572 
MR. R. W. STEWART OX THE ABSOLUTE THERMAL 
between 0° C. and 200° C., strictly proportional to the ditierence between the tempe¬ 
ratures of it's junctions. The galviinometer in use Avith the couple Avas a Ioav 
resistance reflecting galvanometer, adjusted so as to giA^e a deflection of about four 
scale-divisions for a difference of 1° C. between the temperatures of the junctions. 
The scale deflections could be easily read to half a diAusion, so that the difference 
between the temperatures of the junctions could be estimated to about one-tenth of 
a, degree. 
The distribution of temperature along the bars in the stationary state was deter¬ 
mined by inserting one junction successively into the small holes drilled in the bar, 
Avhile the other junction was maintained at a constant temperature^ by inserting it in 
a hole in a short iron bar restinn on the bottom of the zinc troimh T. The deflections 
O O 
obtained were, iii this way, proj^ortional to the difference of temperature between the 
seA-eral points on the bar and the temperature of the surrounding air. It Avas found 
necessary to carefully insulate the ends of the Avires of the couple from one another 
just above the junctions, and from the bars in Avhich they were inserted ; this Avas 
done by filing down the wires, inserting a thin slip of silk between them, and then 
whipping them round with a fine silk thread. The outer surface of the thread 
charred slightly at the higher temperatures, but not sufficiently to destroy its 
efficiency. 
In taking a series of observations of the distribution of temperature along the 
bar, the heating AAms alloAved to go on for fiA'e or six hours, and then the deflections 
corresponding to each hole along the bar Avere observed, up and down the bar, time 
after time, until a set of perfectly constant and concordant readings Avere obtained. 
It was not always possible to attain this end ; a change in the tem23erature of the 
room, or any cause tending to produce draught in the room, destroyed the constancy 
of the results, but by repeated obserA-ations under favourable circumstances, and by 
shielding the bar from draughts by means of the trough TT, it Avas fouiul possible to 
obtain perfectly constant data, except in the case of the copper bar, for the holes 
iiearest the heated end. It Avas found that the temperatures of these holes A\dien 
above 180°C. to 200° C. varied continually, through a range haAuiig a maximum A'alue 
of about 1° for the hole nearest the end, and quicklv diminishing to zero at the 
third or fourth hole from the end. This fluctuation in temperature Avas pi'obably 
due, partly to the want of massiveness of the bar and partly to the formation of a 
film of oxide on the surface of this part of the bar; but by keeping the temperature 
from going much above 200° C. and by taking the mean of ten or tAveh'e successive 
readings the mean temperature of these holes Avas determined Avith certainty to 
* This temperature Avas indicated by a thermometer 25iaced with its bulb in a hole drilled in tlie bar 
close to the small hole in Avhich the junc^oii was placed. In no case aa'us the A-ariation of temperature 
more than a fifth of a degree. The bar Avas a short massive iron bai', not the one used for cooling 
cxporimeiits. 
