154 SCIENTIFIC APPARATUS. 



Electrical Machines. 



1. Acting by friction. The most familiar of these depend upon 

 the friction of two different substances against each other. One 

 of the substances is usually a plate or cylinder of glass, less com- 

 monly of ebonite, to which rotatory motion can be given by a 

 handle, and the other is a semi-solid metal (amalgam) spread upon 

 leather or silk, and pressed against the revolving glass or ebonite. 

 Such electrical machines are too well known to need further 

 description here. 



Armstrong's Hydro-electric Machine. Another example of a 

 machine, of which the action is essentially similar, is Armstrong's 

 Hydro-electric Machine, in which electricity is generated by the 

 friction of water against the sides of a tube, through which it is 

 driven with great velocity by a current of steam. 



2. Acting by electrical induction. An electrified body tends to 

 make the electrical condition of all bodies in its neighbourhood 

 approximate towards its own condition, exerting this action most 

 strongly on those which are nearest to it. Thus, if a positively 

 electrified body, A, is brought into the neighbourhood of two 

 insulated (and previously unelectrified) conductors, B and c, each 

 of these becomes positively electrified, in the sense of tending to 

 impart positive electricity to other bodies, as long as A is present ; 

 if, however, B is nearer to A than c is, B becomes more strongly 

 positive than c, or the tendency of B to give positive electricity to c 

 is greater than the tendency of c to give positive electricity to B.* 

 Consequently, if B and c touch each other, electricity flows 

 from B to c to an extent depending on the difference between 

 the electrifications of the two bodies, and electrical equilibrium 

 is established between them. If now they are separated from 

 each other, and removed beyond the range of the sensible influ- 

 ence of the body A, they are both found to be electrified, B 



* Stated in more technical language, the effect of the body A is to raise the 

 electrical potential of all bodies in its neighbourhood. 



