International projectionist (Oct 1931-Sept 1933)

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m 1.9 \»' INTERNATIONAL PROJECTIONIST VOLUME II NUMBER 1 MARCH 1932 FUNDAMENTALS OF TESTING ELECTRIC CIRCUITS A. C. Schroeder MEMBER I. A. LOCAL UNION ISO, LOS ANGELES, CALIF. Ill IN the last article we discussed the testing of a line and a fuse block very thoroughly, using a 110-volt lamp. If the line had been 550 instead of 110, it would have burned out the lamp the first time it was put across the circuit. Had we used a 12-volt lamp on the 110-volt line, the result would have been the same. Using a 110-volt lamp on a 12-volt line would give no results, the voltage not being high enough to force sufficient current through the light to give an indication. When testing a 110volt circuit with a 220-volt lamp the result is that the lamp glows dimly, but in most cases this is O.K. if the fact that we are using a lamp designed to be worked on a higher voltage is kept in mind. We now realize that testing apparatus must be suitable to the circuit to be tested: different circuits require different apparatus. Often the testing device can be adapted to the circuit. An example of this is the test lamp having two sockets and two 110-volt lamps. The lamps are in series and form the equivalent of a 220-volt lamp. Figure 1 shows such a tester. With it we can test both 110 and 220-volt circuits. Naturally, the lamps will not burn to full brilliancy on 110 volts. Sometimes a lead is brought out between the lamps so that it can be used with one of the outside leads when test ing 110 volts. Only one lamp lights when they are used in this manner, and it burns brightly only if the voltage is normal. Other Methods Let us leave the test lamp for the time being and take up other methods of testing. The voltmeter as a test instrument has certain advantages and certain disadvantages. The chief disadvantages are the delicacy of a good meter and the cost if it is ruined. Replacing a burned-out test lamp is a matter of thirty or forty cents. The cost of the average meter will run from five dollars up. It is true that meters can be obtained for about a dollar, and sometimes less, but these are not used a great deal and then only for testing battery voltages. A DC meter can be used only on DC. Some DC meters are now supplied with a copper-oxide rectifier, allowing them to be used on AC, but it is then considered to be an AC meter. Most of the AC meters can be used on DC as well as on AC, although they usually are not as accurate in such a case as the DC meter. On some types of DC circuits the AC meter cannot be used at all, even though the same meter can be used on other DC circuits. Some of the advantages in using a voltmeter are that it usually shows nearly the exact voltage present. On low resistance circuits — such as power lines, battery circuits, etc. — any ordinary meter [7] will show the voltage; but when testing high resistance circuits found in amplifier and power supply devices the ordinary meter will show only a fraction of the voltage that is normally present. The reason for this is that all meters require a flow of current through the instrument in order to give an indication; they are devices that consume power, and that power must be supplied by the circuit under test. If the circuit we are testing has a comparatively great amount of resistance, there will be a large drop in voltage across it due to the current drawn by the meter. Usually there already is a current flowing: it may be the current taken by the plate of an amplifier tube. The purpose of the test might be to determine the voltage at the plate of the tube under working conditions. The current taken by the plate causes a drop in voltage across th'e resistance, and when the addi Figure 1