Chap. XIV.] G A LI' A NO METER AND THERMO-PILE. 145 



brass, which has a small lid at one end, the lifting of 

 which exposes one set of junctions. The other end is 

 prolonged into a funnel for collecting heat rays and 

 leading them into the other junctions. 



By means of the binding screws r o the thermo-pile 

 p can be connected with a sensitive galvanometer H 

 (Fig. 77), and thus the 

 slightest difference of 

 temperature will 

 cause a deflection of 

 the needle. The elec- 

 tromotive force of 

 thermal currents is 



very small ; and their 



Capacity for overcom- Fig< 77. Galvanometer and Thermo-pile. 



ing resistance is, there- 

 fore, very little. If, consequently, a high -resistance 

 galvanometer, such as that which is used for the de- 

 tection of the muscle currents, be employed, its 32,000 

 turns of wire interpose such resistance that the 

 heat current is so weakened as to be unable to affect 

 the needle. With Wiedemami's apparatus, the sliding 

 arrangement of the coils of wire (page 105) permits 

 of the coils of fine wire being removed, and other 

 coils, usually made of from 5 to 10 metres of insulated 

 copper wire, one mm. thick, being put on instead. 

 Usually with the instrument two sets of coils are 

 supplied, one set of high resistance and another of 

 low, arid thus in a moment the change can be made 

 from a high to a low resistance galvanometer. The 

 sliding device also permits of any number of coils 

 being made of varying resistances for the same instru- 

 ment. If Sir Wm. Thomson's instrument be used, 

 one ought to be obtained which has an arrangement 

 of plugs by which many or few turns of the coils 

 may be interposed in the circuit of the galvanometer, 

 which is thus made of high or low resistance at pleasure. 

 K 7 



