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RESISTANCE 39
required. It may be anything between 12 and 125 amperes and higher in some cases.
Let it be understood that, given a certain resistance, the higher the electrical pressure (voltage), the greater the number of amperes will be forced through. Conversely, the lower the pressure (voltage) the less the current flow will be.
(31) A projector arc light will not operate efficiently unless the supply voltage or pressure is considerably higher than that required to force the requisite amperage through the arc. A steady arc cannot be maintained with insufficient voltage. The arc will flutter. (32) Experience has shown that in order to secure arc stability at least 20 to 25 volts must be broken down. (33) This power dissipation is necessary to obtain the proper results.
(34) If, however, more voltage is broken down than is required to stabilize the light source, wasted power may be saved by the installation of a motor generator set, though this is not always profitable. For example: if the current is d. c. and the voltage 110, and if the voltage drop across the arc is 55, and if we break down 15 volts in the rheostat to attain arc stability, we will require altogether 70 volts supply pressure. If we subtract 70 from 110 we have 40 volts of waste power: we have only to multiply 40 by the amperage to ascertain the wattage loss; further multiply the result by the number of hours of operation per day and that by the price per watt hour, and we arrive at the cost of the power waste for each day of operation. With this we can estimate the saving achieved by the installation of a motor generator set. Any appreciable saving would be doubtful unless the current cost is high and the hours of daily operation long.
On the other hand, if the voltage supply is 220 (the arc stability breakdown remaining the same, as of course it would), we then would have a waste of 220 — 70 == 150 volts times the amperage, which under almost any possible circumstances would demand the installation of a motor generator set. In this case a motor generatoi