The Moving Picture World (June 1907)

THE MOVING PICTURE WORLD. 197 orisists as follows of the three above-named points or actors, viz.: The pressure, the current and the resistance. "he law is simply this, that the current multiplied by the esistancc equals the pressure, so, therefore, if you were iven any of these three factors or units you can readily nd the third, as, for instance: To find the pressure required, multiply the current by ie resistance. To find the current that will flow, divide the pressure «the resistance. V To find the resistance necessary to absorb the pressure nd current, divide the pressure by the current. -^ To put these in a still better form and one easily re- membered as a formula, we will call the pressure by the ;tter P. The current by the letter C. The resistance by ie letter R. . .-....". ... ..... I Thus: p = Pressure, or Volts. C = Current, or Amperes. R = Resistance, or Ohms. Then, as above stated, = Ohm's Law. CXR To those who do not understand a simple formula of lis kind I may here explain it. When two or more letters presenting some certain quantities as per the heading of ie formula, as shown by P, C, and R, for example, fey are to be either multiplied, added or subtracted, xording to the sign given between them, such as ; for multiply, -f- i° r addition and — for subtraction, id sometimes two letters may be placed together with- it a sign between them, which means they are to be ultiplied together, unless otherwise stated in the head- g of the formula. The placing of two or more letters ith their signs above a line as shown with one or more iters below that line shows that what is above the line ust be divided by what is below the line, first working it by the signs given the top lines and then working out e bottom line and then dividing the result of the bottom ne into the result of the top line and you get the is\ver. So much for this simple lesson in algebra algebraic formula, which is the most convenient form not only remembering formulas, but of working em out. Remember whenever you see any formula this nature there always precedes it the description what each letter represents, so that by referring to at description all becomes clear. Remember also that ese and all formulae that follow are merely simple sums do and will only be shown as such. Therefore, re- entering the foregoing method, Ohm's Law simplifies self into three formulas, viz.: To find the Pressure required, C X R = P, THE PRESSURE: For example, what pressure (volts) will be required make a current of 12 amperes flow through a resist- ce of 10a ohms? •. v !C=i2 amperes in this example, and R =100 ohms; therefore, as per above formula, if we multiply the irrent, 12 amperes, by the resistance, 100 ohms, we get 200 volts pressure, the answer. To find the Current that will flow, ■F- ■ ■ '■ ■:' > — ==C, THE' CURRENT. R Taking above example again to find what current will w with a pressure of 1,200 volts and a resistance of »ohms: > - ..' • P='iy20Q ohms in this example, and R= 100 ohms; so, therefore, as per formula above, if we divide the press- ure, i,aoo volts, by the resistance, 100 ohms, we get 12 amperes as the answer. . To find the Resistance required, P .. _ = R, THE RESISTANCE. C Again the above example; to find the resistance re- quired to absorb the pressure of 1,200 volts, with a cur- rent flowing of 12 amperes: P = 1,200 volts in this example, and C = 12 amperes; so, therefore, as per the above formula, if we divide the pressure, 1,200 volts, by the current, 12 amperes, we get 100 ohms resistance. By remembering the above formula any operator can work out for himself the various units he may require to adjust accurately for all practical purposes any ap- paratus, so that it will work properly; also he can find out either the resistance of any apparatus, say a lamp, for instance, or the pressure required for it or the quantity of current it will require by knowing any one of the two factors or units, as, for example, an incandescent lamp of 100 volts pressure and a resistance of 200 ohms. By- Ohm's I^aw we have: P 100 " - — — C = .5 amperes, or % ampere. R 200 It is presumed that all of you know the use of the decimal point through its use in dollars and cents, but to those to whom it is not quite clear remember all numbers to the right hand of the decimal point, which is always a dot, are as follows: The first figure starting from the decimal point going to the right represents tenths, the next hundredths, the next thousandths and so on, but there are usually not more than three decimals shown, so in working out the last example .5 would read five-tenths, which is exactly Yz (one-half) in fractions; .05 would read five one-hundredths, and .005 would read five one-thousandths, in fractions. We will now pass on to a brief description of the two forms of electrical energy that we shall have to deal with in bur every-day work and the rest of these articles, viz.: The direct cur- rent or D. C. and the alternating current or A. C. The direct current is that current which flows steadily in the one direction, as upon referring to the following diagram, * * 7*10- 5. Fig. 3, it would be that current which would flow steadily from A to B through the lamp. A and B representing the two points Of electrical supply, such as the two ter- minals of an electrical generator or the two terminals or connections on a switchboard or. panel and extending^ from points A and B are two wires or conductors to a lamp, for sake of a simple illustration. Remember a direct current is that current which flows constantly in