Showmen's Trade Review (Oct-Dec 1944)

66 SHOWMEN'S TRADE REVIEW November 4, 1944 PROJECTIO]\ CLII^IC {Continued from Page 60) first turn of wire. Evidently, since current does flow, the induced voltage is not as strong as the apphed voltage, but the induced voltage does reduce the applied voltage and as a result it delays the current so that the peak current will occur at a period of time later than the peak voltage. This same induction of voltage will occur between every adjacent turn of wire of the coil. Considering that there are hundreds of turns of wire to even one layer of a coil, and that there are many, many layers, one can appreciate the magnitude of the opposition to current flow that there is in a coil of wire. Thus the greater the number of turns of wire there is on a coil of wire and the greater the amount of iron that forms the core, the greater will be the inductance of the coil. Now this inductance of the coil also acts in the opposite direction when the voltage across the coil is decreasing. When the voltage does decrease the magnetic field around the wire will now move across the wire in the opposite direction and this will mean an inducted voltage that will be in the opposite direction. Since the applied voltage is decreasing, the induced voltage will tend to prevent the applied voltage from decreasing — hence the induced voltage will now be in the direction of the applied voltage when it was first applied or increasing. So we may sum up by saying that when a voltage is first applied to a coil of wire there will be an induced voltage resulting which will oppose the applied voltage. Should the applied voltage tend to decrease in value the induced voltage Why Wait Until They Break Down? If your SIMPLEX HEADS are old stylenoisy — show jumpy picture — NOW is the time to replace 'em. We have available for IMMEDIATE DELIVERY 10 REBUILT SIMPLEX MECHS All 5 numbered heads complete with genuine Simplex or Simplex type rear shutters — double bearing movements— removable film gates — heavy cast iron traps — micrometer focusing attachments — formica gears. No, they are not brand new but try to tell the difference . . . Price EA. *425 Liberal trade-in allowance on your old Simplexes. Give us full description including serial numbers. S.O.S. CINEMA SUPPLY CORP. 449 West 42nd St. New York 18, N. Y. CONTINENTAL ELECTRIC COMPANY GENEVA, ILL. GET READY FOR THE SIXTH WAR LOAN SELL MORE WAR BONDS will again oppose the applied voltage and tend to keep the voltage across the coil to a maximum value. Thus if a fluctuating voltage is applied to the coil of wire, it will be varying as it enters the coil but will be unvarying as it leaves the coil. This smoothing out efifect of the coil is called filtering action, and the coil may be called a filter coil (or choke) coil. Now it often happens that a certain amount of inductance is needed in an electrical circuit but there is no single coil that has the required amount of inductance. It is then possible to use several coils in series and accomplish the desired result. The method of doing this is. pictured in fig. 7. The letter "L" means the inductance of the coil in Henries. There are also occasions where we may wish to have a current flow that will be greater in. value than will be permitted, without heating, by the size of wire comprising the coil. In this case we will want to hook up our coils in parallel. Two wires in parallel will conduct a greater amount of current. The same holds true with the inductance coils. When we connect these coils in parallel we figure the total inductance of the combination according to figure 8. These inductance coils are used in radio power supplies, as plate loads in vacuum tubes, and as excitation fields in the loud speakers of our sound systems and radio receivers — so we will have plenty to do with them as we progress. There is much more to be said about them but we will take that up under the specific applications when we come to them. Now let us look at figures No. 9 and No. 9A. First, we have a sensitive meter that has a zero in the center of the scale. If the pointer moves to the right it will indicate a voltage of one polarity and if it moves to the left of center it will indicate a voltage of the opposite polarity. Having the coil of wire connected as shown and holding the permanent magnet as shown and moving the magnet into the coil of wire, the meter will move to the right. As we reach the limit of travel into the coil and stop the movement of the magnet, the pointer of the meter will return to the zero position since there is no longer any movement of the magnet there will be no induced voltage. Now as we withdraw the magnet from the coil we will find that the voltage produced or induced will be opposite in polarity and as the magnet is completely removed the meter indicator will again return to the zero position. Now if we securely mount the magnet on the table and move the coil of wire we will notice a reproduction of the previous results. In practical circuits the magnet is replaced by the magnetic field that surrounds the turns of wire as a result of current flow. New House for Century Circuit The Century Circuit, operating 17 theatres in Brooklyn, has added another house with the opening of the Linden Theatre, Parkside and Nostrand Avenues. Theatre was opened Oct. 12. Entire day's receipts were turned over to the Red Cross. Tennessee House Planned Cowan Oldham, Cumberland, Tenn., has announced his company will remodel the Martin Young building in Sparta, Tenn., into a theatre as soon as building restrictions are lifted. Building was purchased by the company in March with a view of converting it into a theatre. House will seat 800, with 600 on the main floor and others in the balcony. The latest modern equipment will be installed.