Fig. 40
The mercurial thermostat shown in [Fig. 39] has a glass tube T and bulb containing mercury. Into each end is sealed a platinum wire P P. Upon the temperature rising to a predetermined degree, the expanded mercury completes the circuit between P P and the battery C and bell B are put in operation.
[Fig. 40] is the open circuit system most used by the fire alarm companies, only one circuit of six thermostats being illustrated.
It will be seen that if any thermostat closes the circuit between the outer and inner wires of the ring A B, current will flow through the corresponding drop of the annunciator and will attract the armature A of the relay. This will cause the bell to ring. As the relay is connected to the annunciator as before shown for the annunciator bell, it offers a common path for any drop to the battery. Thus the bell will ring for any circuit, but the individual drop only will fall. In a simpler circuit the relay may be dispensed with and a vibrating bell only used.
Thermostats may be operated on open or closed circuits, that is, they may give the alarm by closing a circuit and ringing a bell, or by opening one and releasing a contact spring as in the burglar alarm system to be described later.
Water Level Alarms.
Where it is desired to signal the rising or falling of water in a tank above or below a given point, a water level indicator as in [Fig. 41] may be used.
A hollow ball H is mounted on the end of a rod which slides vertically in guides, not shown. Adjustable stops S S press against a spring arm R, pressing it up or down, according as the water level is rising or falling. If rising, R makes contact with the adjustable screw A, if falling, with D, in both cases completing the electrical circuit of the battery C and bell B.
Fig. 41