Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine (1914)

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492 CURRENT RECTIFIERS [Cn. XIII The small electrode in the bottom of the tube, at C is used in starting the tube. In starting, the tube is first rocked making and breaking a mercury contact. A small amount of current flows through between C and B and starts the arc going, after which it will continue to burn as long as B is the cathode, but if the arc is extinguished even for an instant, it will go out and the tube must be tilted again before it will work. pulsating, but the current is always in one direction and the pulsations are so slight that the crater of the positive carbon remains almost as constant as with the direct current furnished by a motorgenerator set. Both the motor-generator set and the mercury arc rectifier are necessarily expensive. For a small plant to be used much of the time for the arc lamp, and where power is needed for other purposes, like the lighting of the house, pumping water, running machinery, etc., etc., it would be cheaper to install one of the modern forms of engines. The cost of running these is relatively very little, much less than for the current supplied to the rectifier or for the motor-generator set. It is also very easy to care for the modern engine used with the generator. By adapting the generator set for low voltages (60 volts) it is possible to connect the arc lamp directly without a rheostat, thus saving the energy wasted by heating the rheostat. A rheostat may also be used but if so it is called upon to give very slight reduction in voltage, and therefore uses up but little energy. PROJECTION WITH 135 CYCLE AND 25 CYCLE CURRENT ยง 684. In most places where alternating current is used for lighting, the supply has a frequency of 60 cycles per second, and in this chapter it has generally been assumed that the alternating current has this frequency. There are, however, places in which the supply has a frequency of 135 cycles per second and there are others, especially small towns in the neighborhood of large hydroelectric plants, in which the supply has a frequency of 25 cycles. The authors of this book have had practically no experience with other frequencies than 60 cycles. We have reason to believe however, that with 135 cycle current the arc will give as good results as with 60 cycles and will perhaps have less tendency to show a flicker,