Thermo-electric Couples 



251 



was connected in series with the galvanometer and thermo- 

 couple, and by means of this resistance, suitable sensitiveness 

 was obtained. 



Standardization of galvanometer (fig. 7). In order to be able to 

 determine electromotive force by means of a galvanometer it was 

 standardized by the "fall of potential" method, as follows: — 



A primary circuit was made up of (1) a source of electromotive 

 force (dry cell), (2) an adjustable resistance, r (3) a milliammeter. 

 (4) a known resistance, R, such as a bridge wire, or standard ohm. 



The terminals of the galvanometer circuit including the "ad- 

 justing resistance" S were connected across R; then for a given 

 current / flowing around the primary circuit, the electromotive 

 force acting on the galvanometer circuit is equal to Rxl. 



Fig. 7. Arrangement for standardizing galvanometer. 



By adjusting r various values of / are obtained, hence various 

 electromotive forces. 



Results were recorded as follows: 



(a) 



(b) 



(c) 



(d) 



Amperes (M'A) Deflection (G) EMF [{ai)xR] Volts per mm. 

 [(c)/ (b)] (a) and (b) are obtained from readings of instruments 

 MA and G; (c) is obtained by multiplying (a) by the resistance 

 R; and (d) is found by dividing (c) and (b). Example: Re- 

 sistance R = .444 ohm. 



(a) 

 ,00268 



(b) 

 25mm. 



(c) 

 .001189 



(d) 

 ,0000476 



Now, the electromotive force of any thermo-couple acting is 

 at once obtained by multiplying the deflection of the galvanome- 

 ter by the factor (d). 



