ESSENTIAL PARTS OF ELECTRICAL MACHINES. 183 



taken into account; contact between the cloth and the different 

 parts of the surface of the glass, only takes place by friction. When 

 the glass charged with positive electricity is taken to the inside of 

 the cylinder, it would cause an equal layer of electricity, but of the 

 opposite kind, to be formed on the inside of the cylinder; mere 

 contact, however, with a point of the inner surface would not be 

 sufficient for neutralisation of the two charges. This result would 

 be obtained if the inner surface of the cylinder were charged with 

 points : equilibrium would only be established when the density at 

 the end of each of the points was null that is, when the electricity 

 which escaped had neutralised the charge of the glass disc. This is 

 the arrangement ordinarily used in all frictional machines. 



195. ESSENTIAL PARTS OF ELECTRICAL MACHINES. It will be 

 seen that in all cases the electrical machine is reduced to three essen- 

 tial parts one which develops electricity, another which transmits it, 

 and a third which receives it : a producer, a carrier, and a receiver. 

 The potential energy imparted to the collector is furnished by the 

 mechanical work performed when the carrier is moved in a direction 

 opposed to that of the electrical forces, from the producer charged 

 with the contrary electricity which attracts it, to the receiver charged 

 with the same electricity which repels it. 



In frictional machines, the receiver takes the same quantity 

 of electricity at each operation, and its charge increases in arith- 

 metical progression. 



Induction machines may be so arranged that the charge increases 

 in geometrical progression : the two machines must be coupled up in 

 such a manner that the two inductors develop electricity of opposite 

 kinds, and that each of them is in metallic connection with the 

 receiver of the other system. At each operation, the charge of the 

 inducer increases at the same time as that of the receiver, with 

 which it is connected, and induces a greater charge in the re- 

 ceiver at the next operation. This arrangement is made use of in 

 Varley's machine, and in some of the very ingenious apparatus 

 invented by Sir W. Thomson, such as the replenisher and the self- 

 acting reciprocal condenser. Holtz's machine depends on the same 

 principle, but is somewhat more complicated. 



196. LIMIT OF THE CHARGE. Although theory assigns no limit 

 to the charge of the receiver, a practical limit is soon attained, either 

 through the losses by air or by the supports, or by the fact of dis- 

 charges which take place in the form of sparks between the receiver 

 and the other parts of the machine or the adjacent conductors. 



In the latter case, the limit only depends on the shape of the 



