Handbook of projection for theatre managers and motion picture projectionists ([1922])

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MANAGERS AND PROJECTIONISTS 55 result, count an equal number, and place the decimal to the left of the last figure counted. If there are not enough figures in the result to do this, add enough ciphers on the left. When dealing with formulas, E/C means that the quantity represented by E is to be divided by the quantity represented by C, E being the voltage and C amperes. If there be two or more quantities above or below the line, with no sign between them, it means that they are to be multiplied together, thus : E — means that E (volts) is to be divided by C (amperes) mulCR E-15 tiplied by R (ohms). means that after 15 has been sub C tracted from the quantity represented by E (volts), the remainder is to be divided by the quantity represented by C (amperes). The student will be greatly benefited if he will practice writing out formulas of this kind in letters, afterward substituting quantities in figures and working them out. Ohms law sets forth the fact that the number of amperes flowing is equal to the voltage divided by the resistance in E ohms. We therefore have — = C, or in other words, volts R E divided by ohms equals amperes. It then follows that if — = C, R E C multiplied by R must equal E. It also follows that — = R. C It works out as follows: We know that the ordinary 110 volt, 16 c. p., carbon filament incandescent lamp requires approximately one-half ampere of current to bring it up to candle E power. What is its resistance? Using the formula — = R, C 110 volts substituting figures, we have = 220, the number .5 of an ampere of ohms resistance in the filament of the lamp. Again applying E 110 the formula — = C, we have = .5, or }4, as the amper R 220