CHAPTER III
PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS
There are several varieties of oil lamps on the market, but in practically every case they take the same general form, a metal reservoir sliding in grooves in the lantern body and holding approximately a pint of oil with (usually) four wicks nearly parallel, but slightly converging from rear to front, these enclosed in a flame chamber of Russian iron, with loose well-annealed ends of sheet glass and an adjustable reflector at the back, or sometimes the reflector itself forms the rear end of the flame chamber. The chimney must be tall and is now usually made adjustable, though I have never been able to trace any real advantage from this complication
(Fig. 1). The whole secret of obtaining the best results from these lamps may be summed up—good oil and perfect cleanliness; and it is wonderful what can be done when these points are properly attended to.
Care should be taken in trimming the wicks to see that no charred parts fall down between the wick holders, but it makes little difference whether the trimming is done with scissors or by rubbing with the finger. Special lamp scissors are sold by all makers with a large flat on one side to catch the portions cut off.
These lamps should be well rubbed over the last thing before use, as paraffin-oil is apt to 'creep,' and the operator does not want to be told that his apparatus is suggestive of a fried fish shop. In working with these lamps it is difficult to avoid a dark streak down the centre of the sheet, representing the space between the two centre wicks; to a certain extent this can be obviated by adjusting the reflector, and in any case is not very obvious when the slide is in place. Lamps constructed with either three or five wicks are better in this respect, but the former are usually considered to be too poor in illuminating power, and the latter are apt to crack the sheet-glass ends by excessive heat.
Incandescent Gas.—Incandescent gas burners do not need much description, as they are practically similar to those in general use for house lighting. They may be either of the erect or inverted forms, the latter being preferable owing to the light being more concentrated, and a reflector is provided to increase the illumination (Fig. 2).
These reflectors should be spherical and so adjusted that the radiant is in the centre of curvature, thus ensuring that the light from the reflector passes again through the original source. If this point is not attended to, we shall be dealing with essentially two sources of light instead of one, to the detriment of the definition.
The same remark applies to every lantern illuminant
which is supplemented by a reflector, and it is extraordinary how often it is neglected by the manufacturer. Of course the opacity of the illuminant destroys much of the efficiency of the reflector, and hence in the case of incandescent gas mantles there is not much real gain in making use of them, but with these comparatively weak illuminants every fraction tells, and the reflector does not add much to the cost.
In light the inverted gas burner is very little superior to oil, but it is whiter, slightly more concentrated, and freer from smell, and therefore to be regarded as preferable if a supply of gas is available.
Methylated Spirit Burners.—Incandescent mantles heated by methylated spirit are also largely used, and provide a light decidedly superior to gas and nearly equal to acetylene. Some arrangement must be made for volatilising the spirit and driving the vapour out under pressure, and the most usual contrivance is somewhat as illustrated in Fig. 3.
In this apparatus the spirit is contained in a metal reservoir at the rear and air pressure is provided by a pair of rubber balls and valves after the manner of a medical spray. Sufficient
pressure having been obtained, the liquid spirit is forced into a vaporising chamber immediately behind the mantle, and a kind of miniature pitchfork, with its prongs wrapped in asbestos wool, is soaked in spirit, and pushed over the brass fitting of the burner in such a way that when lighted the flame heats the chamber and volatilises the spirit. The burner can now be lit, and although the fork burns out in the course of a minute or so, the heat from the mantle itself is thereafter sufficient to vaporise the spirit as rapidly as required. This lamp works exceedingly well in practice, but has one drawback, viz. that it is possible to obtain too much pressure and squirt liquid spirit through the burner, when it naturally catches fire and may even run on to the floor.
An accident of this sort is rare and usually harmless even if it does occur, but an audience is easily frightened, and hence this burner should only be used by an operator with experience. An altogether better arrangement is that made by Messrs. Hughes of Kingsland and known as the 'Luna' Lamp (Fig. 4).
In this burner there is no pump and no volatilising chamber;
the spirit is contained as before in a metal reservoir and a separate burner underneath is used to keep this sufficiently hot to both vaporise the spirit and provide the necessary pressure. The heat can be regulated by means of an adjustable sheath to the burner, and a simple safety valve provides against an excess of vapour.
I do not say that an accident of the sort previously referred to is impossible even with this burner, but I have never heard of it happening, and the lamp is certainly the best apparatus of its kind that I am acquainted with.
Incandescent Electric Lamps.—Incandescent electric lamps of the ordinary metal or carbon filament type are also frequently used in small class-rooms, and should be mentioned here, as they provide approximately the same illumination as a gas mantle, or in some cases rather better. It will, however, be more convenient to deal with the question of electric lighting as a whole in the chapter devoted to it.
It will suffice here to say that lamps are made for the purpose with a special filament arranged to provide a concentrated light, the ordinary type being almost useless in this respect, and that small battery lamps, worked by a suitable accumulator, can also be used, but except under very special circumstances are hardly worth the trouble of keeping the batteries charged.