


342 C. G. KNOTT AND J. G. MACGREGOR ON THE 
(2.) Thermo-Electric Circuit containing M and N. 




Resistance of galvanometer, as before, : = 22°588 ohms. 
Resistance of M, N, and other wires in circuit, . = S41 7= 
Temperature of Temperature of : 
M-N Junction. other Junctions. Lene 
obo: C. Deas OP 46°5 
149°5 11:3 160 






From these data it is found that the arbitrary unit which we have used in 
drawing our diagram has the value 105 x 10~° of a DANIELL’s cell of the above 
construction. 
Professor Tait has given the following formula for the electromotive force 
of a circuit consisting of two substances a and 80, viz. :— 
E = (th) (ta) (Ta — $4), 

where ¢ and ¢, are the temperatures of the junctions, T,, the “neutral point ” of 
the substances, and k,, *, constants whose magnitude depends on the peculiarity 
of the substance and on the units employed in the measurement of electromotive — 
force and temperature. It may be easily shown that / for any metal is numerically 
equal to the tangent of inclination to the ¢ axis of its line on the diagram. Hence 
we obtain the electromotive force of any pair in terms of our arbitrary unit by 
substituting in the above formula the various temperatures in °C., and writing for 
k,, k, the coefficients of ¢ in the equations to the diagram lines of the metals a, 0. 
The neutral point of any two substances is found from the equations to their 
diagram lines as given above by the elimination of y. To obtain the electro- 
motive force in terms of any other unit is a mere matter of arithmetic. For con- 
venience we give in Table XV. the values of 4, which, when substituted in the 
above formula, give the electromotive force in terms of 10~° of a DANIELL’s cell. 
dy 
dt 
tions to the diagram lines of the various substances (the equations being of the 
Columns (1) and (2) contain the values of the constants — and A in the equa- 
d. 
form y == t+ A). The fourth column contains the values of & (after Tarr*) 
expressed in terms of a unit equal to “nearly 10-° of a Grove’s cell.” 
* “Trans, R. 8. E.” vol. xxvii. 1872-73, pp. 126 and 139. 

