om the instrument shown in Fig. 9 was
remarkably distinct, but its great defect consisted in the fact that it
could not be used as a transmitting instrument, and thus two telephones
were required at each station, one for transmitting and one for
receiving spoken messages.

[Illustration: Fig. 10]

It was determined to vary the construction of the telephone shown in
Fig. 8, and I sought, by changing the size and tension of the membrane,
the diameter and thickness of the steel spring, the size and power of
the magnet, and the coils of insulated wire around their poles, to
discover empirically the exact effect of each element of the
combination, and thus to deduce a more perfect form of apparatus. It was
found that a marked increase in the loudness of the sounds resulted from
shortening the length of the coils of wire, and by enlarging the iron
diaphragm which was glued to the membrane. In the latter case, also, the
distinctness of the articulation was improved. Finally, the membrane of
goldbeaters' skin was discarded entirely, and a simple iron plate was
used instead, and at once intelligible articulation was obtained. The
new form of instrument is that shown in Fig. 10, and, as had been long
anticipated, it was proved that the only use of the battery was to
magnetize the iron core, for the effects were equally audible when the
battery was omitted and a rod of magnetized steel substituted for the
iron core of the magnet.

[Illustration: Fig. 11]

It was my original intention, as shown in Fig. 3, and it was always
claimed by me, that the final form of telephone would be operated by
permanent magnets in place of batteries, and numerous experiments had
been carried on by Mr. Watson and myself privately for the purpose of
producing this effect.

At the time the instruments were first exhibited in public the results
obtained with permanent magnets were not nearly so striking as when a
voltaic battery was employed, wherefore we thought it best to exhibit
only the latter form of instrument.

The interest excited by the first published accounts of the operation of
the telephone led many persons to investigate the subject, and I doubt
not that numbers of experimenters have independently discovered that
permanent magnets might be employed instead of voltaic batteries.
Indeed, one gentleman, Professor Dolbear, of Tufts College, not only
claims to have discovered the magneto-electric telephone, but, I
understand, charges me w