Magnitudes to which we attribute like dimensions may be qualitatively
irreducible one to the other. Thus the different forms of energy are
measured by the same unit, and yet it seems that some of them, such as
kinetic energy, really depend on time; while for others, such as
potential energy, the dependency established by the system of
measurement seems somewhat fictitious.
The numerical value of a quantity of energy of any nature should, in
the system C.G.S., be expressed in terms of the unit called the erg;
but, as a matter of fact, when we wish to compare and measure
different quantities of energy of varying forms, such as electrical,
chemical, and other quantities, etc., we nearly always employ a method
by which all these energies are finally transformed and used to heat
the water of a calorimeter. It is therefore very important to study
well the calorific phenomenon chosen as the unit of heat, and to
determine with precision its mechanical equivalent, that is to say,
the number of ergs necessary to produce this unit. This is a number
which, on the principle of equivalence, depends neither on the method
employed, nor the time, nor any other external circumstance.
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