International projectionist (Jan-Dec 1949)

VOLUME XXIV JANUARY 1949 NUMBER 1 Sound System Components ONE of the most conspicuous features of an electric current is the phenomenon of polarity, the distinction between positive and negative potentiality. This characteristic of electricity may be ascribed to the fact that the flow of current in a circuit is unidirectional at any specified instant. As an illustration, consider the terminals of a dry cell: when a circuit is established by connecting the two terminals, current leaves one terminal and enters the other. It is absurd to think of electricity as consisting of two separate kinds, positive and negative, which seek to join or "amalgamate," as one writer of projection texts quaintly expresses it. Electricity is a single entity. Polarity is merely the result of direction of flow. Until the latter part of the last century electrical engineers had no way of knowing which way electricity moves. The early supposition that current flows from positive to negative was incorrect; we now know that the reverse is true. The older incorrect theory nevertheless still survives in our custom of tracing d-c circuits from positive to negative, and also in the practice of grounding the negative side of amplifier circuits. A study of the action of electron tubes and photoelectric cells proves conclusively that electric current flows from negative to positive. For the present, however, let us ignore these purely scientific considerations and delve into the eminently practical problem of determining which of two wires conducting d-c is positive and which is negative. Many electrical appliances — incandescent lamps, heaters, d-c motors, bells, By ROBERT A. MITCHELL III. Alternating Currents buzzers, etc. — do not require specified polarity connections. Their operation is not affected in the slightest degree when their leads are reversed. But certain other d-c-operated devices — arc lamps, . photocells, amplifier tubes, etc. — must be properly connected with respect to polarity if they are to operate. Tests for Polarity There are several good tests for polarity at the disposal of projectionists. A d-c voltmeter, the terminals of which are correctly marked + and — , provides the simplest and most rapid test. When correctly "poled" (positive wire to + terminal; negative wire to — terminal) the voltage will be indicated by the meter. If, however, the wires be reversed, no reading will be obtained. (In such a case the pointer will be impelled to swing over to the off-scale side of zero.) Another test requires the acquisition of a fresh raw potato. The white part of a thick slice of the potato is touched by the two copper wires charged with d-c. For best results the two wires should be thrust into the slice about half an inch apart. In a minute or more, depending on the voltage, the potato will be stained green around the positive wire. In the aforementioned test the slice of potato serves as a crude sort of "test strip," but those who wish may easily prepare professional polarity test paper. This is accomplished by soaking white blotting paper in a solution of salt water to which a small quantity of phenolphthalein in alcohol has been added.1 The paper is allowed to dry and is stored for future use. To test polarity, a strip of the test paper is moistened with water and touched by the two wires carrying d-c. A bright red stain will develop on the paper surrounding the negative wire. A fourth test for polarity involves the electrolysis of water. Introduce the two uninsulated wires into a glass of water containing a pinch of salt. The wires should be held about an inch apart. In a short time — depending on the strength of the current — bubbles of hydrogen gas will collect around the negative wire and tend to cling to it. Alternating Currents; Frequency If the circuit connections to the terminals of a dry cell or other source of d-c be continually reversed, the e.m.f (electromotive force, or voltage) will be periodically reversed in polarity. Instead of a continuous d-c, a reversing and pulsating, or alternating, current will flow through the circuit. The laws which govern the flow of a-c are much more complicated than those which obtain with continuous d-c, but so important is a-c in sound amplification that its study, though difficult, is well worth our time and effort. The rate of periodic reversal of a-c is called the frequency of the current. It is usually measured in cycles per second. For 60-cycle a-c, the current flows in one direction for 1/120 of a 1 Dissolve 15 grains of colorless phenolphthalein in a small quantity of alcohol. Add the alcoholic solution of phenolphthalein to 4 fluid ounces of water in which 75 grains of salt (sodium chloride) have been dissolved. INTERNATIONAL PROJECTIONIST • January 1949