balance thermometer

Century Dictionary and Cyclopedia

1. n. A device in which mercury inclosed in a balanced tube is caused to make one or the other of the ends preponderate, thereby opening or closing a window or damper, or touching an alarm.

GNU Webster's 1913

1. n. a thermometer mounted as a balance so that the movement of the mercurial column changes the inclination of the tube. With the aid of electrical or mechanical devices adapted to it, it is used for the automatic regulation of the temperature of rooms warmed artificially, and as a fire alarm.