n of an electromotive force to the
dielectric produces a current through it which rapidly falls in value,
as if the electric resistance of the dielectric were increasing. The
current, however, does not fall continuously but tends to a limiting
value, and it appears that if the electromotive force is kept applied
to the cable for a prolonged time, a small and nearly constant current
will ultimately be found flowing through it. It is customary in
electro-technical work to consider the resistivity of the dielectric as
the value it has after the electromotive force has been applied for one
minute, the standard temperature being 75 deg. F. This, however, is a
purely conventional proceeding, and the number so obtained does not
necessarily represent the true or ohmic resistance of the dielectric. If
the electromotive force is increased, in the case of a large number of
ordinary dielectrics the apparent resistance at the end of one minute's
electrification decreases as the electromotive force increases.

_Practical Standards._--The practical measurement of resistivity
involves many processes and instruments (see WHEATSTONE'S BRIDGE and
OHMMETER). Broadly speaking, the processes are divided into _Comparison
Methods_ and _Absolute Methods_. In the former a comparison is effected
between the resistance of a material in a known form and some standard
resistance. In the _Absolute Methods_ the resistivity is determined
without reference to any other substance, but with reference only to the
fundamental standards of length, mass and time. Immense labour has been
expended in investigations concerned with the production of a standard
of resistance and its evaluation in absolute measure. In some cases the
absolute standard is constructed by filling a carefully-calibrated tube
of glass with mercury, in order to realize in a material form the
official definition of the ohm; in this manner most of the principal
national physical laboratories have been provided with standard mercury
ohms. (For a full description of the standard mercury ohm of the Berlin
Physikalisch-Technische Reichsanstalt, see the _Electrician_, xxxvii.
569.) For practical purposes it is more convenient to employ a standard
of resistance made of wire.

Opinion is not yet perfectly settled on the question whether a wire
made of any alloy can be considered to be a perfectly unalterable
standard of resistance, but experience has shown that a platinum
silver alloy (66