Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

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CH. XIII] DYNAMO FOR ARC LAMPS 4§7 decrease in current will be met by an increase in the voltage generated. § 679. Shunt generator. — The connections for a shunt generator or dynamo are shown diagrammatically in fig. 257. A is the revolving armature from which the current is drawn. N and S are the poles of the field magnet and F is the field coil which keeps it strongly magnetized. The stronger the magnetization of this field magnet the higher the voltage furnished by the machine. As usually operated the field rheostat R must be continually adjusted so that the right current is supplied to the field coil F to keep the machine at the desired voltage. § 680. Adaptability of a shunt generator for direct connection to an arc lamp. — If instead of continually adjusting the rehostat R so that the dynamo will supply a constant potential, the machine is left to itself it will be found that when no current is supplied, i. e., the dynamo is running on no load, the potential difference between the terminals a and b is greatest and consequently the current flowing in the field coil F is greatest. If now current is drawn from the dynamo the potential difference between a and b will drop slightly. This will result in a decrease in the current flowing in the field coil F, a decrease in the magnetization of the field magnets and hence a decrease in the voltage generated. The result is in the direction desired, namely, that an increase in the current will be met by a decrease in the voltage. Whether or not a shunt generator connected directly to an arc will work satisfactorily, or whether the arc will be unstable and want to either "run away" or "die out" will depend upon the details of the design of the dynamo ; that is, the voltage at no load, the resistance of the shunt field coils and the resistance of the armature and also on the resistance of the wiring to the arc. Some dynamos have been designed which will work satisfactorily when connected directly to the arc without any intervening resistance. Such dynamos may be run directly by some form of engine or they may be part of a motor-generator set in which high voltage, direct current or alternating current is used to furnish the power. (See also § 682, 684).