26 ELEMENTARY LESSONS ON [CHAP. i. 



is not obtained at the expense of any part of the original 

 charge, it is a matter of some interest to inquire what 

 is the source from whence is drawn the energy of this 

 apparently unlimited supply ; for it cannot be called 

 into existence without the expenditure of some other 

 form of energy, any more than a steam-engine can work 

 without fuel. As a matter of fact it is found that it 

 is a little harder work to lift up the cover when it 

 is charged with the + electricity than if it were not 

 charged ; for, when charged, there is the force of the 

 electric attraction to be overcome as well as the force 

 of gravity. Slightly harder work is done at the ex- 

 pense of the muscular energies of the operator ; and this 

 is the real origin of the energy stored up in the separate 

 charges. 



23. Continuous Electrophori. The purely me- 

 chanical actions of putting down the disc on to the 

 cake, touching it, and lifting it up, can be performed 

 automatically by suitable mechanical arrangements, 

 which render the production of these inductive charges 

 practically continuous. The earliest of such contin- 

 uous electrophori was Bennet's " Doubler," the latest 

 is Holtz's machine, described in Lesson V. 



24. "Free" and "Bound" Electricity. We 

 have spoken of a charge of electricity on the surface of 

 a conductor, as being " bound " when it is attracted by 

 the presence of a neighbouring charge of the opposite 

 kind. The converse term " free " is sometimes applied 

 to the ordinary state of electricity upon a charged con- 

 ductor, not in the presence of a charge of an opposite 

 kind. A "free " charge upon an insulated conductor 

 flows away instantaneously to the earth, if a conducting 

 channel be provided, as will be explained in the next 

 lesson. It is immaterial what point of the conductor be 

 touched. Thus, in the case represented in Fig. 10, 

 wherein a + electrified body induces ~ electricity at 

 the near end, and + electricity at the far end of an 



