640 



SCIENCE. 



[N. S. Vol. II. No. 46. 



trical cui-reut depends upon its temperature, 

 and advantage is taken of the fact that dif- 

 ferent metals have different electrical tem- 

 perature coefBcients. Thus the resistance 

 of a copper wire increases about one per 

 cent, for each 5° Fahr., while in the case of 

 German silver the increase is only about 

 one-tenth as great. It is a curious fact that 

 the coeificients of most pure metals are 

 almost the same as that of copper, but that 

 alloys have coefficients which are much 

 lower. 



L' 



/} B 

 Fig. I. 



The arrangement of the electrical parts 

 of the thermophone is shown in Figure I. 

 Students of electricity will recognize it as 

 being a modification of a AVheatstone's 

 Bridge. Two coils of resistance wires, A, 

 B, one of which is copper and the other 



German silver, are made to form two arms 

 of the bridge. These two coils are joined 

 together and placed at the point where the 

 temperature reading is desired. They are 

 usually drawn inside a long brass tube of 

 small diameter, coiled into a helix and 

 hermeticallj' sealed, the space between the 

 wires and the walls of the tube being filled 

 with oil to prevent corrosion and to hasten 

 the transmission of heat between the out- 

 side of the tube and the resistance wires. 

 The sensitive coils are connected by the 

 leading wires L and L' to the ends of a 

 circular slide wire C, D, and at these points 

 connection is also made with the battery M. 

 A third leading wire, K, extends from the 

 junction of the two coils to a movable con- 

 tact, Y, on the slide wire. In this circuit 

 there is interposed either a galvanometer or 

 a telephone in connection with a current 

 interrupter, the latter being operated by an 

 independent battery connection. This com- 

 bination of telej)hone and current inter- 

 rupter is used in all the portable forms of 

 the instrument and has been found to be a 

 very cheap and etficient substitute for a 

 galvanometer. The presence of a current 

 is indicated by a buzzing sound in the tele- 

 phone; silence coi-responds to the ' zero de- 

 flection ' of a galvanometer. 



Bearing in mind the principle of the 

 Wheatstone's Bridge it will be seen that 

 the galvanometer will indicate ' zero deflec- 

 tion' when A:B = CY:DY. The coils A 

 and B being made of metals having very 

 different temperature coefficients will vary 

 in resistance at different rates as their tem- 

 perature changes, and consequently there 

 will be a different value of the ratio of A to 

 B for each degree of temperature. Thus it 

 will be seen that with the bridge balanced 

 there must be a diiferent position of the 

 contact Y for everj- degree of temperature, 

 and a graduated scale may be constructed 

 corresponding to the temperature of the 

 sensitive coils. The slide wire is wound 



