WHEATSTONE BRIDGES.
For use in measuring the temperature of the air and of the copper wall of the calorimeters, as well as the rectal temperature of the subject, a series of resistance thermometers is employed. These are so connected on the observer's table that they may be brought into connection with two Wheatstone bridges, W1 and W2. Bridge W1 is used for the resistance thermometers indicating the temperature of the wall and the air. Bridge W2 is for the rectal thermometer. Since similar thermometers are inserted in both calorimeters, it is necessary to introduce some switch to connect either set at will and hence the double-throw switches S1, S2, and S3 allow the use of either the wall, air, or rectal thermometer on either the bed or chair calorimeter at will. Since the bridge W1 is used for measuring the temperature of both the wall and the air, a fourth double-pole switch, S4, is used to connect the air and wall thermometers alternately. The double-contact key, K1, is connected with the bridge W1 and is so arranged that the battery circuit is first made and subsequently the galvanometer circuit. A similar arrangement in K2 controls the connections for the bridge W2.