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MANAGERS AND PROJECTIONISTS 427
slightly overloaded (we speak of rheostats made for use on a projection circuit) because of the fact that such rheostats are made to operate in conjunction with an arc, the resistance of which is figured in the total, so that if the arc produced one ohm resistance and the rheostat two ohms, then the total would be three ohms when connected singly, but when connected in series instead of having six ohms there would only be five, because the resistance of one arc would be absent.
You may use a rheostat built for a certain voltage on that voltage or on anything less than that voltage, but you cannot use a rheostat on a higher voltage than it is built to withstand, unless it be placed in series with additional resistance; though this latter is qualified to the extent that a rheobtat built for a certain voltage may usually be used or. current 5, 10 or even 15 volts in excess of that pressure.
A. C. and D. C. RHEOSTATS.— The novice often asks: What is the difference between an A. C. and a D. C. rheostat?
There is no such thing as an A. C. or D. C. rheostat. Any rheostat will work either on A. C. or D. C, but a rheostat which will deliver 30 amperes to a projection arc when connected to 110 volt D. C. supply wires, will deliver a somewhat higher amperage on the same voltage A. C. This is because of the fact that an A. C. projection arc is shorter, hence offers less resistance than the D. C. projection arc of equal amperage, therefore the total resistance opposed to the current by the rheostat and the arc is reduced.
This is, however, qualified by the fact that there is some tendency to inductive effect when a wire coil rheostat is used on A. C, which has the effect of adding inductive resistance, or in other words "magnetic kick" to the ordinary resistance offered. The amount of inductive restisance thus set up will vary with the size of the coils, their length and the spacing of the spirals. It amounts to something, though not very much. The inductive effect, however, does cause vibration in the coils, with the result that some wire coil rheostats are very noisy when used on A. C. This noise may be reduced by packing the center of the coils tightly with threaded asbestos forced in at the end of each coil.
The use of rheostats on alternating current is extremely bad practice. It is entirely unnecessary and very wasteful. Where alternating current is used, rheostats should be replaced with low voltage transformers, (see Page 596) or better still with a motor generator set (see Page 493) or mercury arc rectifier (see Page 567).