THE ALTERNATOR. 21 



electromotive force developed by the alternator must be used to 

 produce the electrical stress which is created in the insulating 

 material DD between the metal plates CC as these plates are 

 electrically charged first in one direction (upper plate positive and 

 lower plate negative) and then in the reverse direction (upper 

 plate negative and lower plate positive). 



The metal plates CC Fig. 20 with the insulating material DD 

 between them constitute what is called a condenser, and the action 

 of this condenser is exactly analogous to the action of the chamber 

 CC, Fig. 19, with its elastic diaphragm DD. The elastic dia- 

 phragm permits an alternating current of water to surge back and 

 forth through the circuit of pipe, but it does not permit the flow 

 of a steady current of water. Similarly, the condenser CC, Fig. 

 20, permits an alternating electric current to surge back and forth 

 through the circuit of wire, but it does not permit the flow of a 

 steady electric current. 



Concentrated capacity and distributed capacity. Figure 21 rep- 

 resents a valveless pump which causes an alternating current of 

 water to surge back and forth through a circuit of distensible 

 rubber tube. When the piston starts upwards the rapidly in- 

 creasing pressure in the portion a of the rubber tube causes this 

 portion of the tube to swell, and the rapidly decreasing pressure 

 in the portion b of the rubber tube causes this portion of the 

 rubber tube to shrink. The swelling and the shrinking of the 

 rubber tube extends with decreasing intensity to the middle point 

 c, as shown by the tapering of the tube, and as indicated by the 

 positive and negative signs of decreasing size. One result of the 

 elasticity of the tube is that the whole of the water current which 

 enters the tube at a does not flow around to b, and the whole of 

 the water current that enters the pump at b does not flow around 

 from a, but the value of the current (amount of water passing a 

 point of the tube per second) decreases from a to c and increases 

 from c to b. The current of water does not have the same value 

 all along the circuit of tube. Similarly, when the alternator in 

 Fig. 22 starts a pulse of electric current in the direction of the 



