PARAFFIN-OIL LAMPS, INCANDESCENT GAS AND SPIRIT BURNERS
[Illustration: FIG. 1.--Oil Lamp.]
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 {7}
(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 {8} 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.
[Illustration: FIG. 2.--Inverted Incandescent Lamp.]
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 {9} 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.
[Illustration: FIG. 3.--Methylated Spirit Burner.]
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; {10} 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.
[Illustration: FIG. 4.--Luna Lamp.]
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.
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