We shall limit ourselves here to pointing out the simplest and most
practical of the numerous models of such apparatus that have been
constructed up to the present time. All those that we shall describe
are based on the incandescence of a platinum wire. A few have been
constructed based on the induction spark, but they are more complicated
and expensive, and have not entered into practical use. Before
commencing to describe these apparatus, we shall make a remark in regard
to the piles for working them, and that is that we prefer for this
purpose Leclanche elements with agglomerated plates and a large surface
of zinc. In order to bring about combustion in any given substance, it
is necessary to bring near it an incandescent body raised to a certain
temperature, which varies with the nature of the said substance, and
which is quite low for illuminating gas, higher for petroleum, and a
white heat for a wax taper or a candle. We have said that we make use
exclusively of a platinum wire raised momentarily to incandescence by
the passage of an electric current. The temperature of such wire will
depend especially upon the intensity of the current traversing it;
and, if this is too great, the platinum (chosen because of its
inoxidizability and its elevated melting point) will rapidly melt;
while, if the intensity is too little, the temperature reached by the
wire will itself be too low, and no inflammation will be brought about.
Practice soon indicates a means of obviating these two inconveniences,
and teaches how each apparatus may be placed under such conditions that
the wire will hardly ever melt, and that the lighting will always be
effected. For the same intensity of current that traverses the wire,
the temperature of the latter might be made to vary by diminishing or
increasing its diameter. A very fine wire will attain a red heat through
a very weak current, but it would be very brittle, and subject to break
at the least accident. For this reason it becomes necessary to employ