Fig. 59.
§ 58. Another useful application of "contact" is for the notification of any rise or fall of temperature beyond certain fixed limits. The devices used for this purpose are known as "fire alarms," "frost alarms," and "thermometer alarms." The thermometer alarm is at once the most effective and trustworthy of the forms known, as, besides its delicacy, it has the advantage of being able to give notice of low, as well as of abnormally high temperature. The form usually given to the electric alarm thermometer, is well shown at [Fig. 59]. It consists in an
ordinary thermometer with a wire projecting into the tube to a certain point, say 100 degrees. The mercury in the bulb being also connected with another wire. When the temperature is within the usual climatic range, the mercury does not reach the upper wire. If by reason of fire or any other abnormal heat, the temperature rises beyond that to which the instrument is set, the mercury rises and touches the upper wire, contact is thus established, and the bell rings.
By giving the thermometer the shape of a letter U, it is possible to notify also a fall below a certain degree, as well as a rise beyond a certain fixed point. These thermometers are specially used by nurserymen and others, to warn them of the too great lowering of temperature, or vice versâ, in the houses under their charge.
Other forms of fire alarms are shown at Fig. [60] and [61]. If a strip be built up of two thin layers of dissimilar metals riveted together, as the two metals do not expand at the same rate, the strip will bend to the right if heated, and to the left if cooled. In the instrument shown at [Fig. 60], the application of heat causes the flexible strip carrying the contact screw, to bend over till it touches the lower stop, when, of
course, the bell rings. If two stops are employed instead of the lower one only, the bell will ring when a low temperature is reached, which causes the strip to bend in the opposite direction.
