192 APPLIED SCIENCE 



Repeat the reasoning for the second half of the revolution. Notice 

 that for every complete revolution, the current reverses its direc- 

 tion twice. It is accordingly called alternating current. As the 

 strength of the current depends upon the number of lines of force 

 cut, so the induced electromotive force starts at zero, goes to a 

 maximum, and then back to zero in the first half-turn. That is, 

 the induced electromotive force reaches its maximum when the 

 loop is in a horizontal position because it cuts the most lines of 

 force at this position. It cuts the least number of lines of force 

 at the beginning and at the end of each half- vertical revolution. 



229. Commutator. We have seen that the current 

 generated in the coil is alternating. Alternating current 

 is very valuable for lighting and power, but there are cases 

 in electroplating and charging storage batteries where it is 

 absolutely necessary to have the current flow in the same 

 direction. To do this, it is necessary to add to the dynamo a 

 device called a commutator, the object of which is to make 

 the current flow in one direction in the external circuit, 

 regardless of the fact that the current reverses twice in every 

 revolution. 



A commutator consists of copper bars which are arranged 

 in circular form and separated or insulated from each other 

 by thin plates of mica. The bars connect with the arma- 

 ture wires, so that the current, as fast as it is generated, flows 

 from the armature to the segments of the commutator. 



230. Armature Brushes. The electricity is taken off 

 the commutator by strips of carbon which touch or lean 

 upon it. There are usually two brushes on the opposite 

 sides of the commutator. The brushes, when adjusted, 

 can shift sections on the commutator just when the loop 

 is in a vertical position, so that the current will flow out of 

 the positive brush and in at the negative brush. 



