SOUECES OF ILLUMINATION 



105 



B, a double pole-switch for making and breaking the circuit. 

 One wire is continued from either terminal it matters not 

 which to one end of the resistance coil C. The other terminal 

 of the resistance is then connected up directly on to the 

 proper terminal of the lamp. The other wire from the double- 

 pole switch is then connected directly from the switch through 

 to the other terminal of the lamp in the same way, except that 

 it does not connect to the resistance. 



When the lamp is to be started, then if it be automatic, 



r_ 



c 



FIG. 33. Method of connecting up Electric-arc Lamp. 



it is only necessary to switch on the current, and the arc is 

 at once made. With the hand-feed lamp, after the current 

 is switched on, the two carbons have to be brought together 

 momentarily and drawn apart, as already explained, when the 

 lamp will immediately light. 



If desired, an ammeter and voltmeter can be connected 

 up in the circuit. The ammeter is arranged so that the whole 

 of the current passes through it in the same way as through 

 the resistance, i.e. ' in series.' The voltmeter must, however, 

 be connected up as a shunt-circuit, so that it indicates the 

 difference of potential on the terminals of the lamp. These 



