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40 MOTION PICTURE PROJECTION
path exists. Hence to produce a current it is necessary to have an electromotive force and a closed circuit. The current continues to flow only as long as the electromotive force and closed circuit exists.
The strength of a current in a conductor is defined as the quantity of electricity which passes any point in the circuit in a unit of time. Current is designated by the letter C or 7.
Resistance. — Resistance is that property of matter, in virtue of which bodies oppose or resist the free flow of electricity. Water passes with difficulty through a small pipe of great length or through a pipe filled with stones or sand, but very readily through a large, clear pipe of short length. Likewise, a small wire of considerable length and made of poor conducting material offers great resistance to the passage of electricity, but a good conductor of short length and large cross-section offers very little resistance.
Resistance is designated by the letter R.
Volt, Ampere and Ohm. — The volt is the practical unit of electromotive force.
The ampere is the practical unit of current.
The ohm is the practical unit of electrical resistance. The microhm is one millionth of an ohm, and the megohm is one million ohms.
The International ohm, as nearly as known, is the resistance of a uniform column of mercury 106.3 centimeters in length by one square milimeter in cross-section at a temperature of zero centigrade.
The ampere is the strength of current which, when passed through a solution of silver nitrate,