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ELECTRICAL ACTION 13
5 ohms, and the current flowing through it is known to be 2 amperes, the voltage must be 5 X 2 = 10 volts ; but if the voltage is known to be 10 volts and the current to be 2 amperes, the resistance of the conductor must be 10-r-2 = 5 ohms; if the resistance were anything else the current would be greater or less than 2 amperes. The formula above given is
E with equal accuracy written as R = — . Similarly,
I when both resistance and voltage are known the amperage can be found, since it will always be that current which that particular voltage will drive through that resistance; the formula for finding the
E current value being I = — . All three forms of this
R formula constitute what is called Ohm's Law.
Every projectionist should memorize and know its three forms as well as he knows the spelling of his own name. He should be able to use them in electrical arithmetical calculations as easily as he counts change, since he will need to use Ohm's Law very frequently in his work. Unless he has it at his fingertips he will not be able to do the work expeditiously.
Practical D. C. Circuits
(30) The simplest form in which electricity enters the projection room is through the common twowire, 110-volt d.c. circuit or "line." Physically, the line consists of two insulated wires which, after being serviced by suitable fuses, terminate at a doublebladed switch. To the opposite end of the switch two insulated wires are attached, which receive the current when the switch is closed, conveying it to various circuits where its use is required.
(31) Electrically, the incoming wires possess "polarity." That is to say, one is positive, the other negative, according to their connection with the