In practice any current from 100 volts to 250 volts may be considered as satisfactory for lantern work with a suitable resistance. Less than this involves feeding the arc rather frequently, and more may give a nasty shock, should the operator inadvertently touch a live wire, though I have worked an arc lamp on a current of as much as 500 volts.
The resistance usually consists of a suitable length of wire of high resistance (Iron, German Silver, or those alloys known as Platinoid, Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on a frame, and is generally supplied adjustable (Fig. 28), so that more or less current may be used as desired. These resistances get pretty hot in use, and care must be taken that they are placed where they cannot scorch woodwork, &c., and in cases where the lantern is a fixture it is a good plan to have the resistance bolted up against a wall once and for all. The resistance may be placed anywhere in the circuit, so long as the current passes through it, then through the arc lamp (or vice versâ), and back to the other
pole of the supply main; it does not matter in the least whereabouts it comes.
In cases, however, where one pole of the supply main is earthed, it is a good thing to place the resistance in the 'live' side, as this keeps the arc lamp within 45 volts of earth potential while it is working, to the comfort of the operator should he touch a terminal or wire, though with an ordinary lighting main there is no real fear of a dangerous shock in any case.
The amount of current required depends of course on the size of the sheet, length of the hall, and density or otherwise of the slides; but it is usually accepted in practice that the efficient light from a continuous current arc lamp equals 100 candles per ampère, and therefore a 10-ampère arc will give 1000 candles. This is sufficient for all ordinary halls and slides, but where these latter are very dense, as for example with the Lumière three-colour process, as much as 20 or 25 ampères may be required.
In these cases some special precautions must be taken for keeping the slides cool, or the result may be disastrous, but this is a question that will be referred to in a later chapter. A current of 10 ampères is pretty safe for all ordinary slides, and may be taken as the normal current used in large halls, though in arranging for the wiring it is as well to stipulate for at least 12 or even 15 ampères, especially as there must necessarily be a momentary increase of current at the instant the arc is 'struck.'
Varieties of Hand-fed Arc Lamps.—The pattern of hand-fed arc lamp illustrated in Fig. 26 is only typical of many of the same general design, and there are others in which the design itself is fundamentally different. Of these the 'Scissors' arc lamp made by several firms deserves mention on account of its simplicity and cheapness. As its name implies, the mechanism resembles a pair of scissors, the carbons being attached to the ends of a pair of levers hinged together
(Fig. 29). In this lamp centring movements are usually dispensed with, the arc being clamped on to a tray pin as in the case of a limelight jet. This is not, of course, so convenient, and a further disadvantage of this pattern arc lamp is that the feeding process gradually alters the position and angle of the carbons. In fact, the one great merit of the lamp is cheapness, and where expense is an object, it should certainly be considered.
Yet another arc lamp deserving of mention is the 'Parallel,' a name again very aptly chosen, as the two carbons are either exactly parallel to each other or very slightly inclined. In the former case the arc has to be 'struck' by touching the ends of the carbon rods with a piece of metal or carbon. Of the actual manipulation of this lamp I have had very little practical experience, but I have heard it well spoken of, though I believe it has so far only been made for currents of 5 ampères or so.