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CURRENT RECTIFICATION 309
Effect of Resistance
( 105) One very important item in operating efficiency is that there shall be but the minimum possible ballast resistance interposed in the line, since all resistance above what is strictly necessary to steady the arc is wasted. Unfortunately, in the types of arcs in use at present the stability of the light source depends upon a certain amount of resistance in series with it. This is termed "ballast resistance." If it is too little the arc will be very unsteady and give poor screen illumination.
(106) Experience shows that the steadiness of a projection arc is assured only when there is sufficient ballast resistance to avoid the excessive variation in current Mow that comes from irregularities in the arc feed and the consequent arc length and the variations in carbon resistance. (107) The voltage of the generator of a motor generator set above that of the arc is usually not less than 1 5 volts and often .as much as 30.
(108) The skillful projectionist who keeps his arc feed mechanism working perfectly will secure satisfactory results with less ballast resistance.
Oil and Dust
Keep the motor-generator set scrupulously clean. (109) Oil acts as an insulator, but it will soften the armature insulation, collect dust, grit' and metallic particles, and these, if permitted to remain, may in time produce a short circuit with serious damage to the equipment.
Do not permit dust to accumulate around the armature and pole pieces. (110) It is especially important that all dust be blown out from between the pole pieces and armature once each week in all-day houses, and once every two weeks in two-a-day theatres, because the magnetism of the pole pieces gradually collects bits of metal dust from the air and thus impairs the efficiency of operation. Managers should provide a hand bellows of ample size for this purpose.
Worn Bearings
(111) Many motors and generators have but little