Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
[Ch. 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 man¬ 
ner 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 rheo¬ 
stat 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 neces¬ 
sary 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


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