THERMO-ELECTRIC PROPERTIES OF CHARCOAL AND CERTAIN ALLOYS. 323 
other than either M or N. The hot junction was then quadruple, since in all 
cases in which the heated hollow cylinder was used, the M-N circuit was 
employed as the thermometer. After the completion of each experiment, 
which lasted until the temperature of the hot cylinder fell to very little above 
the temperature of the room, the junction was heated in olive-oil, and a series 
of measurements of temperature and corresponding M-N deflections was made. 
The temperature measurements were made by means of.a mercury thermo- 
meter corrected by comparison with a Kew standard centigrade thermometer. 
We thus calibrated our galvanometer, and could at once substitute for the ° 
M-N deflections their equivalents in degrees centigrade, and thus obtain the 
law of variation of the electromotive force of the given thermo-electric pair 
with temperature. The observations of temperature of the cold junctions were 
also corrected by a similar comparison. It was found convenient to make the 
resistances of the circuits which were to be compared equal. 
The results given below are subject to the following sources of slight mac- 
curacy :—(1.) The variation of temperature of the cold junctions, which, however, 
never exceeded ‘2° C., and which in almost all cases was very small compared 
to the difference of temperatures of the hot and cold junctions. The assump- 
tion of a constant temperature of cold junction could not appreciably affect the 
interpretation of results. (2.) A necessary error in the estimation of the higher 
temperatures—due to the merely approximate parallelism of the M-N diagram 
lines. For the lower and really important temperatures, however, this error was 
so small as to be inappreciable. (3.) The variation in resistance of the circuits 
due to heatmg. The thickness of the M-N wires, and the small portions of 
them which were in the heated cylinder, and which were perhaps 5,55 of the 
whole resistance in the circuit, warrant us in believing that this variation in 
the M-N circuit was within the unavoidable errors of observation. Most of 
' the wires we examined, however, were very thin, and had a great resistance per 
unit length, so that we could not put very great lengths in circuit ; consequently 
a comparatively great proportion of the whole resistance was subject to increase 
from rise of temperature. For high temperatures then we do not consider our- 
selves warranted in placing more than a guarded confidence in the determina- 
tions, and consequently we give below no measurement made above 400° C. 
Further, for these high temperatures, the thinness of the wires rendered them 
peculiarly sensitive to currents of air, which could not be wholly screened off. 
I. Charcoal. 
The charcoal which we examined was an ordinary piece of gas-coke, such as 
is used in the Bunsen cell. It was cut in the form of a cylinder about 15 cm. 
in length and 1°5 cm. in thickness, and was arranged exactly as above described. 
