The somewhat rapid consumption of the carbons in air (about 2 in. per hour on an average) renders necessary frequent regulation of the arc, especially as the positive carbon wears away at twice the rate of the negative carbon, unless its section is twice as great (diameters in the relation of i to 1-4).
The use of mineralized white-flame carbons is particularly advantageous in arc lamps fed with alternating current, as the arc then works inure regularly and quietly. Also, as the arc is here the principal source of the light there is no trouble (Inc to the periodic inversion of the current, which, with ordinary carbons, causes the two carbons to be alternately the sources of light, so that under good conditions only one alternance in two can be made use of.
793. Arcs with convergent carbons are gener ally used for projection work, the two carbons forming an angle of about 120° (6o on each side of the optical or, failing this, lamps in which the carbons in line at a slope of about 30° to the vertical are used.
In spite of numerous attempts to construct arcs with an automatic feed as used for street lighting, no simple and completely satisfactory pattern has yet been devised, but some do allow of much less attention on the part of the Ian ternist in the course of a lecture.
The regulation of the arc requires that it be examined very frequently. This examination must be made through sight-holes fitted with red or very dark smoked glass. With a fixed installation it is possible to avoid looking at the arc, the glare of which is more serious the more frequently it is experienced, by piercing in one of the sides of the lantern a very small hole which acts as a pin-hole (§ 38), and projects on the side of the projection cabin a greatly en larged image of the arc and of the tips of the carbons. It is then an easy matter to trace on the wall (or on a sheet of card hung there if necessary) the normal position of the carbons, thus allowing the arc to be constantly watched (F. Dogilbert, 1914).
The lamp having been fitted with carbons of suitable diameter and of sufficient length to last for the required duration of the lecture, the set screws are fixed and the carbons are drawn apart until ready to start. After closing the circuit they are momentarily brought close to each other 1 to start the arc, and are then drawn apart to the distance which experience has shown to be suitable for the normal functioning of the arc. If it becomes necessary to touch the
carbon holders or the carbons during the lecture, either pincers or thick folds of rag must be used.
794. To supply an arc with continuous current from mains requires that about 6o per cent of the energy be absorbed in resistances with current at a pressure of 'no volts the loss amounts to 8o per cent with a current of 220 volts. In the case of an installation of large size, this difficulty can be got over by using the current to operate a motor and in turn a dynamo supplying current at about 5o volts, but such an outlay is justified only for continuous use.
With alternating current 2 it is possible to use a static transformer, receiving the current from the mains and supplying it at about 5o volts, or an induction coil can be used which intro duces a de-phasage between the electro-motive force and the intensity such that the efficient voltage can be reduced to any required value (except for a very short moment after closing the circuit). The efficiency of an induction coil is a little less than that of a transformer, but it costs very much less.
In all cases it is necessary to have a slight excess of voltage, to be absorbed in a rheostat of variable resistance, so as to ensure perfect regulation. Over-running of the arc can be of advantage when projecting a very dense slide, as is usually the case with dyed slides or with colour photographs on screen plates.
795. The Condenser. The condensers of pro jection lanterns are similar to those already described for use in enlarging lanterns. The precautions to be taken against overheating the condenser are all the more necessary, as the light-sources usually employed for projection work are very much more powerful than those used in enlarging.
In some cinema projectors, compound con densers (A. Koehler, 1915 ; L. Lenouvel, 1922) have been used. With these an auxiliary lens projects on the gate a sharp image of the rear surface of the condenser. Such devices are not of general importance in " still " projection.