electromotive forces in the
windings varies the pull of the permanent magnet of the receiving
instrument upon its diaphragm.

[Illustration: No. 10 SERIES MULTIPLE SWITCHBOARD _Monarch Telephone
Mfg. Co._]

[Illustration: Fig. 3. Magneto Telephones without Permanent Magnets]

Fig. 3 illustrates a similar arrangement, but it is to be understood
that the cores about which the windings are carried in this case are
of soft iron and not of hard magnetized steel. The necessary magnetism
which constantly enables the cores to exert a pull upon the diaphragm
is provided by the battery which is inserted serially in the line.
Such an arrangement in action differs in no particular from that of
Fig. 2, for the reason that it matters not at all whether the
magnetism of the core be produced by electromagnetic or by permanently
magnetic conditions. The arrangement of Fig. 3 is a fundamental
counterpart of the original telephone of Professor Bell, and it is of
particular interest in the present stage of the art for the reason
that a tendency lately is shown to revert to the early type,
abandoning the use of the permanent magnet.

The modifications which have been made in the original magneto
telephone, practically as shown in Fig. 2, have been many. Thirty-five
years' experimentation upon and daily use of the instrument has
resulted in its refinement to a point where it is a most successful
receiver and a most unsuccessful transmitter. Its use for the latter
purpose may be said to be nothing. As a receiver, it is not only
wholly satisfactory for commercial use in its regular function, but it
is, in addition, one of the most sensitive electrical detecting
devices known to the art.

Loose Contact Principle. Early experimenters upon Bell's device, all
using in their first work the arrangement utilizing current from a
battery in series with the line, noticed that sound was given out by
disturbing loose contacts in the line circuit. This observation led to
the arrangement of circuits in such a way that some imperfect contacts
could be shaken by means of the diaphragm, and the resistance of the
line circuit varied in this manner. An early and interesting form of
such imperfect contact transmitter device consisted merely of metal
conductors laid loosely in contact. A simple example is that of three
wire nails, the third lying across the other two, the two loose
contacts thus formed being arranged in series with a battery, the
line, and