Fig. 19.—Diagram of wiring of differential circuit with its various shunts, used in connection with resistance thermometers on water-circuit of bed calorimeter.
Provision is made for automatically moving the contact q by electrical means and thus the complete balance of the two differential circuits is maintained constant from second to second. As the contact q is moved, it carries with it a stylographic pen which travels in a straight line over a regularly moving roll of coordinate paper, thus producing a permanently recorded curve indicating the temperature differences. The slide-wire J is calibrated so that any inequalities in the temperature coefficient of the thermometer wires are equalized and also so that any unit-length on the slide-wire taken at any point along the temperature scale represents a resistance equal to the resistance change in the thermometer for that particular change in temperature. With the varying conditions to be met with in this apparatus, it is necessary that varying values should be 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 S1 constitutes a variable shunt to permit slight variations of r to finally adjust 0 after S' is fixed and t is a permanent shunt across the thermometer T1 to make the temperature coefficient of T1 equal to that of T2.
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 predetermined so as to affect the value of the wire J and thus halve its influence in maintaining the balance. Similarly, when the lower range, i. 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 S1 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 S1, and also of S', is necessary in order to have the apparatus draw a curve to represent truly the temperature differences. These slight alterations are determined beforehand by careful tests and the exact value of the resistances in S' and in S1 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 Pl, fig. 21. The galvanometer pointer P swings freely below the silver contacts S1 and S2, just clearing the ivory insulator i. The magnet plunger makes a contact depending upon the adjustment of a clock at intervals of 2 seconds. So long as both galvanometer coils are influenced by exactly the same strength of current, the pointer will stand in line with and immediately below i and no current passes through the recording apparatus. Any disturbance of the electrical equilibrium causes the pointer P to swing either toward S1 or S2, thus completing the circuit at either the right hand or the left hand, at intervals of 2 seconds. The movement of the pointer away from its normal position exactly beneath i to either S1 on the left hand or S2 on the right, results from an inequality in the current flowing through the two coils in the galvanometer. The difference in the two currents passing through these coils is caused by a change in temperatures of the two thermometers in the water circuit.
Fig. 20.—Diagram of galvanometer coil used in connection with recording apparatus for resistance thermometers in the water-circuit of bed calorimeter. A, anti-vibration tube; P, pointer.