[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 always flow in the same direction. The secondary currents are also
always of the same sign, as are the currents in a Ruhmkorff coil, and
as are the currents in high vacua with which Crookes has made us so
familiar. Moreover, the frequency of these currents is a very variable
quantity, not only due to the various tones of voices, but to the
various styles of articulation. Hence the laws of periodic alternate
currents following the sine function of the time fail when we come to
consider microphones and telephones. It is important to bear this in
mind, for nearly everything that has hitherto been written on the
subject assumes that telegraphic currents follow the periodic sine
law. The currents derived from Bell's original magneto-transmitters
are alternate, and comply more nearly with the law. The difference
between them and microphones is at once perceptible. Muffling and
disturbance due to the presence of electromagnetic inertia become
evident, which are absent with microphones. I tested this between
London and St. Margaret's, and found the effect most marked.
7. _Lightning._--A metallic telephone circuit may have a static charge
induced upon it by a thunder cloud, as shown in Fig. 6. Such a charge
is an electric strain which is released when the charged cloud flashes
into the earth or into a neighboring cloud. If there be
electromagnetic inertia present, the charge will surge backward and
forward through the circuit until it dies out. If there be no E.M.F.
present it will cease suddenly, and neutrality will be attained at
once. Telephone circuits indicate the operation by peculiar and
characteristic sounds. An iron wire circuit produces a long swish or
sigh, but a copper wire circuit like the Paris-London telephone emits
a short, sharp report, like the crack of a pistol, which is sometimes
startling, and has created fear, but there is no danger or liability
to shock. Indeed, the start has more than once thrown the listener off
his stool, and has led to the belief that he was knocked down by
lightning.
[Illustration: Fig 6.]
8. The future of telephone working, especially in large cities, is one
of underground wires, and the way to get over the difficulties of this
kind of work is perfectly clear. We must have metallic circuits,
twisted wires, low resistance, and low capacity. In Paris a remarkable
cable, made by Fortin-Herman, gives an exceedingly low capacity--viz.,
only 0.069 [phi] per mile. In the United States they are using a wire
insulated with paper which gives 0.08 [phi] per mile. We are using in
London Fowler-Waring cable giving a capacity of 1.8 [phi] per mile,
the capacity of gutta-covered wire being 3 [phi] per mile.
* * * * *