Motion Picture News (Mar-Apr 1923)

1084 Motion Picture New National Anti-Misframe League Forum : iMPLiFiED Electricity for Projectionists Electrical Units Lesson 1 — Part II n HE terms of mathematical equations usually consist of letters as their use simplifies and condenses the equations. These letters have a definite meaning and stand for some one definite thing. Thus I ordinarily means electrical current; E is used to denote voltage or electrical pressure; R stands for resistance, etc. These are the meanings we shall give to the use of such letters in future articles. Electricity is a very peculiar thing. What it is no one really knows. We know what it does but do not know what it is. To be sure, there are numbers of theories concerning it, but these are merely scientific guesses. However, its actions are pretty well understood, and these are the things we are most interested in. Current The word current is used to describe flowing electricity or electricity in motion. Coulomb (coo-lomb) is used to denote the quantity of electricity. A coulomb, however, means a certain quantity of electricity at rest. Should the electricity be put in motion so that one coulomb passes a given point in one second, then we say one ampere is flowing in the conductor. The ampere takes its name from Andre Marie Ampere, a French professor, in honor of his important electrical discoveries. Coulomb per second is rather an awkward phrase, so we say so many amperes, remembering that one coulomb per second equals one ampere. Hence, if a certain motor draws 10 amperes from the line, we know that 10 coulombs per second are being used to drive the motor and its load. Voltage In order to cause this current to flow some kind of a force must be applied to drive it. A pressure must be exerted on the current to make it flow through the carrying wires and electrical machinery. This pressure is called " electro-motive-force," or E. M. F., for short, and is caused by what is known as the difference in potential between two points. We often hear electric power lines referred to as high potential lines or low potential lines, which means that these wires are charged by the generators in the power station to either a high or low potential. It is the potential difference between two wires of an electric circuit which determines the E. M. F. and hence the amount of work which can be done under a given set of conditions. This E. M. F. is measured in volts. The volt is named in honor of Allesandro Volta, an early Italian professor in Physics. If a difference of potential is set up between any two points on the same conductor, electricity will flow until the difference no longer exists, or in other words, until the potential difference is zero. This, however, does not apply where the difference in potential is caused by a generator, for if such is the case, then electricity will flow in the circuit continuously. The earth has been chosen as representing zero potential. Usually certain wires of elec tric circuits are grounded, or electrically connected to the ground, in order to limit the potential difference between the wires to a certain determined figure. In the case of alternating current circuits, such grounding prevents the accidental existence of very great differences in potential and is a safety measure. Thus certain wires of residence electric currents are grounded to prevent persons receiving fatal shocks should they accidentally make contact with the wires. If the circuit was not permanently grounded and an accidental ground was caused elsewhere, such a high potential difference might exist as would cause instant death if contact with the wires was made. From what has been said it can be seen that in order for current to flow in a circuit, the circuit must first be " closed/' that is, have no breaks in it, and secondly, voltage must be impressed or applied across the feed wires. If the resistance of the circuit is not changed, then the higher the voltage is the greater will be the current. There can be no current without voltage, but there can be voltage without current, as in the case of an " open circuit." As stated before, the potential difference, or E. M. F., is measured in volts and the everyday name applied to it is voltage. If this distinction is kept in mind, it will help the student understand many electrical actions later to be described. Resistance When a given E. M. F., or voltage, is driving a current of electricity through a conductor such as a wire, it does so against the resistance of the wire. That is, the wire partially obstructs or hinders the current from flowing through it peacefully, so to speak, just as friction slows down the motion of a revolving wheel. Electrical resistance acts in the following manner: It causes a reduction in the voltage applied to a circuit, and the reduced voltage in turn permits a smaller current to flow Change of Policy JUST because the policy of running a great many of the letters received from the various members of the N. A. M. L. in this Forum has been discontinued, is no indication that the interest in the League is decreasing. The editor is in receipt of ever increasing membership applications and interesting letters telling about the efforts made by the projectionists to correct the evil of mutilating film. The present series of articles is being run in an attempt to present the most useful technical information to projectionists, thus allowing him to secure comprehensive technical knowledge for his work. It is highly advisable that each article as it appears in the NEWS be filed for future reference both because references will be made to data in past articles and also for the purpose of compiling a complete booklet on electrical and optical subjects. through the circuit. This reduction in voltag is called " voltage drop " or just " drop." Thru if a certain current is required to flow througl an electrical machine, proper allowance must be made for this " drop " and a higher voltage must be " impressed," or applied, across the machine than would be necessary if no lc due to " voltage drop " were present. Ohm's Law Resistance is measured in ohms (named after George Simon Ohm, who discovered the] relation between current, voltage and reskJ tance, now called Ohm's Law.) Now the current flowing through a circuit; the voltage driving this circuit and th resistance offered to the current are all relat to each other. That is, if we know any two o these quantities, we can easily find the third.] This relation can be, and usually is, expres as an equation as follows: E = IR Remember that E means voltage, I means current and R means resistance. This, then, simply means that the voltage (E) is equal to the current (I) multiplied by the resistance (R). We know, from last week's article, that these terms can be changed around in almost] any manner we require. Thus R = E or I = E R So we see that the voltage divided by the current is equal to the resistance and also the voltage divided by the resistance is equal to the current. It must be remembered, how-; ever, that these relations hold only where these quantities are taken from the same circuit. As an example, suppose we close the switch of an electric circuit and then conpect a voltmeter across the two wires and it reads 100 volts. Then we break one of the wires and connect an ammeter across the broken parts and it reads 4 amperes. By simply dividing the voltage, 100, by the current, 4, we find that the resistance of the circuit was 25 ohms. This equation, which is known as Ohm's Law, is really the most fundamental one of electricity. It combines the three principal quantities, voltage, current and resistance, and is applied to all circuits whether of direct or alternating currents When studying this law ask yourself these questions : 1. If the voltage is increased and the resistance is not changed, does the current increase or decrease? 2. If the voltage is decreased and the resistance is not changed, what happens to the current? 3. If the resistance is increased and the voltage is not changed, how is the current affected? 4. If the resistance is decreased and the voltage is not changed, how does the current act? 5. When the current is changed and the resistance is not, how is the voltage affected? 6. When the current is changed and the voltage is not, how is the resistance affected? 7. If the resistance is zero, what happens to the current when voltage is applied? (Continued on page 1086)