International projectionist (Jan-Dec 1935)

Testing Electric Circuits By A. C. SCHROEDER 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 restored 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 but elsewhere. Do not assume 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 essential. 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 re From I. P. for Dec, 1931; Feb. and March, 1932 STB X lationship 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 of 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 assumption is never true, but the resistance of the wires in this case is very low and can be ignored. 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 ignored. The current in such a circuit will be found by dividing 10 by 5, which shows that 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 caused this. Placing the voltmeter across the battery, we see that there still are 10 volts at this point. We place the meter across the resistance and get only 5 volts. Apparently some voltage has been lost between the battery and the resistance. Taking the voltage across the resistance, which is 5, and dividing by 5, the number of ohms, again gives us 1 amp. as the current. To illustrate a different angle, we draw 2^ ^.figure 1 tne circuit shown in Figure 2. R has been moved to the next battery, where it is connected by a wire having practically no resistance. The loop of wire extending from R to X and back to the battery is 200 feet long and has 5 ohms resistance, just as the two wires had in Figure 1 after R had been moved 100 feet from the battery. An ammeter will show a flow of 1 amp. A voltmeter across the battery shows 10 volts. The meter is now placed across R, and the reading is 5 volts as before. One lead from the meter is then touched to point 2 at the lower end of the battery, and with the other lead placed on 3 at the far end of the resistance, a reading of 5 volts is obtained. The drawing will show that we are measuring the voltage across the wire that connects the battery and the resistance, that is, around the 200-foot loop. It takes 5 volts of the battery potential to force the current through the loop of wire. We know that the resistance of the wire is 5 ohms. Dividing the voltage drop in the wire by the resistance of the wire gives us 1, which is the number of amperes flowing. As the current remains constant so long as no change is made in the circuit and the battery is not discharged, then our answer in amps, must be 1, regardless of how the calculation is made; and right here we must watch our step. Notice the italics in the previous paragraph. To apply Ohms-law we must be very careful not to get the various parts of the circuit mixed up. Had we taken the voltage of the battery and divided it by the resistance of the long loop of wire, we should have had a wrong answer. Mistakes such as this are very easy to make when dealing with circuits that are more or less complicated, but this is no fault of the Ohms-law. The law always holds good, and when it seems as though it will not work in some cases, it is because we do not use it properly. The voltage of the battery (Fig. 2), is also the voltage across that part of the circuit starting at 1, through the resistance, R, out on the long stretch of wire to X, and back again to point 2 at the other end of the battery. In applying Ohms-law, when we consider the voltage across the entire circuit we must also consider the resistance of exactly the same circuit. Before passing on from Figure 2, let us make a different application of Ohms R 3 Figure 2 [5]