So8 POLARITY TESTS [CH. XIII 



wire. The method in practice is to watch the burning carbons 

 through smoked glass or smoky mica. The positive one is markedly 

 brighter than the negative one (fig. 271). 



If the upper carbon is positive the lamp is correctly installed, if 

 the lower carbon is positive then it is improperly installed for 

 ordinary projection. If the positive wire goes to the lower carbon, 

 turn off the light by opening the switch or pulling the separable 

 plug apart. Now reverse the position of the wires in the binding 

 posts of the lamp, and this will bring the positive wire in connection 

 with the upper carbon, and the negative wire in connection with the 

 lower carbon (fig. 2, 270). 



If a non-polarized separable plug is used (fig. 268 B), the simplest 

 way to reverse the polarity is to pull the cap off, turn it half way 

 round and insert it again. When found to be in the correct posi- 

 tion mark the socket, the plug and the cap in some way so that the 

 connections can be made at some future time with certainty. 

 There are polarized plugs (fig. 268A) in which the connections are 

 so arranged that the attachment plug can be inserted only in one 

 way, thus avoiding the change of polarity when once the wiring is 

 correctly installed. 



When the polarity is found to be correct it is advantageous for 

 future work to mark the insulation of the positive wire near the 

 switch with red paint. The positive side of the table switch 

 should also be marked with a + sign or with P. using black or red 

 paint. In like manner the insulation of the wire near where it is 

 connected with the binding post of the arc lamp should be marked 

 red, and a + or P. should be put alongside the binding post for the 

 upper carbon unless it is so evident that no mistake is likely to 

 occur. 



(B) Testing the polarity with a direct current voltmeter 

 To do this connect the voltmeter with both wires (fig. 272). 

 Turn on the current by closing the switch and if the positive wire 

 is connected with the positive binding post the voltmeter will 

 record the voltage in the line. If the wires are wrongly connected 

 then the hand will try to move off the dial face below zero. If the 

 hand does not register the voltage, open the switch, and reverse the 



