Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine (1914)

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506 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 manner 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 rheostat 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 necessary 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