The torch body and tubes are usually made of brass, although copper is
sometimes used. The joint must be very strong, and are usually threaded and
soldered with silver solder. The nozzle proper is made from copper, because
it withstands the heat of the flame better than other less suitable metals.
The torch must be built in such a way that it is not at all liable to come
apart under the influence of high temperatures.
All torches are constructed in such a way that it is impossible for the
gases to mix by any possible chance before they reach the head, and the
amount of gas contained in the head and tip after being mixed is made as
small as possible. In order to prevent the return of the flame through the
acetylene tube under the influence of the high pressure oxygen some form of
back flash preventer is usually incorporated in the torch at or near the
point at which the acetylene enters. This preventer takes the form of some
porous and heat absorbing material, such as aluminum shavings, contained in
a small cavity through which the gas passes on its way to the head.
_High Pressure Torches._--Torches are divided into the same classes as
are the generators; that is, high pressure, medium pressure and low
pressure. As mentioned before, the medium pressure is usually called the
high pressure, because there are very few true high pressure systems in
use, and comparatively speaking the medium pressure type is one of high
pressure.
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