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each cycle of 120 times for a 60 cycle circuit. (4) The alternating current arc has a greater tendency to travel around the ends of the electrodes, which constantly shifts the incandescent area of the carbon tips, thus producing "ghosts" on the screen. (5) The light produced by alternating current has a preponderance of the violet end of the light spectrum, which is not so desirable as the yellow, from an optical point of view. (6) The cost of power for the alternating current arc is high, because the current required to produce a given illumination is about three times greater than the direct current required. From a carbon point, the disadvantage is chiefly in operation. The very highest grade, and therefore costly, carbons are required. The very short arc required for satisfactory operation means constant attention on the part of the operator. Further, the high currents necessary to get the degree of illumination now considered desirable, renders such an arc very noisy, which in itself is a hardship on the operator.
2. (a) The advantages of the direct current arc can be briefly stated as being converses of the disadvantages which have been cited for the alternating current arc. Careful experiments have demonstrated that the light resulting from a direct current arc is produced by the crater on the positive carbon, by the incandescent tip of the negative carbon, and by the arc stream proper, in the proportions of 85 per cent., 10 per cent, and 5 per cent., respectively. The crater is of comparatively small area and can therefore be located at the proper focal point of the condenser lenses. This explains the fundamental advantages of the direct current arc. The ease of operation, requiring little attention from the operator and the inherent stability and good operation of the direct current arc over wide range of arc voltages, makes this form of current ideal. Very high arc wattages can be made use of on direct current to obtain the illumination essential for very large pictures, but which would hardly be feasible to attempt with alternating current, so that in the field of projection in the large picture houses, direct current is supreme.
2. (b) The disadvantage of using direct current is primarily in the cost of converting appliances to change from alternating current to direct current, or if operating directly from 110 volts direct current, the entire line voltage enters into the cost consideration, since whatever energy is not used in the arc must be dissipated in resistance.
3. (a) If an alternating current supply system only is available and direct current is desired for the projecting machines, it is necessary to install equipment which will rectify or convert the alternating current to direct current. There are numerous types of equipment available for this survice, such as Mercury Arc Rectifier, Motor Generator Sets (of which there are two classes, namely, constant potential and constant current). Rotary Converters and Mechanical Rectifiers. When any one of the various types of converting equipments is used, with the exception of the Mercury Arc Rectifier, it is advisable to provide for "break down" service, so