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30 HANDBOOK OF PROJECTION FOR
GLASS. — A substance made by melting together sand or silica with lime, potash, soda or lead oxide. By varying the proportions of these ingredients different kinds of glass are obtained, such as bottle, plate, flint, crown, etc.
GRAPHITE. — One of three forms in which carbon occurs in nature. Also called plumbago. Useful to the projectionist as a lubricant for the arc lamp, since it is an excellent lubricant and is but little affected by high temperature.
GRID. — As applied to projection, one resistance of a grid rheostat. See figure 132, page 416.
GROUND. — (a) Broadly it is a. term used to designate a current carrying connection of such high resistance that it is not a short circuit, but which nevertheless enables the current to reach opposite polarity without traveling its allotted path, (b) An electrical contact of one or both polarities with earth, (c) An electric contact of one polarity with something it is not intended shall be electrified.
GROUND WIRE. — In projection a wire connecting a projector frame with earth.
HARD SOLDER.— A solder which melts at red heat only. May be made from zinc and copper.
HIGH COMMUTATOR BAR.— A condition where one or more bars or segments of a commutator are higher than the others. Unless remedied it will work very serious harm to the commutator.
HORIZONTAL CANDLE POWER.— Illuminating power of a light source in a horizontal direction.
HORSE POWER.— One horse-power (h.p.) equals 33,000 foot-pounds of work per minute. It is the theoretical amount of work one strong draft horse is supposed to perform if a block and tackle be attached to a weight of 33,000 pounds and the tackle be of such proportion that the horse can, by exerting his full strength, just raise the 33,000 pounds one foot while walking outward pulling on the rope for a period of one minute. Under these conditions one horse-power has been exerted during that minute. That is the theory of the thing. One horse-power-hour is the amount of work exerted by one horse during one hour, or by 60 horses during one minute, or by 3600 horses during one second. In electrics 746 watts is supposed to represent the raising of 33,000 pounds one foot in one minute, or, in other words, one horse power. The unit was established as follows : 1 watt is equivalent to 1 joule per second (the joule is the practical C. G. S. unit of electrical energy. One joule is equal to .73734 of a foot