a product which indicates that speech should be very good.
2. _Trials of Apparatus._--The preliminary trials were made during the
month of March between the chief telegraph offices of the two
capitals, and the following microphone transmitters were compared:
Ader. Pencil form.
Berliner. Granular form.
D'Arsonval. Pencil "
DeJongh. " "
Gower Bell. " "
Post office switch instrument. Granules and lamp filaments.
Roulez. Lamp filaments.
Turnbull. Pencil form.
Western Electric. Granular.
The receivers consisted of the latest form of double-pole Bell
telephones with some Ader and D'Arsonval receivers for comparison.
After repeated trials it was finally decided that the Ader,
D'Arsonval, Gower-Bell (with double-pole receivers instead of tubes),
Roulez, and Western Electric were the best, and were approximately
equal.
These instruments were, therefore, selected for the further
experiments, which consisted of using local extensions in Paris and
London. The wires were in the first instance extended at the Paris end
to the Observatory through an exchange at the Avenue des Gobelines. The
length of this local line is 7 kms. The wires are guttapercha-covered,
placed underground, and not suitable for giving the best results.
The results were, however, fairly satisfactory. The wires were
extended to the Treasury in London by means of the ordinary
underground system. The distance is about two miles, and although the
volume of sound and clearness of articulation were perceptibly reduced
by these additions to the circuit, conversation was quite practicable.
Further trials were also made from the Avenue des Gobelines on
underground wires of five kilometers long, and also with some renters
in Paris with fairly satisfactory results. The selected telephones
were equally efficient in all cases, which proves that to maintain
easy conversation when the trunk wires are extended to local points it
is only necessary that the local lines shall be of a standard not
lower than that of the trunk line. The experiments also confirm the
conclusion that long-distance speaking is solely a question of the
circuit and its environments, and not one of apparatus. The
instruments finally selected for actual work were Gower-Bell for
London and Roulez for Paris.
3. The results are certainly most satisfactory. There is no circuit in
or out of London on which speech is more perfect than it is between
London and Paris. In fact, it is better than I anticipated, and better
than calculation led me to expect. Speech has been possible not only
to Paris but through Paris to Bruxelles, and even, with difficulty,
through Paris to Marseilles, a distance of over 900 miles. The wires
between Paris and Marseilles are massive copper wires specially
erected for telephone business between those important places.
4. _Business Done._--The charge for a conversation between London and
Paris is 8 s. for three minutes' complete use of the wire. The demand
for the wire is very considerable. The average number of talks per
day, exclusive of Sunday, is 86. The maximum has been 108. We have had
as many as 19 per hour--the average is 15 during the busy hours of the
day. As an instance of what can be done, 150 words per minute have
been dictated in Paris and transcribed in London by shorthand writing.
Thus in three minutes 450 words were recorded, which at 8 s. cost five
words for a penny.
5. _Difficulties._--The difficulties met with in long-distance
speaking are several, and they may be divided into (a) those due to
external disturbances and (b) those due to internal opposition.
(_a._) Every current rising and falling in the neighborhood of a
telephone line within a region, say, of 100 yards, whether the wire
conveying it be underground or overground, induces in the telephone
circuit another current, producing in the telephone a sound which
disturbs speech, and if the neighboring wires are numerous and busy,
as they are on our roads and railways, these sounds became confusing,
noisy, and ultimately entirely preventive of speech. This disturbance
is, however, completely removed by forming the telephone circuit of
two wires placed as near to each other as possible, and twisted around
each other without touching, so as to maintain the mean average
distance of each wire from surrounding conductors the same everywhere.
Thus similar currents are induced in each of the two wires, but being
opposite in direction, as far as the circuit is concerned, they
neutralize each other, and the circuit, therefore, becomes quite
silent.
In England we make the two wires revolve completely round each other
in every four poles, but in France it is done in every six poles. The
reason for the change is the fact that in the English plan the actual
crossing of the wires takes place in the span between the poles, while
in the French plan it takes place at the poles. This is supposed to
reduce the liability of the wires to be thrown into contact with each
other by the wind, but, on the other hand, it diminishes the
geometrical symmetry of the wires--so very essential to insure
silence. As a matter of fact, contacts do not occur on well
constructed lines, and I think our English wires, being more
symmetrical, are freer from external disturbance than those in France.
[Illustration: FIG. 1.]
(_b._) The internal opposition arises from the resistance, R, the
capacity, K, and the electromagnetic inertia, L, of the circuit. A
current of electricity takes time to rise to its maximum strength and
time to fall back again to zero. Every circuit has what is called its
time constant, _t_, Fig. 1, which regulates the number of current
waves which can be transmitted through it per second. This is the time
the current takes to rise from zero to its working maximum, and the
time it takes to fall from this maximum to zero again, shown by the
shaded portions of the figure; the duration of the working current
being immaterial, and shown by the unshaded portion.
The most rapid form of quick telegraphy requires about 150 currents
per second, currents each of which must rise and fall in 1/150 of a
second, but for ordinary telephone speaking we must have about 1,500
currents per second, or the time which each current rises from zero to
its maximum intensity must not exceed 1/3000 part of a second. The
time constant of a telephone circuit should therefore not be less than
0.0003 second.
Resistance alone does not affect the time constant. It diminishes the
intensity or strength of the currents only; but resistance, combined
with electromagnetic inertia and with capacity, has a serious
retarding effect on the rate of rise and fall of the currents. They
increase the time constant and introduce a slowness which may be
called retardance, for they diminish the rate at which currents can be
transmitted. Now the retardance due to electromagnetic inertia
increases directly with the amount of electromagnetic inertia present,
but it diminishes with the amount of resistance of the conductor. It
is expressed by the ratio L/R while that due to capacity increases
directly, both with the capacity and with the resistance, and it is
expressed by the product, K R. The whole retardance, and, therefore,
the speed of working the circuit or the clearness of speech, is given,
by the equation
L
--- + K R = t
R
or L + K R² = R t
Now in telegraphy we are not able altogether to eliminate L, but we
can counteract it, and if we can make Rt = 0, then