This is done in telephony, and hence we obtain the law of retardance,
or the law by which we can calculate the distance to which speech is
possible. All my calculations for the London and Paris line were based
on this law, which experience has shown it to be true.
How is electromagnetic inertia practically eliminated? First, by the
use of two massive copper wires, and secondly by symmetrically
revolving them around each other. Now L depends on the geometry of the
circuit, that is, on the relative form and position of the different
parts of the circuit, which is invariable for the same circuit, and is
represented by a coefficient, [lambda]. It depends also on the
magnetic qualities of the conductors employed and of the space
embraced by the circuit. This specific magnetic capacity is a variable
quantity, and is indicated by [mu] for the conductor and by [mu]_{0}
for air. It depends also on the rate at which currents rise and fall,
and this is indicated by the differential coefficient dC / dt. It
depends finally on the number of lines of force due to its own current
which cut the conductor in the proper direction; this is indicated by
[beta]. Combining these together we can represent the electromagnetic
inertia of a metallic telephone circuit as