wo
pieces of osmium iridium alloy. The contact is separated to a minute degree
partly by the action of the local current from F, which flows through it
and also through the winding W of the two magnet coils. The local current
from F assists in forming the microphone by rendering the space between the
contacts conductive. The vibrating reed P is fastened to the metal frame
(not shown) which carries a micrometer screw by which the distance between
the contacts can be accurately regulated. It will be seen from Fig. 25 that
the local circuit consists of a battery F (about 1.5 volts), the microphone
contacts C, the windings W, milliampere meter B, and the terminals T, for
connecting to the galvanometer or telephone, all in {50} series. On the top
of the magnet cores N, S is a smaller magnet D, wound with fine wire for a
resistance of about 4935 ohms, the free ends of the coils being connected
to the detector terminals. The working is as follows. Supposing the current
from the detector flows through D in such a way that its magnetism is
increased, the reed P will be attracted, the contacts opened, and their
resistance increased. It will be seen that the current from F is passed
through the coils W, in such a way as to increase the magnetism of the
permanent magnet, so that any opening of the microphone contact increases
their resistance, causes the current to fall, and weakens the magnets to
such an extent that the reed P can spring back to its normal position. On
the other hand, if the detector current flows through D in such a direction
as to decrease the magnetism in the permanent magnets, the reed P will rise
and make better contact owing to the removal of the force opposing the
stiffness of the reed. Owing to the decrease in the resistance of the
microphone, the strength of the local current will be increased, the
magnets strengthened, and the reed P will be pulled back to its original
position. This relay gives a greatly magnified current when properly
adjusted, the current being easily increased from 10^{-4} to 10^{-2}
amperes. It is also very sensitive, but needs careful adjustment in order
that the best results may {51} be obtained. A greater range of
magnification can be obtained by placing two or more relays in series.

[Illustration: FIG. 26.]

A very sensitive receiver designed by the writer is given in the figures 26
and 27. To the centre of a telephone diaphragm is fastened a light steel
point P, and the moveme