Replace the wires in the binding posts, clean the platinum on both contact screw and armature spring and try the adjustment. Troubles in the bell will be mostly similar to those before mentioned.
If no current has been obtained at either bell or push, and the battery is in good working order, the line must be tested for a cross or break.
If the wires are touching each other (Fig. 35) at some bare spot _S_ between the bell and the battery, it will be shown by the metallic taste upon detaching one wire from the battery and laying it on the tongue _T_, together with another wire _W_ from the disconnected terminal of the battery. The current will travel from the battery to the cross at _S_, then back along the second circuit wire to the tongue and through the short wire to the battery.
If no current is obtained in this way it is probable that the wire is broken.
The easiest way to find this is to take a bell to the battery and connect it between the circuit wires and the battery (Fig. 36).
Then with a sharp knife carefully cut away a little piece of the insulation from each wire beyond the bell and battery and short circuit the bared spots with the knife blade _K_. Keep working towards the push. The bell will ring each time at _K K_ until the break _D_ is passed, at _C_ it will not. It becomes an easy matter then to locate it.
If the bell and push are far apart, as in Fig. 37, a break between the push and the bell may be found as shown. With the knife blade _K_ at different points the bell will ring, but after passing the break _D_ it will not ring.
Such simple tests as are here given can be carried out by any one, but far better results will be obtained if the reason for each is first learned.
This can be readily done by a careful study of the diagrams and text.
=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.
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.