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54 HANDBOOK OF PROJECTION FOR
10 = 1100 watts, which, divided by 746, equals 1.47 h. p. If, on the other hand, we use 110 amperes at 10 volts, the result in power would be the same. But if we use 10 amperes at 10,000 volts we then would have 10,000 x 10 = 100,000 watts, which, divided by 746 equals 134 h. p.
USE OF ELECTRICAL TERMS IN CALCULATIONS.— In
projection rooms not equipped with a reliable voltmeter and ammeter it will be difficult for the projectionist to make intelligent calculations, since in order to find a desired quantity, be it voltage, amperes or ohms, he must know the value of the other two. In order to accurately calculate the number of amperes flowing in a circuit it is necessary to know exactly the number of ohms resistance the circuit offers (including wires, appliances, etc.) and the exact voltage.
It is, however, highly desirable that the projectionist understand how to make electrical calculations, at least insofar as applies to his projection circuit, since under some circumstances such knowledge will be necessary to intelligent work.
The projectionist must firmly fix in his mind the fact that where the projection circuit is concerned the resistance does not lie wholly in the rheostat, or whatever takes its place. The wires, carbon-arms and carbons usually offer comparatively slight resistance, but a very considerable portion of the total resistance of a projection circuit is in the arc itself. Under usual conditions the resistance of the wires, carbonarms and carbons may, for the purposes of calculation, be neglected, but unless the resistance of the arc itself be taken into consideration, very serious error will result.
When making electrical calculations it is customary, for the sake of brevity, to use the letters E, C, and R, in which E stands for "electromotive force," which is merely another name for voltage ; hence E stands for voltage ; C stands for current flow, meaning amperes; hence C stands for amperes; R stands for resistance in ohms; hence R stands for ohms.
The projectionist should also remember that in a common fraction the horizontal line always means "divided by," thus 1/2 really means 1-^-2. To divide 1 by 2 we put down the 1, followed by a period, called a "decimal point" and then add ciphers thus: 1.00. We now have 1.00, with a decimal point between the one and the two O's. In dividing 1 by 2 we only need one cipher, thus 1.0 and 1.0 -* 2 = .5, which is exactly the same thing as 5/10, or 1/2. The rule is to count the figures or ciphers to the right of the decimal point in the number being divided, and then, beginning at the last figure of the