Head-light, Electric.
An electric head-light for locomotives has been experimented with. It
includes the parabolic reflection of the regular light with an arc-lamp
in place of the oil lamp. An incandescent lamp may be used in the same
place, but has no great advantage over oil as regards illuminating
power.
Heat.
A form of kinetic energy, due to a confused oscillatory movement of the
molecules of a body. Heat is not motion, as a heated body does not
change its place; it is not momentum, but it is the energy of motion. If
the quantity of molecular motion is doubled the momentum of the
molecules is also doubled, but the molecular mechanical energy or heat
is quadrupled.
As a form of energy it is measured by thermal units. The calorie is the
most important, and unfortunately the same term applies to two units,
the gram-degree C. and the kilogram-degree C. (See Calorie.) Calories
are determined by a calorimeter, q. v.
Independent of quantity of heat a body may be hotter or colder.
Thermometers are used to determine its temperature.
Heat is transmitted by conduction, a body conducting it slowly for some
distance through its own substance. Bodies vary greatly in their
conductivity for heat. It is also transmitted by convection of gases or
liquids, when the heated molecules traveling through the mass impart
their heat to other parts. Finally it is transmitted by ether waves with
probably the speed of light. This mode of transmission and the phenomena
of it were attributed to radiant heat. As a scientific term this is now
dropped by many scientists. This practice very properly restricts the
term "heat" to kinetic molecular motion.
The mechanical equivalent of heat is the number of units of work which
the energy of one unit quantity of heat represents. (See Equivalents,
Mechanical and Physical.)
Heat, Atomic.
The product of the specific heat of an element by its atomic weight. The
product is approximately the same for all the elements, and varies as
determined between 5.39 and 6.87. The variations are by some attributed
principally to imperfection of the work in determining them. The atomic
heat represents the number of gram calories required to raise the
temperature of a gram atom (a number of grams equal numerically to the
atomic weight) one degree centigrade.