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302 RICHARDSON'S BLUEBOOK OF PROJECTION
to permit the use of sufficient ballast resistance to stabilize the light source to burn steadily. Without such resistance in series an arc will not burn steadily; also, lack of sufficient resistance will cause an immediate dead short-circuit as soon as the carbons are brought into contact to strike an arc. By reduction of voltage the waste in "breaking down" the line voltage to arc voltage is reduced to the minimum consistent with good operation.
(86) To drive the dynamo there is a motor, mounted on the same "bed plate" (often referred to as a "subbase"). This motor may be either a. c. or d. c, but usually is a. c, and for two reasons: first, in almost all localities the supply lines are charged with a. c, usually 60 cycles at 110 volts, or 110-220 volts if it is a 3-wire system. (87) Secondly, as will be explained, it is unprofitable to install a motor generator set where d. c. power is available at 110 volts, as it generally is where d. c. is available from power lines at all.
(88) Where the power lines supply a. c. there is a double purpose served by using a motor generator set: first, it reduces the line voltage to a point about 25 volts above the voltage required at the light source, thus reducing the waste in rheostative resistance to a minimum ; secondly, it permits the taking of a. c. power from the line to drive a d. c. dynamo. It thus accomplishes both voltage reduction and current rectification.
Y\ nere d. c. is available from the power lines, only one purpose is served by installing a motor generator set, the reduction of voltage. Considering that a certain amount of ballast resistance is necessary and that motor generator sets at best have an efficiency rating of only about 70 percent (including the loss through ballast resistance) and considering also that unless they are expertly and carefully handled they may drop below that figure, it is readily understandable that the savingis too small to justify installation of a motor generator when 110 volts d. c. is available.
We therefore do not advise the installation of a motor-generator set where 110 volt d. c. is available.