After a great deal of preliminary experimenting, in which the Leeds & Northrup Company have most generously interpreted our specifications, they have furnished us with an apparatus which meets to a high degree of satisfaction the conditions imposed. The thermometers themselves have already been discussed. (See page 30.) The recording apparatus consists of three parts: (1) the galvanometer; (2) the creeper or automatic sliding-contact; (3) the clockwork for the forward movement of the roll of coordinate paper and to control the periodic movement of the creeper. Under ordinary conditions with rest experiments in the chair calorimeter or bed calorimeter, the temperature differences run not far from 2° to 4°. Thus, it is seen that if the apparatus is to meet the conditions of the specifications it must measure differences of 2° C. to within 0.01° C. Provision has also been made to extend the measurement of temperature differences with the apparatus so that a difference of 8° can be measured with the same percentage accuracy. FUNDAMENTAL PRINCIPLE OF THE APPARATUS. The apparatus depends fundamentally upon the perfect balancing of the two sides of a differential electric circuit. A conventional diagram, fig. 19, gives a schematic outline of the connections. The two galvanometer coils, _fl_ and _fr_, are wound differentially and both coils most carefully balanced so that the two windings have equal temperature coefficients. This is done by inserting a small shunt _y_, parallel with the coil _fl_, and thus the temperature coefficient of _fl_ and _fr_ are made absolutely equal. The two thermometers are indicated as T_{1} and T_{2} and are inserted in the ingoing and outgoing water respectively. A slide-wire resistance is indicated by J, and _r_ is the resistance for the zero adjustment. Ba, Z, and Z_{1} are the battery and its variable series resistances. If T_{1} and T_{2} are exactly of the same temperature, _i. e._, if the temperature difference of the ingoing and outcoming water is zero, the sliding contact _q_ stands at 0 on the slide-wire and thus the resistance of the system from 0 through _fl_, _r_, and T_{1} back to the point C is exactly the same as the resistance of the slide-wire J plus the coil _fr_ plus T_{2} back to the point C. A rise in temperature of T_{2} gives an increase of resistance in the circuit and the sliding contact _q_ moves along the slide-wire toward J maximum until a balance is obtained. [Illustration: FIG. 19.--Diagram of wiring of differential circuit with its various shunts, used in connection with resistance thermometers on water-circuit of bed calorimeter.]