146 A RESPIRATION CALORIMETER. 



those at the beginning, will be obtained. Obviously, the direction of 

 the current through the galvanometer and the direction of the deflection 

 for increasing or diminishing resistances in C depend upon the direction 

 of the current from the battery and also upon the connections through the 

 galvanometer terminals, for by interchanging the wires leading from 

 the galvanometer to G' and G" the current through the galvanometer 

 may be made to pass in the opposite direction. 



Of the four resistances in the bridge system, but one is affected by 

 temperature changes, i. e. , that composed of the copper coils ; hence any 

 differences in the deflection of the galvanometer may be ascribed directly 

 to variations in resistance of the copper coil, provided the electro-motive 

 force of the battery remains constant. To insure a constant electro- 

 motive force, we have relied upon the establishment of an arbitrarily 

 adjusted bridge system, in which the coils C and D are slightly out of pro- 

 portion, i. e. , coil C has a somewhat lower resistance than coil D. Both 

 coils (C and D) in this system are made of wire with zero temperature 

 coefficient. These coils are mounted on the mercury contact switch (fig. 

 40), one on each side of the rows of mercury cups in the oak base and 

 about midway of the sides. The coils are similar in form to the two 20- 

 ohm coils mentioned before, and while one has a resistance of exactly 20 

 ohms, the other is a small fraction of an ohm lower in resistance. Inas- 

 much as these coils belong to a bridge system that is used to standardize 

 the electro-motive force from the battery, they are called standard coils, 

 or standard resistances, and on figure 43 they are marked St'd res. 



With the bridge system thus arranged, the closing of the battery and 

 galvanometer circuits should result in a deflection of the galvanometer, 

 owing to the inequality of arms C and D of the bridge. The deflection 

 of the galvanometer is then approximately proportionate to the electro- 

 motive force, and by adjusting the number of cells of the battery and 

 varying the resistance in the battery circuit it is possible to produce a 

 deflection of any definite magnitude. Since all coils are of wire with 

 zero temperature coefficient, no changes in temperature will affect the 

 bridge system, and the amount of current necessary to produce a deflec- 

 tion of 100 mm., for example, will be constant, provided there are no 

 variations in the galvanometer constant. This latter factor can be tested 

 readily, and in fact varies but slightly, so that we have by this bridge 

 system an excellent method of compensating for variations in the electro- 

 motive force of the battery. As a source of current, we use ordinary 

 dry cells, with a small variable resistance in series with them. With 

 this arrangement and with the connections as now made, the amount 

 of current required to produce a deflection of 120 mm. is used as the 

 standard for all our work. Inasmuch as the temperature of the cal- 



