CH. XIII] 



POLARITY TESTS 



the one commonly employed for weak currents for it is very 

 sensitive; it is, however, equally good for large currents). 



(C) Salt and water polarity indicator. Make a y% solution 

 of common salt (NaCl) in water. Place the solution in a glass or 

 porcelain dish about 10 cm. (4 in.) across. Insert the two separ- 

 ated wires to be tested in the liquid and turn on the current. When 

 the current is on, many small bubbles will appear at the negative 

 pole. In making this test remember the precautions ( 701). 



(D) Raw potato polarity indicator. Cut an ordinary uncooked 

 potato in half. Insert the wires into the potato having the wires 

 as far apart as possible. Turn on the current. The potato 

 around the positive pole will turn greenish. If the potato is quite 



FIG. 274. THREE-WIRE ARC LAMP OF THE BAUSCH & LOME OPTICAL COMPANY 

 For a full explanation see fig. 14.5 and 704. 



