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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 position 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