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Equipment for D.C. Electrical apparatus may be supplied for use on A.C. or D.C. mains, or be of the type known as "universal", which implies that it can be used on either A.C. or D.C. with no apparent after-effects. A truly universal model, however, is one which can be used on either type of mains without employing special equipment. The majority of projectors are for A.C. supplies only, a few are universal, and D.C. models (in Britain) are a rarity. Most of the troubles experienced by operators occur when using A.C. machines on D.C. mains, but after discovering that the audience won't put up with a strong smell of burning and no picture, they at once switch off and retire to a corner to read their operators instructions. They may then read, for instance, that the motor is universal and that the amplifier is A.C. only, in which case there are two methods of dealing with the con- nections to the mains. The first method, which is by far the simplest, is to connect up the whole of the equipment to a rotary convertor so that it may all operate on the A.C. supplied by it. The second method would be to use the convertor to supply A.C. only to the amplifier, the motor (and the lamp) being connected direct to the D.C. mains. But note that this method may entail using a resistance for the lamp and/or motor, as a transformer must not be used on D.C. The transformer may be used, if required, by connecting it up between the convertor and amplifier. (Fig. 13). Providing your rotary convertor is of sufficient power, it is usually best to take full advantage of it by connecting the whole of your equipment to it. Other D.C. equipment, such as vibrator packs and polarity changers, are supplied with one or two makes of machines, but the rotary convertor is considered as standard equipment for professional use. It is actually a combination of a D.C. motor driving an alternator (or A.C. generator) and it is more econo- mical to run than other equipment which includes a mains resistance. Those supplied for use with sound projectors average between 50 and 60 lbs. in weight (their only drawback) and handle up to 2,000 watts—giving ample margin for two pro- jectors employing high-wattage lamps, etc. But a wide range 57