8 4 



ARC LAMPS WITH SMALL CURRENTS 



[CH. Ill 



The rheostat needed for the small-current, arc lamp is small and 

 inexpensive. It need not be adjustable. One has only to be cer- 

 tain that it will not deliver a current above five or six amperes. 



In purchasing a rheostat for the house circuit, tell the manufac- 

 turer the kind of current (direct or alternating) and the voltage 

 (no or 220). If one does not know the character and voltage of 

 his house circuit the information can be obtained at the office of 

 the company furnishing the current. 



Lamp Socket s P 



FIG. 45. WIRING AND CONNECTIONS OF THE ARC LAMP USED ON THE 

 HOUSE LIGHTING SYSTEM. 



130. Polarity with the arc lamp. With alternating current 

 both wires are the same (see 103 and 653), but with direct current 

 one of the wires is positive and one negative, and the positive wire 

 must be connected with the binding post for the upper carbon. 

 The most practical ways of determining the polarity are described in 

 Ch. I, 80; Ch. XIII, 702. 



In case the lower carbon shows the brightest crater it is positive 

 and hence the polarity wrong. If the separable attachment plug is 

 of the polarized form, separate the two parts thus turning off the 

 current. Then reverse the position of the wires in the binding 

 posts of the lamp. This will connect the positive wire with the 

 upper carbon as it should be. A simple way, if non-polarized plugs 



