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8 MOTION PICTURE HANDBOOK pass in one second through a resistance of one ohm, under a pressure of one volt.). A current of such strength as would deposit .005084 grains of copper per second. The unit rate of flow per second. [NOTE.—Some writers say Uiat the term amijcre does not represeut quantity, but only indicates tUe strengtli of tlie current; quantity being* represented in coulombs, which means the quantity passing a point in a given time (coulombs equal the amperes times and secouds). Thij is splitting hairs altogether too line for the average operator. Technically the above statement is quite true, but for the operator's purpose it Is well enough to say that amperes represent quantity.] OHM (there are several standards, viz., the Board of Trade Ohm, English Ohm, British Association Ohm, Legal Ohm and the Standard Ohm): The "Legal" ohm is the standard used in the United States, and it is defined as follows: The resistance of a column of mercury (the resistance such a body of mercury would offer to current) 106 centimeters in length, having an area of cross-section of one square milli- meter at 0 degrees Centigrade or 32 degrees Fahrenheit. This is now the international value of the ohm. WATT: The unit of electrical activity or power. The number of watts is numerically equal to the amperes times tile voltage. One volt times one ampere equals one wait or 1/746 horsepower. Sometimes called the "Volt-Ampere." HOW TO MAKE CALCULATIONS. Knowing the voltage and number of ohms resistance the number of amperes flowing may be determined by dividing the volts by the ohms; as, for instance, having no volts pres- sure and a rheostat offering 3 ohms resistance how many amperes will we get? no divided by 3 equals 36 2/3, the number of amperes. Knowing the voltage and number of amperes flowing, the ohms resistance offered may be deter- mined by dividing the volts by the amperes; as, for instance, with 220 volts pressure and 40 amperes flowing how many ohms resistance have we? 220 divided by 40 equals 5^/l>, the ohms resistance offered to current passage. Knowing the number of amperes flowing and the ohms resistance we may find the voltage by multiplying the amperes by the ohms; as, for instance, if we had 3 ohms resistance and were getting 30 amperes of current we would find the pressure by multiplying 30 by 3, which would tell us the voltage was 90