d at 1, 2, and 3,
these having the wave-length 1, 2, and 3. In 4, the three are combined
into one compound wave, and better show the form of a transverse section
of such a sound-wave in the air. The organ-pipe called the principal
gives out such a compound wave as is seen by referring to the table on
p. 91. The second overtone, however, is quite weak in that pipe, which
would so modify the form as to lessen somewhat the density at _b_, and
increase it at _a_.

[Illustration: FIG. 10.]

In like manner the space in the length of the fundamental sound,
whatever it may be, is divided up into a number of minor condensations
and rarefactions, which may strengthen each other, or so interfere as to
change the position of both; as is seen in the figure at _b_, where the
condensation due to wave 2 interferes with the rarefaction of 3.

CORRELATION.

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