592 



ELECTRICAL MACHINES. 



of which is weighted by some heavy object to prevent its being 

 upset ; or it may be fixed by a screw clamp to the edge of the table. 

 The net should be 20 or 25 cm long ; it may be starched so as to make 



FIG. 314 (an. proj. | real size). 



it somewhat stiff. Two threads of pure silk are tied to the point of 

 the bag, each 40 cm long, one being inside, the other outside. 



After electrifying the bag it is seen by means of the proof-plane, 

 that the electricity is only on the exterior, and that there is more 

 electricity at the point of the bag than anywhere at the wider portion 

 near the opening, and that if the sides are reversed by drawing the 

 point of the bag through the opening (without, of course, touching 

 it with the fingers) so as to turn the bag inside out, the electricity 

 will still be found only on the outside. 



The experiments with the gauze bag must be made rapidly, 

 because the charge is soon dispersed by the action of the numerous 

 fine fibres of the fabric. The net must be starched before the silk 

 threads are tied to it, as the starch would make the silk conductive. 



On the action of points depend essentially various 

 apparatus for generating and collecting larger supplies 

 of electricity, called electrical machines. The essential 

 parts of a machine in which the electricity is generated 

 by friction, are the ' rubber/ a cushion of leather or silk, 



