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510 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 indicators.— (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 moistened 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 (]4 oz.) of dry starch (corn starch, laundry starch or wheat flour) with 300 cc. (10 oz.) of cold water. Add ^ 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 current, one cannot determine polarity with them, for they register whichever way they are connected with the circuit.