ical with itself, and to fix our ideas let us choose
mercury, taken at the temperature of 0 deg. C., which completely fulfills
this condition. We may determine by several methods the specific
electric resistance, [rho], of mercury in absolute electrostatic
units; [rho] is a specific property of mercury, and is consequently a
magnitude absolutely invariable. Moreover, [rho] is _an interval of
time_. We might, therefore, take [rho] as a unit of time, unless we
prefer to consider this value as an imperishable standard of time.

In fact, [rho] is not simply a quantity the measure of which is found
to be in relation with the measure of time. It is a concrete interval
of time, disregarding every convention established with reference to
measures and every selection of unit. It may at first sight, appear
singular that an interval of time is found in a manner hidden under
the designation _electric resistance_. But we need merely call to mind
that in the electrostatic system the intensities of the current are
speeds of efflux and that the resistances are times, i.e., the times
necessary for the efflux of the electricity under given conditions. We
must, in particular, remember what is meant by the specific
resistance, [rho] of mercury in the electrostatic system. If we
consider a circuit having a resistance equal to that of a cube of
mercury, the side of which = the unit of length, the circuit being
submitted to an electromotive force equal to unity, this circuit will
take a given time to be traversed by the unit quantity of electricity,
and this time is precisely [rho]. It must be remarked that the
selection of the unit of length, like that of the unit of mass, is
indifferent, for the different units brought here into play depend on
it in such a manner that [rho] is not affected.

It is now required to bring this definition experimentally into
action, i.e., to realize an interval of time which may be a known
multiple of [rho]. This problem may be solved in various ways,[1] and
especially by means of the following apparatus.

[Footnote 1: In this system the measurement of time is not
effected, as ordinarily, by observing the movements of a
material system, but by experiments of equilibrium. All the
parts of the apparatus remain immovable, the electricity alone
being in motion. Such appliances are in a manner clepsydrae. This
analogy with the clepsydrae will be perceived if we consider the
form of the