THE CALORIMETER. 39 



assigned at times to J and to r. This necessitates the use of shunts, and the 

 recording range of the instrument can be easily varied by simple shunting, 

 i. e ., by changing the resistance value of J and r, providing these resistances 

 unshunted have a value which takes care of the highest obtained temperature 

 variations. 



Fig. 19 shows the differential circuit complete with all its shunts. S is 

 a fixed shunt to obtain a range on J ; S' is a variable shunt to permit very 

 slight variations of J within the range to correct errors due to changing of 

 the initial temperatures of the thermometers ; y is a permanent shunt across 

 the galvanometer coil fl, to make the temperature coefficients of fl and fr 

 absolutely equal; Z is the variable resistance in the battery-circuit to keep 

 the current constant; r is a permanent resistance to fix the zero on varying 

 ranges; S" plus S x constitutes a variable shunt to permit slight variations 

 of r to finally adjust after S' is fixed and t is a permanent shunt across 

 the thermometer T x to make the temperature coefficient of T 1 equal to that 

 of T 2 . 



The apparatus can be used for measuring temperature differences from 

 0° to 4° or from 0° to 8°. When on the 0° to 8° range, the shunt S is 

 open-circuited and the shunt S' alone used. The value of S, then, is pre- 

 determined so as to affect the value of the wire J and thus halve its influence 

 in maintaining the balance. Similarly, when the lower range, t. e., from 

 0° to 4°, is used, the resistance r is employed, and when the higher range is 

 used another value to r must be given by using a plug resistance-box, in the 

 use of which the resistance r is doubled. 



The resistance S" and S x are combined in a slide-wire resistance-box and 

 are used to change the value of the whole apparatus when there are marked 

 changes in the position of the thermometric scale. Thus, if the ingoing 

 water is at 2° C. and the outcoming water at 5° C. in one instance, and in 

 another instance the ingoing water is 13° and the outgoing water is 15°, a 

 slight alteration in the value of S x , and also of S', is necessary in order to 

 have the apparatus draw a curve to represent truly the temperature differ- 

 ences. These slight alterations are determined beforehand by careful tests 

 and the exact value of the resistances in" S' and in Si are permanently 

 recorded for subsequent use. 



THE GALVANOMETER. 



The galvanometer is of the Deprez-d'Arsonval type and has a particularly 

 powerful magnetic field, in which a double coil swings suspended similar 

 to the marine galvanometer coils. This coil is protected from vibrations 

 by an anti-vibration tube A, fig. 20, and carries a pointer P which acts to 

 select the direction of movement of the recording apparatus, the movable 

 contact point q, fig. 19. In front of this galvanometer coil and inclosed in 

 the same air-tight metal case is the plunger contact PI, fig. 21. The gal- 



