play. The two contact points are attached to a toothed wheel in which
the pawls play, and these pawls are so arranged that they drive the
wheel whenever moved by their magnets; thus the primary contact is
broken.

[Illustration]

In the receiver there is a similar toothed wheel carrying the hand of
the indicating instrument, and actuated at the same moment as the
transmitter. The primary contacts are so arranged that the contact is
made for each degree of temperature to be indicated. This series of
operations leaves the instruments closed and the pawls home in the
toothed wheel. To break the circuit another wire and separate set of
contacts are employed.

These are arranged on the arms carrying the pawls, and so adjusted
that no contact is made until after the toothed wheel has moved a
degree, when a circuit is closed and a magnet attracts an armature
attached to a pendulum. This pendulum, after starting, breaks the
circuit of the magnets which hold the pawls down, as well as of the
short-circuiting device. As the pendulum takes an appreciable time to
vibrate, this allows all the magnets to drop back, and breaks all
circuits, leaving the primary contacts in the same relation as at
first. The many details of the instruments are carefully worked out.
All the contacts are of a rubbing nature, thus avoiding danger from
dirt, and they are made with springs, so as not to be affected by jar.

The receiving instruments can be made recorders also by simple
devices. Thus, having only a most delicate pressure in the primary
instrument, a distinct ink record may be made in the receiver, even