Heat, Irreversible.
The heat produced by an electric current in a conductor of identical
qualities and temperature throughout. Such heat is the same whatever the
direction of the current. The heating effect is irreversible because of
the absence of the Thomson effect, q. v.) or Peltier effect, q. v.
Heat, Mechanical Equivalent of.
The mechanical energy corresponding to a given quantity of heat energy.
Mechanical energy is generally represented by some unit of weight and
height, such as the foot-pound; and heat energy is represented by a
given weight of water heated a given amount, such as a pound-degree
centigrade. Joule's equivalent is usually accepted; it states that
772.55 foot pounds of mechanical energy are equivalent to 1 pound-degree
F. (one pound avds. of water raised in temperature one degree
Fahrenheit). Other equivalencies have also been deduced.
Heat, Molecular.
The product of a specific heat of the compound by its molecular weight.
It is approximately equal to the sum of the atomic heats of its
constituent elements.
The molecular heat represents the number of gram calories required to
raise the temperature of a gram-molecule (a number of grams equal
numerically to the molecular weight) one degree centigrade.
The molecular heat is approximately equal for all substances.
Heat, Specific.
The capacity of a body for heat; a coefficient representing the relative
quantity of heat required to raise the temperature of an identical
weight of a given body a defined and identical amount.
The standard of comparison is water; its specific heat is taken as
unity. The specific heats by weight of other substances are less than
unity. The specific heat varies with the temperature. Thus the specific
heat of water is more strictly 1+.00015 tº C.
Specific heat is greater when a substance is in the liquid than when it
is in the solid state. Thus the specific heat of ice is 0.489; less
than half that of water. It differs with the allotropic modifications of
bodies; the specific heat of graphite is .202; of diamond, .147.
The product of the specific heat by the atomic weight of elements gives
a figure approximately the same. A similar law applies in the case of
molecules. (See Heat, Atomic-Heat, Molecular.)
The true specific heat of a substance should be separated from the heat
expended in expanding a body against molecular and atomic forces, and
against the atmospheric pressure. So far this separation has not been
possible to introduce in any calculations.