THERMO-ELECTRIC PROPERTIES OF CHARCOAL AND CERTAIN ALLOYS. 327 
We thus know the ratio of the distance from the N line of the point corre- 
sponding to 200° C. on the charcoal line, to the distance between the M and 
N lines ; and it only remains to determine their general relative position on the 
diagram. This is indicated by the signs of the deflections given in Table I. 
These show that the arrangement M, N, galvanometer, gives a current in the 
same direction.as N, C, galvanometer, 7.¢., if a current flows across the heated 
M-N junction from M to N, it flows across the heated N-C junction from N 
to C, and thus on the diagram the C line must be on the same side of the N 
line as the N line is of the M line. 
The above data enable us to draw the C ie from 30° or a up to 230°. 
To fix the line for higher temperatures, we have determined = ° graphically as 
follows :— 
fort =290° C., < = = 1‘01 
dé 
= 2 pee types. >) 
£=340° C., a ae 
The line thus determined will be found lettered C in the supplementary 
thermo-electrical diagram which we give in Plate X XI. It will be noticed that 
we have found the value of “ for <=0° C., although 0° C. is beyond the limits of 
our experiments. Consequently the line at and somewhat above 0° C. is dotted. 
‘The diagram which we have prepared is simply an extension of that of Professor 
Tarr. It is drawn to exactly the same scale. In the equations to lines on this 
diagram, the point at which the 0° C. line cuts the lead line is taken as origin, 
and the lead line as the axis of temperature. The equation to that part of the 
carbon line which is straight is 
y= —029¢ + 20°54, 
where the ordinate y is expressed in terms of a unit of electromotive force, the 
uumber of which corresponding to different lengths of ordinates is given on the 
“Margin of the diagram, and ¢ in degrees centigrade. 
For the value of the above equation in determining the electromotive force 
im a circuit containing carbon, see the conclusion of this paper. 
The position which is thus assigned to the charcoal line agrees generally 
with the results which E. BecquerEL and Marruatrssen have obtained for the 
same substance. The position in thermo-electric series which the former finds 
to hold for the low temperatures is indicated as follows*:—(+) cadmium, silver, 
platinum, charcoal, tin, lead (—). Taking the lines for these metals as given in 
Professor Tarv’s diagram, our order for. low temperatures is :—(+) cadmium, 
 * Ann, de Chim. et de Phys.” (4) T, viii. p. 415. 
