Lewis Wright

“ In the case of a glow-lamp, the entire resistance is provided by the filament of the lamp itself, and that is why an ordinary metal or carbon filament lamp, for say 200 volts, has to be manufactured with an extremely long and slender, and therefore fragile, filament, while with an ordinary pocket-torch, which is usually supplied with current from a dry battery of some 3 or 4 volts only, the filament can be short and thick.
Speaking generally, glow-lamps on a low voltage current can be made more efficient than on a high one, and are also longer lived for very obvious reasons ”

“ It is true that if too much oxygen is turned on the jet may suddenly go out with a loud snap or pop, and this is in reality a miniature explosion in the mixing chamber; but it can in any case hardly be serious enough to matter, though I have found after such a snap that the gauze packing, inside the chamber above referred to, has been pierced right through, and, when first lit afterwards, the jet has for a few minutes burnt with a characteristic green flame, denoting the presence in the gas of fine copper or brass particles. ”

“ By spherical aberration we mean that the light falling upon the centre of a lens is not brought to a focus at exactly the same spot as the marginal rays, and a general want of definition is the result, usually accompanied also by a want of 'flatness' in the image, that is to say the edges of the picture do not focus at the same time as the centre. ”

“ So great is the advantage of the electric arc that attempts have been made to use it from accumulators in places where a current supply is not available, but this cannot be seriously recommended, except in special cases. Where an electric supply is, however, available there can be no real choice, whether the lantern is required for use in a large hall or a small class-room. The advantages of using the arc are so great that no other method need be seriously considered. ”

“ Accidents.—These will occur sometimes, even in the best managed exhibition; the rubber tubing feeding a limelight jet gets kinked or trodden on, or a fuse melts if electric light is being used, &c., and out goes the light. In such cases a loud request such as, 'Would you mind turning up the light for a minute, please,' accompanied by a good-humoured laugh, usually allays the fears of 'nervy' people. An operator must never get 'nervy' himself. ”

“ With an 'Injector' jet there is no need for the acetylene gas to be under pressure at all, and a simple generator such as described on page 12 will answer perfectly, though in practice very seldom used. With such a generator the pressure is so low that in many cases the jet will not even burn until some oxygen is turned on; but this introduces no real difficulty, as with a good 'Injector' a snap is practically impossible, provided the generator is large enough to evolve sufficient acetylene. It is far better in every way, however, to use the acetylene from a cylinder, just as with coal gas. ”

“ The power of this jet is indeed almost unlimited, and those made with large bores, such for example as used for cinematograph work, provide a light amounting often to some two or three thousand candles, and consume an enormous amount of gas; but the ordinary pattern, with a nipple of one-twentieth to one-sixteenth of an inch bore, and using some 5 feet of each gas per hour, or perhaps slightly more for the coal gas, will suffice for all ordinary work. ”

“ Arc Lamps on Alternating Currents.—The alternating current is not so good as the continuous for lantern work with arc lamps: the light per ampère is not so great, the light has an irritating habit of travelling round the carbons and there is always a slight 'hum.'
The sum total of these drawbacks is nothing very serious, provided that proper arrangements are adopted, and I have frequently manipulated arc lamps on alternating circuits with such good results that professional lecturers have at first refused to believe that the circuit really was alternating. ”

“ Having now discussed all the essential parts of a lantern, the next points to be considered are those of lantern boxes and stands. It is best to take these together, as more often than not a lantern is arranged to stand upon its box during use, and the plan is both convenient and simple. The whole question is one to be settled upon its own merits in each individual case. Sometimes neither box nor stand is wanted at all. ”

“ The Fallot Acetylene Light.—This light consists of a jet of acetylene under pressure, without oxygen, but producing its own air blast from the atmosphere by suction, much as the 'Injector' jet does, but the reverse way round.
The light is better than with an ordinary acetylene jet, though not quite so good as with a 'blow-through' jet; but as it only requires a cylinder of dissolved acetylene, or even a 'Pressure' generator, it is fast coming into favour. ”

“ So-called 'soft' limes used to be recommended for 'blow-through' jets as giving a better light than 'hard' limes, but the advantage, if any, is very little, and these limes are now very seldom heard of, possibly because 'blow-through' jets themselves are becoming less and less used, and 'soft' limes will not stand the heat of a mixed or 'Injector' jet for long.
'Hard' limes are turned out of the hardest stone lime, and must be kept in sealed tins until used, as they rapidly disintegrate when exposed to the air. ”

“ A bi-unial lantern consists essentially of two different instruments, each complete with its limelight jet or other illuminant—front, condensers, objective, &c., usually mounted on one body—and with some arrangement for 'dissolving' or turning the light in each lantern gradually on and off. ”

“ Coal gas or hydrogen is very seldom home generated; a gas-bag can, if necessary, be filled a few miles away and brought full to the place of exhibition, or filled on the spot if gas is laid on; or, failing this, acetylene or ether, or even methylated spirit may be utilised instead.
The bags in use are placed between double pressure boards (if both gases are required under pressure) and weights sufficiently heavy placed on the top (Fig. 14) , or with a 'blow-through' jet only the oxygen need be stored in a bag and the coal gas used from the supply main. ”

“ In deciding upon the type of body to be purchased the main considerations to be borne in mind are: (1) The type or types of illuminant to be used, a powerful arc lamp for example requiring a larger body than is necessary for a weaker radiant; (2) the size and position of the lens to be carried, a Petzval objective of say 3 inches diameter which has to be supported at the end of a long brass mount for long-range work obviously demanding a body of greater strength and rigidity than is required with a 6-inch focus lens of 2 inches diameter ”

“ Gas-Bags and Generators.—It has already been remarked that there are two alternative methods of obtaining gas under pressure for limelight purposes, viz. gas-bags and generators (the latter for oxygen alone: there is no good hydrogen generator that I know of) . In both these cases the oxygen is generated by heating a mixture of chlorate of potash and manganese black oxide. In the case of gas-bags the gas is prepared beforehand and passed through suitable purifiers into a rubber gas-bag. With a generator the oxygen is evolved during the exhibition itself ”

“ The best plan of all, however, is to use an automatic regulator, which not only reduces the pressure so as to permit of the required adjustments being made at the jet-taps, but also maintains a practically steady supply as the cylinder empties, thereby obviating continual readjustments. Regulators are now so inexpensive that they have come into almost universal use, and are generally reckoned an indispensable part of a limelight lantern equipment. The form of regulator in most common use is that usually known as 'Beard's,' having been originally designed and patented by Messrs. ”

“ There is no doubt that at present acetylene holds second place to electric light in popularity for optical lantern work. The light is good; not, it is true, so good as limelight or the electric arc, but still sufficient for a picture up to 12 feet in diameter at a working distance from the screen of not more than 30 feet, and this suffices for the large majority of halls. ”

“ 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. ”

“ Whatever method of lighting we use therefore, enough resistance must always be kept in the circuit to ensure {41}that no more current can pass than has been provided for, and in the case of an arc lamp this usually means a resistance or rheostat being retained in the circuit in addition to the arc itself, through which the current is passed and absolutely wasted, though fortunately the waste in money is negligible, and for reasons to be discussed later such a resistance is necessary with an optical lantern arc lamp in any case. ”

“ A good hard stone lime must be used—'soft' limes are useless for this jet—and the coal gas flame should be lit first, and the lime thoroughly heated with this before the oxygen is slowly turned on. As the oxygen increases the flame will gradually disappear and the light increase, until it is at a maximum for that particular amount of coal gas. This latter can then be turned on a little more, and more oxygen passed to balance it until the jet begins to 'roar,' when we are getting the maximum light for that particular sized nipple. ”

“ An acetylene flame is very smoky, and care must be taken that the burners are not turned too high. A nipple cleaner, consisting of a fine wire in a short handle, can usually be obtained from any dealer, and is very handy.
Acetylene gas can also be used for lantern illumination in quite another way, viz. by a blast from a blowpipe, in combination with either air or oxygen, on to a special 'Pastille' provided for the purpose, or an ordinary limelight jet can be used. ”