5o6 POLARITY TESTS [Cn. XIII 



DETERMINING THE POLARITY WITH DIRECT CURRENT 



701. General statement and precautions. With direct 

 current it is necessary, in most cases, to install the apparatus, like 

 the ammeter, the voltmeter, the lamp, etc., in a very definite man- 

 ner so that the current extends through the instrument in a given 

 direction. That is, the positive end of the wire must be attached 

 to the positive binding post. But when ready to install any piece 

 of apparatus with direct current one rarely knows which is the 

 positive and which the negative wire. It is necessary to find out 

 by experiment. 



Precautions in making polarity tests. If possible, have a rheo- 

 stat in the circuit before making the tests. One of the small 

 rheostats for use with the small current arc lamp can be very easily 

 introduced into the circuit (see fig. 188, 270 for wiring). If an 

 adjustable rheostat is already in the circuit, set it for the least 

 current. 



In making the tests never allow two naked wires to come in 

 contact for that would complete the circuit and might burn out a 

 fuse or do something worse. 



Never use a piece of metal, or a metal dish for holding the testing 

 materials. Always use glass, porcelain or wood or some other 

 non-conducting material. The tests are perfectly definite and safe 

 if applied with due care. 



Remember also that when repair work on the line is done, the 

 polarity of the supply wires may be changed. This would of course 

 change the polarity of the arc lamp and a good light could not be 

 obtained. One must be on the lookout for every possible trouble 

 and have the knowledge and the resourcefulness to make the neces- 

 sary modifications. 



DETERMINING THE POLARITY WITH AN ARC LAMP, WITH A 

 VOLTMETER OR AN AMMETER 



702. (A) If an arc lamp and rheostat are available the 

 simplest test is to connect the arc lamp, large or small, and rheostat 

 as directed above ( 700). With proper carbons in place turn on 



