( 321 ) 
XV.—On the Thermo-Hlectric Properties of Charcoal and certain Alloys, with 
— a Supplementary Thermo-Electric Diagram. By C. G. Knort, B.Sc., and 
J. G. MacGrecor, D.Sc. (Plate X XI) 
(Received 24th January 1878. Read 4th March 1878.) 
The determination by experiment of the thermo-electric relations of any 
one substance belonging to the electromotive series to all other such sub- 
stances is sufficient to fix all mutual thermo-electric relations among these. 
The first endeavour of the experimenter is then to obtain as convenient a 
substance for this purpose as possible. In investigating charcoal and certain 
alloys; we have in almost all cases employed one or other of two alloys of 
platinum and iridium, which have been already used by Professor Tarr for a 
like purpose. The wires we used were the same which he discusses in his 
“First Approximation to a Thermo-Electric Diagram,”* under the names 
of M and N. Their complete freedom from oxidation, their elasticity, and 
the high temperatures of their fusing points, rendered them peculiarly suitable 
_ for thermo-electric investigations through long ranges of temperature. 
Generally both the M and N wires were firmly bound, each by its one 
extremity to the end or ends of the wire or wires respectively which were 
under investigation, in a multiple junction. This triple or, as it was in some 
cases, quadruple junction constituted the “hot junction.” The free extremities 
of the wires thus united were each bound to a moderately thin copper wire by 
' very thin wire of the same metal, and the copper wires were led from these 
“cold junctions” to a commutator, which was in connection with a galvanometer. 
The commutator consisted of an arrangement of small mercury pools, into 
which the galvanometer and circuit wires, carefully amalgamated to ensure 
contact, dipped. All the junctions were formed in the manner indicated above, 
namely, by tightly binding the extremities of the wires together by thin copper 
wire. 
The hot junction was made to vary in temperature according to the method 
referred to by Professor Tarr on page 135 of the paper cited above. A wrought- 
iron tube, four inches long, two inches in diameter, with a bore of one inch 
diameter, which opened only below, was heated to the required temperature, 
and suspended by a hook fixed in its upper and closed end over the multiple 
* “Trans, R.S.E.” 1872-3, p. 125. 
VOL. XXVIII. PART IL. fe 
