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ELECTRICAL ACTION
circuit the projectionist must remember that the re sistance is not confined to the rheostats, or whatever machine takes its place. (104) A considerable proportion of it is in the light source itself, including the carbons, wires, switches and other elements. If these elements are in good condition the resistance they offer singly or collectively is small enough to be disregarded in calculations dealing with an arc light source. The resistance of the light source itself must not be ignored or it will result in a serious error.
(105) In making electrical calculations it is cu-t« mary, for the sake of brevity, to use the letter E, I and R to express respectively, electromotive force (voltage), current flow in amperes and resistance in ohms, thus: E = Voltage, I = Amperes and R = Ohms.
E
(106) In formulas, — means that the number E
I represents is to be divided by the number represented by I, Suppose E equals 110 volts, and I represents 25 amperes. We then have 110-^25 = 4.4, which would represent ohms. (107) Should two or more letters with nothing between them be above or below the line,
EI
it means they are to be multiplied thus : means that
R the number represented by E is to be multiplied by the number represented by I and the product divided by the
E-20
number represented by R. would mean that after
I 20 has been subtracted from the number represented by E (voltage), the result is to be divided by the number represented by I (current in amperes).
Ohm's Law
(108) Ohm's Law is a very simple and easily understandable formula. It reads: The number of amperes flowing will always be equal to pressure in volts divided by resistance in ohms. That is the basis, but it naturally