THE CALORIMETER SYSTEM AND MEASUREMENT OF HEAT. 145 



ances in the two arms of the bridge, C and D, and consequently a small 

 current of electricity passes through the galvanometer. The current 

 may pass from G' to G" if the coil C has a greater resistance than D, or 

 it may pass from G" to G' if the coil C has less resistance than coil D. 

 It is important to bear in mind that the resistances of the coils, plus the 

 resistance of connecting wires, must be taken into consideration rather 

 than the resistance of the coils alone. Unless the resistance of the coil 

 C and all its connections with the points G' and B" (fig. 44) is exactly 

 equal to the resistance of the coil D and its connections with the points 

 G" and B", a current of electricity will pass through the galvanometer. 



In the ordinary form of Wheatstone bridge, provision is made for 

 altering the resistance of coil D until the equilibrium is again established, 

 the degree of alteration in coil D being an index of the resistance change 

 (temperature change) in coil C. The so-called " slide-wire " form of 

 Wheatstone bridge alters the position of the point B", thus varying the 

 total resistances between B" and G', and B" and G", until the equilib- 

 rium is established and no current flows through the galvanometer. 

 With either of these two methods of resistance adjustment, elaborate 

 and delicate apparatus is required and the continuous use of such instru- 

 ments is accompanied by a constantly increasing inaccuracy in their use, 

 due to the wearing of parts. In the mercury contact switch and bridge 

 described here, the use of a Wheatstone bridge and resistance box, or a 

 slide-wire bridge, is obviated. 



If in the system shown in figure 44 the resistance of coil C varies 

 and is not identical with coil D, a current will pass through the gal- 

 vanometer and produce a deflection. This deflection will, in general, 

 be nearly proportional to the amount of current flowing through the 

 galvanometer, and, as the current is equal to the electro-motive force 

 divided by the resistance, it follows that with a constant electro-motive 

 force the current varies inversely as the resistance. Assuming that 

 the resistance, coil C (fig. 44), at a given temperature is such as to 

 cause a current of electricity to pass through the galvanometer and 

 produce a deflection of 100 mm. on the scale, if the resistance of the coil 

 C is now decreased a larger current will pass through the galvanometer 

 and the deflection will become larger. Conversely, if the resistance of 

 C increases, a smaller current of electricity will flow through the galva- 

 nometer and the deflections will grow smaller. If, now, the resistance 

 of C is increased further, there will be a point at which the resistance 

 of C is equal to D and no current will pass through the galvanometer, 

 and if the resistance is still more increased, the current will tend 

 to flow through the galvanometer in the opposite direction, and con- 

 stantly increasing deflections, though in the opposite direction from 



