Motion Picture News (Mar-Apr 1923)

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1480 Motion Picture Hews National Anti-Misframe League Forum Electricity for Projectionists Electrical Inductance ill Lesson II|HE subject of induction plays an important part in electricity. In fact, the generation of electricity by means of dynamos would not be possible were it not for induction. Transformers rely on it for their operation; it is present in motors; its effects are seen when a switch, through which current is flowing, is opened; in fact, wherever current flows induction is almost certain to be present. It has already been shown that if a piece of iron is placed near the pole of a magnet another pole will be induced in the iron which will be of opposite polarity to that of the magnet. That is, if the pole of the magnet is south, the induced pole will be north. This is described as magnetic induction. It is also possible to induce voltages in conductors under certain conditions. In order N Figure 12 to give a clear explanation of the action, a roundabout method will be used to describe it. Fig. 12-A shows a magnetic field set up between the poles of two magnets. The lines of force, as we have already learned, go from the north, across the air gap, to the south pole. To the right of this, Fig. 12-B shows the magnetic field set up by a wire carrying an electric current. As the black dot in the center of the wire indicates that the current is coming out, the fines of force, according to the rule of thumb, must move in the direction of the arrow. Now suppose we place this wire, surrounded by its magnetic field, between the two poles shown to the left. What happens? The magnetic field on top of the wire is going to the left, the field between the two poles is going to the right, so that if the wire is placed in the position shown in Fig. 12-A, the field of the wire will buck that of the poles and tend to wipe it out. On the bottom, however, the two fields are moving in the same direction so they will add up and make a much stronger field. This is illustrated in Fig. 13, which shows the weak field on top and the strong one on bottom. This figure shows the shape of the lines of force only at the instant the wire is placed in the field of the magnets. It will be noticed that the lines, after leaving the north pole, bend down under the wire and then curve up to enter the south pole. Now these lines, due to their odd shape, tend to straighten out and go back to their original position as shown in Fig. 12-A. They act just like rubber bands, stretched between the north and south poles, which have been drawn back, or rather down, by the wire. When the wire is released, the tension of the bands forces it up in the direction of the arrow. This principle is used in the electric motor. -Part III When voltage is applied and current flows through the armature, the fields of the wires combine with the field of the poles, as shown in Fig. 13. The result is that the wires on one side of the armature are forced up while those on the opposite side are forced down and, since the armature is pivoted, it must revolve. To summarize what has so far been said: If a conductor, carrying an electric current, is placed in a magnetic field, a force will be exerted on it which will cause the conductor to move in the direction of the weakest part of the field. Electro-magnetic Induction If a conductor, which is carrying no current due to an outside source of E. M. F., be moved through a magnetic field so as to cut the lines of force, an E. M. F. will be induced in it, and if the two ends of the wire are connected together while moving through the field a current will flow in the wire due to the induced E. M. F. or voltage. This is shown in detail in Fig.14, where the wire, represented by the circle, is moving across the lines of force flowing from the north to the south pole, in the direction of the arrow. The cross in the center of the wire indicates that the current is going into the wire and, hence, away from the reader. Compare this with Fig. 13. The direction of the field is the same in case ; the wires are both moving in the same direction; but the current in Fig. 14 is going in the opposite direction to that in Fig. 13. This induced current will, in turn, set up a magnetic field which will move in the direction of the arrows on the dotted circle. This can be verified by applying the rule of thumb. The magnetic field is now strong on top of the wire and weak below it. This means, of course, that the wire will try to move in the direction of the weakest part of the field, which is down. But the wire is being forced up in the direction of the arrow, and hence work is being done in order to obtain this induced current in the wire. This, in fact, is the principle of the electric generator. The armature, carrying the wires, must be turned between the poles in order to make it generate electricity. The electrical action just described is called electro-magnetic induction. The law which governs induced currents is as follows: The induced E. M. F. tends to send an electric current in such a direction as to oppose the change of flux which produces it. Self-induction It is not essential that the conductor be moved for a current to be induced in it. (When speaking of induced currents it should be remembered that the thing which is induced is the voltage and this, in turn, causes the current to flow.) If the conductor is stationary and the lines of force, or magnetic field, around it changes, the same result will be obtained. That is, an N Figure 14 Figure 13 induced current will flow in such a direction as to oppose the change in the magnetic field. We can see from this, then, that since a wire, carrying an electric current, is surrounded by a magnetic field, every time the current changes in value an induced current flows in the wire in such a direction as to oppose the change in current. (The flux varies directly as the current, so what affects the current also affects the flux.) This is known as selfinduction. In Fig. 15 are shown ten sections of the same wire. In number 1 no current is flowing and, hence, no flux surrounds the wire. In number 2 the switch has been closed and current is just beginning to flow. The Mux is just commencing to increase. In numbers 3, 4 and 5 the flux is still increasing along with the current. Between 6 and 8 the current is constant, as is also the field around the wire. Then the switch is opened and 9 and 10 show the current and field decreasing to zero. Now between 2 and 6, when the current is increasing the direction of the induced current in the same wire will be such as to try to prevent the current from increasing. In order to do this it can be easily seen that the induced current will flow in the opposite direction to that of the circuit current. Between 6 and 8, when the current is constant, no current is induced because the flux is not changing; but from 9 on the current is decreasing, as is also the flux, so the induced current will flow in the same direction as the circuit current, in an effort to maintain the circuit current. It is this which causes Hashing and arcing whenever a switch in a circuit is opened. Something like this happens when you turn the crank of your projection machine. As the crank is first turned, the weight of the stationary gears, shutter, etc., try to hold back and prevent you from turning. When you once get up to speed, however, the crank turns easily. When you stop cranking, the weight of the moving parts carry the crank around several times, even though you have let go. (Continued on page 1482)