nfluences.

There is no industrial art in which the advancement of the times has
been followed more closely by practical application than in telephony.
Commercial speech by telephone is possible by means of currents which
so far are practically unmeasurable. In other words, it is possible to
speak clearly and satisfactorily over a line by means of currents
which cannot be read, with certainty as to their amount, by any
electrical measuring device so far known. In this regard, telephony is
less well fortified than are any of the arts utilizing electrical
power in larger quantities. The real wonder is that with so little
knowledge of what takes place, particularly as to amount, those
working in the art have been able to do as well as they have. When an
exact knowledge of quantity is easily obtainable, very striking
advances may be looked for.

The student of these phases of physical science and industrial art
will do well to combine three processes: study of the words of others;
personal experimentation; and digestive thought. The last mentioned is
the process of profoundest value. On it finally depends mastery. It is
not of so much importance how soon the concept shall finally be gained
as _that it is gained_. A statement by another may seem lifeless and
inert and the meaning of an observation may be obscure. Digestive
thought is the only assimilative process. The whole art of telephony
hangs on taking thought of things. Judge R.F. Taylor of Indiana said
of Professor Bell, "It has been said that no man by taking thought may
add a cubit to his stature, yet here is a man who, by taking thought,
has added not cubits but miles to the lengths of men's tongues and
ears."

In observations of many students, it is found that the notion of each
must pass through a certain period of incubation before his private
and personal knowledge of Ohm's law is hatched. Once hatched, however,
it is his. By just such a process must come each principal addition to
his stock of concepts. The periods may vary and practice in the uses
of the mind may train it in alertness in its work. If time is
required, time should be given, the object always being to keep
thinking or re-reading or re-trying until the thought is wholly
encompassed and possessed.

CHAPTER I

ACOUSTICS

Telephony is the art of reproducing at a distant point, usually by the
agency of electricity, sounds produced at a sending point. In this art
the elements of two g