International projectionist (Oct 1931-Sept 1933)

June 1933 INTERNATIONAL PROJECTIONIST 23 Review of FUNDAMENTALS OF SOUND The. probability that projectionists will shortly extend their activities to include the servicing of sound picture equipment has generated renewed interest in the fundamentals of sound recording and reproduction. Each month in this departm,ent will appear material which will serve this interest. This month's installment is presented through the courtesy of RCA — Victor Co., Inc., and is copyrighted by them. Reproduction of all or any part of the following without permission is forbidden. IT WAS explained previously that magnetic lines of force flow from the when a current is passed through north pole to the south pole in parallel a wire, a magnetic field is set up lines across the air gap between the around it, and that a voltage is gen erated across a loop of wire placed in a magnetic field if the number of magnetic lines of force included within the loop are changed. The magnitude of the voltage generated depends upon the rate at which the number of lines of force through the loop are changed, i.e., if a change of ten lines of force per second produces a certain voltage, a change of twice ten, or twenty, lines per second produces twice the voltage. The total number of lines through the loop at the time of the change has no effect upon the voltage generated. poles. In (a) a loop of wire is shown in a position such that one side of the loop is cutting the lines of force in one direction, and the other side of the loop is cutting the lines in the other direction. The direction in which the loop is turning is shown by the arrow of the crank. Under this condition a "^voltage is generated in side "A" so as to cause current to flow in the direction shown by the arrow at that side of the loop. Side "B" of the loop cuts the lines of force in the opposite direction, there voltage generated in the loop for two positions during each revolution. When the loop is in the position shown in (c), one half turn from position (a), the voltage generated in side "B" is such as to cause the current to flow into that side of the loop, and the voltage generated in side "A" is such as to cause a current to flow out of that side of the loop. Thus the direction of flow of the current through the loop reverses as it changes its position from (a) to (c). When the sides of the loop are in the positions shown in (a) and (c), the number of lines of force cut for a given amount of rotation is greater than when the loop is in any other position. As the loop rotates from position (a) to position (b) the number of lines cut for equal amounts of rotation gradually becomes less until, at position (b), no voltage is generated, because when the loop is in position (b), the sides of the loop are moving along the lines of force instead of cutting them. From this it is seen that the voltage is continuously varying in magnitude and twice during each revolution the voltage is zero ; also, the voltage changes direction twice for each revolution. The current generated by such a machine is called an alternating current because it alternately flows in one direction and then in the other. Two rings, one connected to each side fore the voltage generated in side "B" Specifically stated, this means that causes a current to flow in the oppo the same voltage is produced in a coil site direction, as shown by the arrow of the loop, are provided, together when the number of lines is changed at that side of the loop. with wiping contacts called "brushes" from 100 to 120 in one second, as The voltage generated in the sides to permit external connections to the of the loop causes a current to flow into one side and out of the other. In order for a current actually to flow through the loop, a connection when the number of lines is changed from 1,000 to 1,020 in one second, because the rate of change of the number of lines through the loop is the same (20 lines per second) in each would have to be made between the case. rings shown in the drawing. The The direction of the generated drawings do not show an external voltage depends upon the direction of connection, but the arrows indicate the the lines of force through the loop, direction the current would flow if an and upon whether the number of lines of force through the loop are increasing or decreasing. The process of following through the different changes of position of a rotating loop so as to determine the nature of the generated voltage in it, is complicated and not easily understood. The generation of a voltage in a loop can be more readily understood if the sides of the loop are thought of as cutting the lines of force. When so considered the voltage generated is proportional to the number of lines of force cut per second, and the direction of the voltage generated depends upon the direction in which the sides of the loop cut the lines of force. Figure 11 shows a magnetic field in which there is a loop of wire. The external connection was made. When the loop is in such a position as to be at right angles to the lines of force, as shown in (b), there will be no voltage generated in the loop, because the sides of the loop do not cut any lines of force when in this position. The same thing is true when the loop is rotated through a half turn, and side "B" is at the top and side "A" at the bottom. Therefore there is no loop while it is in rotation. The rings are called "collector rings," or "slip rings." These rings are usually made of copper or brass, but iron is sometimes used. They are insulated from the shaft and from each other. The brushes are usually made of carbon or a combination of carbon and some metal. A DC generator is the same as an AC generator except in the manner of collecting the generated current. Figure 12 shows a simple DC generator consisting of a magnetic field, one loop, and a collector ring split into two parts. These parts, called "segments," are insulated from the shaft and from each other. One segment is connected to one end and the second segment to the other end of the -DIRECTrON OF COIL B0T4TI0N ' Figure 11 — Showing operation of AC generator