How to Install Electric Bells, Annunciators, and Alarms. / Including Batteries, Wires and Wiring, Circuits, Pushes, Bells, Burglar Alarms, High and Low Water Alarms, Fire Alarms, Thermostats, Annunciators, and the Location and Remedying of Troubles

by Norman H. Schneider

Fire Alarms.

Thermostats, heat alarms and fire alarms are all practically the same, the term thermostat being applied principally to the apparatus which closes the electrical circuit.

Fig. 38

Thermostats act on the principle that heat causes expansion whether of substances, liquids, or gases.

The degree in which different substances expand varies for the same increase in temperature. This fact is used in a common form of thermostat shown in [Fig. 38]. A strip of wood or hard rubber R has a strip of thin sheet metal S riveted to it. This compound strip is held at one end by a lug L screwed fast to a baseboard. Upon an increase of temperature the hard rubber expands more than the metal strip and the compound strip bends towards the adjustable contact screw A. Upon touching the latter, the circuit through the bell B, battery C and the metal strip S is completed, and the bell rings. A contact screw can be arranged at the other side of S R, which will give warning of a decrease in temperature, as the rubber contracts more than the metal strip.

Fig. 39

In some thermostats of this character two metals having different coefficients of expansion, such as steel and brass, are used instead of metal and hard rubber.

Thermostats of this nature are much used in incubators, and they can readily be combined with electric apparatus to open or close hot-air valves, dampers, etc., and thus regulate the supply of hot air, hot water, or gas.

A thermostat much used in fire alarm work has a thin metal chamber which is air tight. An increase of temperature causes the air to expand, which swells out the walls of the chamber and closes an electric circuit.

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.