When air is supplied to a fuel, the
temperature must be high enough or ignition will not take place and
burning will not be sustained. Nothing will burn until it is in a
gaseous state.
For example, the wax of a candle cannot
be ignited directly; the wick, heated by the flame of a match, draws
up a little of the melted wax by capillary action until it can be
vaporized and ignited. Fuels that liquefy on heating usually will
melt at a temperature below that at which they ignite.
Solid fuels must be heated to a
temperature at which the top layers will gasify before they will
burn. The ignition temperatures of fuels depend on their
compositions; since the greatest part is carbon, the temperature
given in the table, 870°F, will not be far wrong.
Heat must be given to the fuel to raise
it to the temperature of combustion. If there is moisture in the
fuel, more heat must be supplied before it will ignite, since
practically all of the moisture must be evaporated and driven out
before the fuel will burn.
Temperature may be measured by its
effect in expanding and contracting some material, and is usually
measured in degrees. The mercury thermometer is a familiar instrument
in which a column of mercury is enclosed in a sealed glass tube and
its expansion and contraction measured on an accompanying scale.
Two such scales are in common use, the
Fahrenheit (F) and the Centigrade or Celsius (C). The former has the
number 32 at the freezing point of water and 212 at the boiling
point; thus 180 divisions, or degrees, separate the freezing and
boiling points or temperatures of water.
The latter has the number zero (0) at
the freezing point of water and 100 at the boiling point; thus 100 at
the boiling points thus 100 divisions or degrees separate the
freezing and boiling points or temperatures of water. Both scales may
be extended above the boiling points and below the freezing points of
water.
Other instruments may employ other
liquids, gases, or metals, registering their expansion and
contraction in degrees similar to those for mercury.
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