be several thousands 

 volts per centimeter. If the plates are thoroughly insulated and 

 moved apart, the electric field intensity (volts per centimeter) re- 



into the interior of B and brought into contact with B, and carrier D being moved intQ 

 the interior of A and brought into contact with A. The result is that the bundle 

 of lines of force from A to C in Fig. 123 is stretched across from A to B, and 

 the bundle of lines of force from D to B is stretched across from A to B thus 

 increasing the total number of lines of force from A to B. The revolving doubler 

 of Lord Kelvin is a mechanical device for performing the operations here described, 

 the carriers C and D being mounted upon a rotating, insulated arm. 



