243,796. Freda, G. Sept. 3, 1924. Alarms operated by expansion of solids and gases.-An electric fire alarm, comprising a considerable length of wire 1 having tension adjusting means 3, 4 at one end and a spring-controlled contact 13 at the other, is provided with means whereby no signal is given on rise of temperature due to normal conditions. The spring-controlled contact 13 is adapted to coact with a contact 14 mounted on one arm of a lever 17, the other arm of which is connected by a link 21 to a piston 22 in an air cylinder 23. The. expansion of air in the cylinder 23 compensates for expansion of the wire 1 on slow rise of temperature. The volume of air in the cylinder is regulated by a piston 27 moved by a screw 25. A screw knob 28 is removed from the end of the hollow piston rod 26 when it is required to allow air to enter the cylinder. The contact may take place under oil to prevent sparking. The wire 1 may be conducted through a perforated tube.
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