236 ELECTRICITY AND EXERGY. 



spark may be produced by holding the pistol in the hand and 

 bringing the outer knob near the prime conductor ; or the pistol 

 may be suspended from the prime conductor by a wire or chain and 

 the pistol then touched with the hand. The pistol may be fired by 

 means of the electrophorus ( 343) or Cottrell's Rubber. 



372. Relation of Electricity to Energy. The 

 work necessarily performed in operating an electric machine 

 is not all expended in overcoming inertia and friction. 

 Much of it is employed in producing electric separation. 

 It matters not whether this separation be the separation of 

 two fluids or of something else. Whatever be the nature 

 of the realities separated, mechanical kinetic energy is 

 employed in the separation and converted into the poten- 

 tial variety ( 159). In every case of electric attraction or 

 repulsion we have an evident reconversion of this potential 

 into mechanical kinetic energy. We shall soon see that 

 the sound, heat and light accompanying electric discharges 

 are forms of energy due to the conversion of the potential 

 energy of electric separation. 



EXERCISES. 



1. (a.) If a gold-leaf electroscope be placed within a tin pail which 

 is insulated and electrified, what will be the action of the electro 

 scope? (6.) Explain. 



2. (a.) Why may one obtain a stronger spark from a Leyden jar 

 than from the machine by which it is charged ? (&.) A Leyden jar 

 standing upon a glass plate cannot be strongly charged. Why ? 



3. (a.) A globe that is polished will remain electrified longer than 

 one that is not polished. Why ? (&.) Can you devise an appendage 

 to the outer coat of a Leyden jar, so that it may be charged when 

 standing upon a plate of glass ? 



4. (a.) Can you see any connection between electric induction and 

 the fact that electricity dwells only upon the outer surface of a con- 

 ductor? (6.) Describe the plate electric machine, (c.) Explain its 

 action, (d.) Explain the action of the electrophorus. 



