International projectionist (Oct 1931-Sept 1933)

8 INTERNATIONAL PROJECTIONIST September 1933 but because of the inductive effect of the coil winding. Condenser Capacitance If condensers are wired in parallel, increased current (alternating current only, of course), flows "through" them. The same effect can be produced by increasing the surface area of the plates. If the glass plate is removed and a thinner piece of glass substituted (bringing the metals closer together), the light of the bulb will also increase. Still another influence upon the capacitance of the condenser is the nature of the insulating material. This is most easily shown by removing the glass and separating the plates by means of insulating washers, so that the chief matter between them is air. If plates so spaced are mounted in a box, and the box is filled with castor oil, the illumination of the lamp will increase sharply. Several types of condenser are now in commercial use. One consists of two plates of thin, flexible metal foil, insulated from each other by a strip of paper impregnated with wax. This arrangement is wound up to form a flat coil, thus securing plates of large area in a condenser of very small size. The thickness of the insulating paper will depend upon the voltage the condenser is expected to withstand. In order to secure the maximum capacitance the paper is made as thin as is consistent with safety, and the voltage that can safely be applied to the condenser is often printed upon the case or the wrapping. Another common type of commercial condenser consists of many alternate plates of metal and mica (or paper). Plates 1, 3, 5, and so on, are wired together; so are plates 2, 4, 6, etc. The result is that of many small condensers connected in parallel. The Electrolytic Type A newer type, very efficient for certain purposes, is the electrolytic condenser. The insulating layer here is so thin it cannot be measured, but is estimated at far less than one one-millionth of an inch. It is electroplated on a metal surface, usually aluminum. The two conducting plates of the condenser are the metal and the electroplating solution. The film of insulation separating them is so thin that the capacitance of a small condenser of this kind is comparatively very high. The non-conducting layer, in spite of its thinness, possesses good electrical resistance, and condensers of this type can be made to withstand several hundred volts. They cannot, however, be used in all circuits, but only in those carrying D.C. The electrolytic condenser must be "poled" correctly — the aluminum is always positive and the liquid always negative. If this polarity is reversed, the electroplating action is reversed also and the insulating film disappears, resulting in a short-circuit. These condensers are used as filters, in which relation they act as if alternating current passed through them, though in fact, as will be seen in a moment, that cannot happen. The theory of the action of the condenser is a trifle complicated, and of little practical importance to the projectionist. Briefly, electricity is stored, or accumulated, by the metal plates. The charging voltage causes a concentration of electrons on the negative plate and a corresponding lack of electrons upon the positive one. The charge is "held", in part, by the action of the insulating substance, the atoms of which are so "poled", or twisted around, that their positive sides face the negative metal and their negative sides face the positive metal. Current is thus literally stored in the condenser, remaining -^ven after the circuit is opened. But when the charging voltage declines or is reversed, or if the condenser is short-circuited, discharge takes place. The significant fact with relation to the apparent conduction of alternating Showing inside of paper condenser current through the insulation of the condenser lies in the discharge that occurs when the charging voltage decreases or reverses. Consider what happens in a wire leading to one of the metal plates. Current may be thought of as flowing through this wire toward the condenser, charging the . plate to which the wire is connected. ' When partial or complete discharge occurs current flows outward from the same plate, with the result that the direction of flow through the wire in question is reversed. Thus, alternating current flows through that wire. If a lamp is connected in series with that wire, alternating current will flow through its filament (in spite of the fact that the circuit is broken by the insulation of the condenser), the exact amperage depending upon the amount of charge the condenser can hold under the conditions of operation. Therefore if the condenser is large enough a lamp in series with it will light, an A.C. ammeter will register. Effect of Voltage Change A.C. will also flow through the wires leading to the condenser if those wires are connected across unsteady voltage in a D.C. circuit. Whenever the D.C. voltage changes, the charge of the condenser will either increase or decrease. Current will thus flow in and out through the wires connecting to the plates; there will be A.C. in them, although only unsteady D.C. flows in the circuit they bridge. In the case Of hookups of this kind it is commonly said that the condenser short-circuits the "A.C. component" of the D.C. line — another convenient misstatement. There is actually no A.C. in such lines. But the effect of a rhythmic rise and fall in direct voltage is the same as if a small A.C. voltage were added to a line carrying steady D.C, alternately aiding and opposing the direct potential. Electrolytic condensers are used in circuits of this kind. Their charge increases and decreases but never reverses its polarity. Yet, an A.C. ammeter placed in series with such a condenser would, as explained, correctly (^ register alternating current. Since a condenser connected across an unsteady direct voltage tends to charge and discharge with changes in the voltage, its effect is to compensate for those changes. The condenser "smooths" out the D.C. The same fact, expressed another way, is described by saying that the condenser short-circuits the A.C. component, and therefore filters the D.C. Hum heard in sound systems is often caused by