International projectionist (Jan-Dec 1948)

energy, and the rate at which this form of energy is used up is measured in watts. To calculate electrical power in watts we have only to multiply the electrical pressure (volts) by the amount of electricity flowing per second (amperes) : Watts = Volts X Amperes Suppose that an exciting lamp supplied by current of 10 volts pressure is found to pass lx/i amperes. How much power does the exciter consume? Watts = 10 X 7.5 = 75 watts When the number of amperes passed by a device and the power it conusmes are both known, it is an easy matter to find the voltage at which it operates. We simply divided watts by amperes, as shown by the following formula. Watts Volts = Amperes And to find amperes: Amperes = Volts Electricians use the letter P to represent watts (power), E to represent volts (electromotive force), and I to represent amperes (intensity) . These symbols are used so frequently in electrical work of all kinds that it is a good idea to memorize them. The three formulas just given may accordingly be "abbreviated" in the following way: P P P = EI E = — I = — I E . For certain purposes the ampere is too large a unit of current intensity. When measuring very small currents we use the Bmsans MOTION PICTURE LOCAL NO 376, I. A. T (Hmttttga PROJECTIONISTS SYRACUSE, N. Y. . S. E. Watts milliampere — one thousandth of an ampere. The plate current of voltage amplifiers is measured in milliamperes. Likewise, small electrical pressures are measured in millivolts, and small expenditures of electrical power in milliwatts, or even microwatts. (A microwatt is one millionth of a watt.) But when we are dealing with thousands of watts we use the kilowatt — one thousand watts. Energy Units In the interest of accuracy we must distinguish electrical energy from electrical power. A 100-watt lamp consumes 100 watts of power, of course, but the amount of energy used up depends on how long the lamp is burned. If we burn the 100-watt bulb for 1 second, we use up 100 watt-seconds of electrical energy. A 50-watt bulb burned for 2 seconds also consumes 100 watt-seconds of energy. The watt-second is frenquently called the joule. For most purposes the joule is too small a unit, so the kilowatt-hour (1000 watts for one hour) is employed. Los Angeles Projectionists LOCAL UNION NO. 150 Extends To All Its Friends In The I. A. T. S. E. the Reason 5 Ljreeti InaS r This unit is a favorite with the electric utilities, for their service meters register the number of kilowatt-hours of energy consumed by their customers. Series and Parallel Circuits No matter how efficient an "electron pump" we have, current will not flow in a conductor unless a complete electric circuit be established. A battery or generator forces electrons out, to be sure, but other electrons must move in to take their places if there is to be a flow of current. In other words, current will not flow along a wire unless there be a potential difference (voltage difference) between the ends of the wire. In the series circuit all the currentconsuming components are connected one after the other, hence there is only one path for the electrons to follow. Since there is only one path in a series circuit, the current-intensity (amperes) in each part of a series circuit is the same. But some of the electrical pressure (voltage) is lost in pushing electrons through each component of the circuit. Across the terminals of each device connected in the circuit, therefore, we find a voltage-drop, measurable with a voltmeter. The sum of all the voltage-drops equals the original electromotive force supplied to the circuit. Suppose a series circuit contains a resistance coil, a lamp and an ammeter. Connecting a voltmeter to the terminals of the resistor we find a "drop" of 36 volts. Next we test the lamp and find a drop of 73 volts. The drop across the terminals of the ammeter is 1 volt. What voltage is being supplied to this circuit by the source of current? 36 + 73+1 = 110 volts The parallel circuit has two or more branch circuits which allow electrons to travel in two or more paths simultaneously. The current (amperes) flowing in one branch may be different from the current flowing in another. The branch having the greatest resistance will have the least flow of current, and vice versa. A parallel circuit has much in common (Continued on page 29) INTERNATIONAL PROJECTIONIST • December 1948