The galvanometer is of the Deprez-d'Arsonval type and has a particularly
powerful magnetic field, in which a double coil swings suspended similar
to the marine galvanometer coils. This coil is protected from vibrations
by an anti-vibration tube A, fig. 20, and carries a pointer P which acts
to select the direction of movement of the recording apparatus, the
movable contact point _q_, fig. 19. In front of this galvanometer coil
and inclosed in the same air-tight metal case is the plunger contact Pl,
fig. 21. The galvanometer pointer P swings freely below the silver
contacts S_{1} and S_{2}, just clearing the ivory insulator _i_. The
magnet plunger makes a contact depending upon the adjustment of a clock
at intervals of 2 seconds. So long as both galvanometer coils are
influenced by exactly the same strength of current, the pointer will
stand in line with and immediately below _i_ and no current passes
through the recording apparatus. Any disturbance of the electrical
equilibrium causes the pointer P to swing either toward S_{1} or S_{2},
thus completing the circuit at either the right hand or the left hand,
at intervals of 2 seconds. The movement of the pointer away from its
normal position exactly beneath _i_ to either S_{1} on the left hand or
S_{2} on the right, results from an inequality in the current flowing
through the two coils in the galvanometer. The difference in the two
currents passing through these coils is caused by a change in
temperatures of the two thermometers in the water circuit.
[Illustration: FIG. 20.--Diagram of galvanometer coil used in connection
with recording apparatus for resistance thermometers in the
water-circuit of bed calorimeter. A, anti-vibration tube; P, pointer.]