5io POLARITY TESTS [Cn. XIII 



(C) Testing the polarity with a direct current ammeter 

 The circuit should be connected with a rheostat and an arc lamp 

 or one or more incandescent lamps in series (along one wire) then 

 the switch is opened and the ammeter is inserted in one wire (in 

 series), (fig. 273). Now turn on the current and light the lamp 

 ( 30). If the wires are correctly connected the ammeter will 

 indicate the amount of current flowing; if it is wrongly connected 

 then the hand will try to move off the dial below zero. That is, the 

 positive wire has been inserted in the negative binding post of the 

 ammeter, and the negative wire in the positive binding post. 

 Open the switch, and reverse the position of the wires in the binding 

 posts ; turn on the current and the hand will register the amperage. 

 The positive wire can then be marked red or in some other way. 



CHEMICAL POLARITY INDICATORS 



703. Litmus, iodized starch, salt solution and potato indica- 

 tors. (A) Litmus indicator. Take some blue litmus or other 

 acid-alkaline testing paper, about 10 cm. (4 in.) long and place it 

 on a pane of glass or a porcelain plate. Moisten it well. Separate 

 the ends of the wires as indicated in the testing lamp (fig. 21). 

 Put the two ends about 10 centimeters (4 in.) apart on the mois- 

 tened litmus paper. Turn on the current. The positive wire will 

 turn the blue litmus paper red when the current flows. Turn 

 off the current and mark the positive conductor red, or white. 



(B) Iodized starch polarity indicator. Make some starch paste 

 by mixing 15 grams (y oz.) of dry starch (corn starch, laundry 

 starch or wheat flour) with 300 cc. (10 oz.) of cold water. Add y 

 gram (7 or 8 grains) of iodide of potassium. Now heat the mixture 

 with constant stirring until the starch is cooked. Put some of the 

 iodized paste in a glass or porcelain dish and insert the separated 

 wires to be tested in the paste. Turn on the current and the starch 

 at the positive pole will be turned blue. Turn off the current and 

 mark the positive wire in some way. (The iodized starch test is 



702a. If one uses a voltmeter or an ammeter of the new, soft-core type 

 (Eclipse Volt and Ammeters) which register both alternating and direct cur- 

 rent, one cannot determine polarity with them, for they register whichever 

 way they are connected with the circuit. 



