HAVING treated at some length of the three factors involved in
telephony,--namely, electricity, magnetism, and sound,--it remains to
follow up the various steps that have led to the actual transmission of
musical sounds and speech over an ordinary electric circuit.
It is stated upon p. 31, that, when a current of electricity is passed
through a coil of wire that surrounds a rod of soft iron, the latter is
made a temporary magnet: it loses its magnetic property the instant that
the current ceases. If the rod be of considerable size, say a foot or
more in length, and half an inch or more in diameter, and the current be
strong enough to make a powerful magnet of it, whenever the current from
the battery is broken, the bar may be heard to give out a single
_click_. This will happen as often as the current is broken. This is
occasioned by a molecular movement which results in a _change_ of
_length_ of the bar. When it is made a magnet, it elongates about
1/25000 of its length; and, when it loses its magnetism, it _suddenly_
regains its original length; and this change is accompanied with the
sound. This sound was first noticed by Prof. C. G. Page of Salem, Mass.,
in 1837. If some means be devised for breaking such a circuit more than
fifteen or sixteen times a second, we shall have a continuous sound with
a pitch depending upon the number of clicks per second. Such a device
was first invented by the same man, and was accomplished by fixing the
armature of an electro-magnet to a spring which was in the circuit when
the spring was pressing against a metallic knob, at which time the
current made the circuit in the coil of the electro-magnet. The magnet
attracting the armature away from the button broke the circuit, which of
course destroyed the magnetism of the magnet, and allowed the spring to
fly back against the button, to complete the circuit and reproduce the
same series of changes. The rapidity with which the current may be
broken in this way is only limited by the strength of both spring and
current. The greater the tension of the spring with a given current,
the greater number of vibrations will it make.
[Illustration: FIG. 11.]
Suppose such an intermittent current to pass through the coil
surrounding the soft iron rod, 256 times per second; then the rod would
evidently give 256 clicks per second, which would have the pitch of C.
When these clicks are produced in the rod hold in the hand, the sound is
hardly perceptible, being like that of a sounding tuning-fork when held
thus. In order to strengthen it, it is necessary to place it on some
resonant surface. It is customary to mount it upon an oblong box with
one or two holes in its upper surface, inasmuch as such a form is found
to give a louder response than any other, and is the shape usually given
to AEolian harps. The accompanying cut shows the combination of battery
B, the circuit-breaker, and the rod mounted upon the box. The wire W may
evidently be of any length, the magnetized rod and box responding to the
number of vibrations of the spring S, how long soever the circuit may
be.