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428 HANDBOOK OF PROJECTION FOR
If, however, it is for any reason necessary to use resistance in A. C. projection circuits, we would by all means advise the use of the grid type, since they will be less noisy and there will be less inductive effect than with coil rheostats.
RHEOSTAT WASTEFUL.— The rheostat as applied to the projection circuit, is for the purpose of consuming the difference between line voltage and arc voltage. Put in electrical terms it is to "break down" the line voltage to the value of arc voltage. In still other words it is to reduce the voltage to the pressure necessary to force the desired number of amperes against the resistance of the arc.
This process represents an absolute waste of energy, since the difference between the line voltage and the arc voltage must be and is dissipated in the form of heat generated by the resistance, and since the heat cannot ordinarily be put to any practical use it follows that the energy consumed in its production is wasted, and the energy thus consumed is all registered on the meter, and must be paid for.
For example, let us assume a current supply of 110 volts and that we are using 40 amperes at the arc. Voltage multiplied by amperes equals watts, hence 110 x 40 = 4,400 watts registered by the meter. The voltage of a 40 ampere arc, using modern carbons, would be about 50 (see Page 395), hence the difference between 110 volts and 50 volts, which is 60 volts, must be consumed in the resistance element of the rheostat. It therefore follows that under this condition the waste in the rheostat will be equal to 60 volts x 40 amperes, 2,400 watts. We therefore are using a total of 4,400 watts, and only actually employing 4,400 — 2,400 = 2,000 watts in the production of light. Under this condition the rheostat is a little less than 46 per cent, efficient.
The condition just described is bad enough, but if the voltage be higher, as for instance 220, then the proportion of waste becomes literally enormous. Using 40 amperes from 220 volt lines through rheostats means a total consumption of 220 x 40 = 8,800 watts registered by the meter, whereas the actual wattage used at the arc is, as in the former case, 50 x 40 = 2,000, hence there is wasted in the resistance of the rheostat 8,800 — 2,000 = 6,800 watts, or about 3y2 times as much energy as is actually employed in the production of light, and the rheostat is, under this condition, less than 23 per cent, efficient. On the other hand, if the voltage weit only 60 or 70 then the waste in resistance would be correspondingly less.
Note: It may be observed that the resistance of an arc