THE ELECTRIC CURRENT 145 



166. Divided Circuits. A main circuit may be 

 tapped at different points by short branches (called 

 shunts) which take the current to separate instruments. 

 Each of these branches must of course join the main 

 circuit at some point farther on, in order that the cur- 

 rent may pass through it. 



When a current is divided in this way, the amount 

 of current that each branch gets depends upon its 

 resistance : the more resistance each branch offers, the less 

 current flows through it. This principle also is used in 

 controlling electrical energy. For example, suppose it 

 is desired to run a certain motor at different speeds: 

 a device called a rheostat is put into the same shunt 

 which runs to the motor. The rheostat contains several 

 coils of wire, of different resistances, and a switch for 

 making the current pass through one or more of these 

 coils at will. The more coils the current is made to 

 pass through, the greater the resistance offered by that 

 shunt, and consequently the less current passes through 

 it to the motor. Motormen on electric cars move a 

 switch and control the speed of the car in this way. 



167. Batteries. A group of cells arranged on one 

 circuit is called a battery. One cell does not furnish 

 enough current to do very much work, so that the com- 

 bined strength of two or more is generally used. 



Cells may be combined in two ways, in series or in 

 parallel. In parallel arrangement all the positive plates 

 are joined together and all the negatives likewise ; the 

 two sets are then connected by a wire. This arrange- 

 ment decreases internal resistance, but gives no gain in 



