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MANAGERS AND PROJECTIONISTS 429
operating off 220 volt lines through a rheostat is higher, hence its wattage consumption is higher. This may be true but the total waste of energy is not affected by that fact.
Rheostats are only suitable for use on projection circuits where the supply is direct current and the voltage be such that the waste inherent in the rheostat is less than the waste inherent in a motor generator set. If the pressure be 220 or higher it will be very much more economical to break down the voltage by means of a motor generator set, and the efficiency of modern motor generator sets, or some of them, is such that it may pay to use them on 110 volt current, but in this we must consider the actual efficiency of a motor generator set and the fixed charge of the interest on the capital invested, as well as the deterioration of the machine itself. If these various items are such that there is not sufficient saving as against the rheostat to justify its installation and operation, it therefore follows that the rheostat, although enormously wasteful, is still the most economical thing available, everything considered.
If the current be A. C, then as we have already said, there is no excuse for using a rheostat, because either a motor generator set or a transformer is available, and of late years A. C. to D. C. motor generators have been invented which operate at arc voltage, without any resistance in series between the generator and the arc. Such sets work at maximum efficiency, and there is the added enormous advantage of having direct current at the arc. Also there is the mercury arc rectifier available, see Page 515. If for any reason it is deemed inexpedient to install a motor generator set, then a low voltage transformer (inductor, economizer, compensarc, etc.) may be used, and these devices are very much more efficient than the rheostat (their efficiency is well above 90 per cent, though they have the disadvantage of supplying alternating current to the arc. See Page 544.
RHEOSTAT CONNECTIONS AND RESULTANT AMPERAGE.— In Fig. 138 we see a 110-volt adjustable-grid rheostat, with part of its casing removed, and a Powers 110-volt non-adjustable coil rheostat. The grid rheostat has a 40ampere maximum capacity, which may be reduced to 25 amperes by means of the dial switch. The Powers has a maximum capacity of 25 amperes, which cannot be changed. By tracing the connections and comparing them with Figs. 136 and 137 you will find that it is a multiple connection, so that we will get 25 amperes through the Powers and 25 to 40