THE CALORIMETER. 35 



of resistance thermometers is employed. These are so connected on the 

 observers table that they may be brought into connection with two Wheat- 

 stone bridges, W x and W 2 . Bridge W, is used for the resistance ther- 

 mometers indicating the temperature of the wall and the air. Bridge W 2 

 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 S 1? S 2 , and S 3 allow the use 

 of either the wall, air, or rectal thermometer on either the bed or chair 

 calorimeter at will. Since the bridge W x is used for measuring the tem- 

 perature of both the wall and the air, a fourth double-pole switch, S 4 , is 

 used to connect the air and wall thermometers alternately. The double- 

 contact key, K 1? is connected with the bridge vv\ and is so arranged that the 

 battery circuit is first made and subsequently the galvanometer circuit. 

 A similar arrangement in K 2 controls the connections for the bridge W 2 . 



GALVANOMETER. 



The galvanometer is of the Deprez-d'Arsonval type and is extremely 

 sensitive. The sensitiveness is so great that it is desirable to introduce a 

 resistance of some 500 ohms into the thermal-junction circuits. This is 

 indicated at the top of the diagram near the galvanometer. The maximum 

 sensitiveness of the galvanometer is retained when the connection is made 

 with the Wheatstone bridges. The galvanometer is suspended from the 

 ceiling of the calorimeter laboratory and is free from vibration. 



RESISTANCE FOR HEATING COILS. 



To vary the current passing through the manganin heating coils in the 

 air-spaces next the zinc wall, a series of resistances is installed connected 

 directly with the rheostat E 2 in fig. 17. The details of these resistances 

 and their connection with the rheostat are shown in fig. 18. The rheostat, 

 which is in the right part of the figure, has five sliding contacts, each of 

 which can be connected with ten different points. One end of the rheostat 

 is connected directly with the 110-volt circuit. Beneath the observer's 

 table are fastened the five resistances, which consist of four lamps, each 

 having approximately 200 ohms resistance and then a series of resistance- 

 coils wound on a long strip of asbestos lumber, each section having approxi- 

 mately 15 ohms between the binding-posts. A fuse-wire is inserted in 

 each circuit to protect the chamber from excessive current. Of these re- 

 sistances, No. 1 is used to heat the lamp in the air-current shown in fig. 25, 

 and consequently it has been found advisable to place permanently a second 

 lamp in series with the first, but outside of the air-pipe, so as to avoid 

 burning out the lamp inside of the air-pipe. The other four resistances, 

 2, 3, 4, and 5, are connected with the different sections on the two calorim- 

 eters. No. 5 corresponds to the top of both calorimeters. No. 4 corre- 



