.

In short metallic circuits, say of lengths up to 100 miles, this
negative quantity does not appear, but in the Paris-London circuit
this helpful mutual action of opposite currents comes on in a peculiar
way. The presence of the cable introduces a large capacity practically
in the center of the circuit. The result is that we have in each
branch of the circuit between the transmitter, say, at London and the
cable at Dover, extra currents at the commencement of the operation,
which, flowing in opposite directions, mutually react on each other,
and practically prepare the way for the working currents. The presence
of these currents proved by the fact that when the cable is
disconnected at Calais, as shown in Fig. 5, and telephones are
inserted in series, as shown at D and D', speech is as perfect between
London and St. Margaret's Bay as if the wires were connected across,
or as if the circuit were through to Paris. Their effect is precisely
the same as though the capacity of the aerial section were reduced by
a quantity, M, which is of the same dimension or character as K.
Hence, our retardance equation becomes

R (K - M) = t

[Illustration: Fig 4.]

[Illustration: Fig 5.]

Thus it happens that the London-Paris telephone works better than was
expected. The nature of M is probably equivalent to about 0.0075 [phi]
per mile, and therefore K should be also about 0.0075 [phi] instead of
0.0156 [phi] per mile. This helpful action of mutual induction is
present in all long circuits, and it is the reason why we were able to
speak to Brussels and even to Marseilles. It also appears in every
metallic loop, and vitiates the measurements of electromagnetic
inertia and of capacity of loops. Thus, if we measure the capacity of
a loop as compared with a single wire, the amount per mile may be 50
per cent. greater than it ought to be; while if we measure the
capacity of one branch of a circuit under the conditions of the
London-Paris telephone line, it may be 50 per cent. less than it ought
to be. This effect of M is shown by the dotted line in Fig. 1.

Telephonic currents--that is, currents induced in the secondary wire
of an induction coil due to the variation of microphonic currents in
the primary wire--are not alternating currents. They do not follow the
constant periodic law, and they are not true harmonic sine functions
of the time. The microphonic currents are intermittent or pulsatory,
and alw