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26 RICHARDSON'S BLUEBOOK OF PROJECTION
That is the reason why wire capacity tables are necessary. (94) Having a wire of given diameter, the resistance offered to current flow increases as (a) current flow is increased; (b) as diameter of the wire <yf other conductor is decreased, and (c) as the length of the wire is increased. (95) Conversely, resistance becomes less as (a) current flow is greased; (b) as wire diameter is increased or (c) as the distance the current must flow through the wires is decreased. (96) In considering resistance the length of both wires of a circuit through which the current must flow must be taken into account. (97) Resistance in copper circuits is calculated as follows: Combined length of both wires in feet multiplied by 10.5 (see resistance figures of copper circuits) and the result divided by the cross section area of the wire, expressed in circular mills.
The Watt
(98) Watt is the unit of electrical power. It is the unit used to express the amount of energy expended.
(99) A kilowatt is one thousand watts. It is the term used to express electrical power in large quantities.
(100) A kilowatt hour is one thousand watts (1 KW) used during a period of one hour. (101) Watts are calculated by multiplying the number of amperes flowing by the voltage. Thus, 100 amperes X 110 volts would be 11,000 watts, which make 11 kilowatt hours (11 KWH). A slight correction is sometimes necessary in a. c. circuits. (102) If used for ten hours continuously it would be 1 10 kilowatt hours ( 1 ip KWH ) . Expressed in horsepower 1100 watts would be 1100 -r 746 (watt in one horsepower) = 1.4744 horsepower (HP).
Use of Terms in Calculations
In order to arrive at certain results the projectionist must often make calculations involving electrical terms just explained. This is particularly true in small theatres where reliable measuring instruments (volt meters, ammeters, etc.) are not available.
(103) In calculating the resistance of a projection