International projectionist (Oct 1931-Sept 1933)

INTERNATIONAL PROJECTIONIST VOLUME I NUMBER 3 DECEMBER 1931 FUNDAMENTALS OF TESTING ELECTRIC CIRCUITS A. C. Schroeder MEMBER, I. A. LOCAL UNION 150, LOS ANGELES, CALIF. TESTING is done to find out what conditions do or do not exist in an electric circuit. It is done with electrical instruments rather than "looking" for the trouble, because there are difficulties which cannot be found by looking — they occur somewhere within the apparatus or in some remote corner into which we cannot get to see what has happened. In other instances testing is resorted to because it usually locates the seat of the trouble quickly, whereas if we were compelled to look through all of the parts that are in any one circuit it would take considerable time, and when we finished we might find that the trouble is not in this circuit and the hunt would continue in another direction. Do not get the idea that visual examination is of no value. A short inspection combined with the process of pulling and prying on leads, and so forth, will sometimes reveal the trouble in short order. In some instances visual inspection is required after the electrical tests have been made ; at other times the two methods are used together. Placing the hand on a suspected wire or apparatus often gives an indication of trouble by the amount of heat that is present. This must be done carefully, otherwise a skin burn might result. In order to know what kind of meter to use, what meter would be ruined if used on a certain test, or if the meter would ruin the part being tested, and also to interpret the results of the test, an understanding of ohms-law is essen tial. This need not frighten anyone; it is simple and requires only a rudimentary knowledge of mathematics. Ohms-law is simply a statement of the relation existing in a circuit between the voltage applied, the current flowing, and the resistance in that circuit. If any two of these values are known, the third one can be found either by multiplication or by division. When the current and the resistance are known, the voltage is found by multiplying the two known values. When the voltage and one of the others is known, the voltage is divided by the other known quantity. This relationship exists in all circuits no matter how large or how small the apparatus or the wiring may be. The current is the result of the voltage and the resistance. It cannot be changed unless the voltage, the resistance, or both the voltage and the resistance, are changed. A change of voltage or a change in the resistance always causes a change in the current. If the voltage is increased and the resistance is increased proportionately, the current remains the same. If both are decreased proportionately, the current again remains as it was. Let us consider a few examples in order to make this clear. In Figure 1 we have a battery, B, and a resistance, R, which are connected so as to form a closed circuit. For the first example we will assume that this is a small test circuit on the bench and that the connecting wires have no resistance. This as sumption is never true, but the resistance of the wires in this case is very low and can be neglected. The voltage of B is 10, the resistance of R is 5 ohms. If B is a storage battery in good condition, its resistance will be very small and can also be neglected. The current in such a circuit will be found by dividing 10 by .5, which showsthat 2 amps, are flowing. A voltmeter across the battery will read 10. If the meter be put across R, it will also read 10. An ammeter inserted in the line at X will read 2 amps. Placing the same resistance, which may be an electric light, at a point 100 feet away, we must use two connecting wires, each of which is 100 feet in length. The resistance of these two wires is 5 ohms and cannot be ignored, since it will affect the result a great deal. The total resistance in the circuit is now 10 ohms (5 ohms in R, and 5 ohms in the wires). Ten divided by 10 gives us 1. Only 1 ampere is flowing in the circuit now. One ampere is not sufficient to light the lamp properly. Let us see what conditions have acted to cause this. Placing the voltmeter across the battery we see that there still are 10 volts Figure 1 ^M fi X [7]