barub.] METHODS OF PYROMETRY. 53 
insuperable difficulties, the kinds of pyrometers available dwindle down 
to a very small number. Indeed, the thermo couple is almost the only 
instrument of research left. It is therefore most encouraging- to find 
that, for purposes of high temperature measurement in general, the 
thermo-couple can be made to yield results which, apart from practical 
conveniences and easy manipulation, are warrantably as accurate as 
any known to us. I will summarize the advantages here in question 
as clearly as I can, so that they may be referred to in the bulk of the 
work: 
1. Barring a few corrections, the thermo-couple of known properties 
is available for temperature measurement under all pressures. The cor- 
rections implied are those which become necessary in consequence of the 
changes of thermoelectric property with pressure; but these changes 
are slight and quite negligible in comparison with the thermal sensi- 
tiveness of the couples. 
2. The temperature at the hot junction is dependent on the tempera- 
ture at the cold junction and the constants of the couple only. It is 
independent of the distribution of temperature in the parts of the couple 
between the junctions. This is a great practical advantage, the impor- 
tance of which is realized when temperature is to be measured under 
pressure. 
3. The thermo couple is capable of measuring temperature when the 
dimensions of the hot space scarcely exceed a physical point. Small 
zones of constant temperature and relatively small apparatus for heat- 
ing are therefore sufficient for thermal comparisons of relatively great 
accuracy. In this respect the thermo-couple deserves preference to 
the resistance thermometer, particularly when material fusible witli 
great difficulty is operated upon. 
4. The upper limit of temperature measurement is practically infinite, 
aud lies much above the melting point of platinum; for by using re- 
fractory alloys of platinum, iridium, rhodium, and inclosing wires of 
these metals in tubes of calcined lime, distinct and powerful thermo- 
electric effects are obtained even when the contents of the tubes are 
fused. 
5. The electromotive forces of suitable thermo-couples are easily meas- 
urable with an accuracy of 1 in 1,000. Almost with the same accuracy 
may the indications observed at the beginning of any year be compared 
with those at the beginning of the next or any other subsequent time. 
The secular errors of a thermo-electric pyrometer, when properly cared 
for, need not be larger, relatively, than the secular errors of a mercury 
thermometer. 
6. Many couples are known which, in addition to the desideratum 
of thermal sensitiveness, possess great tenacity and ductility, and are 
unalterable under ordinary conditions of heating. 
7. The thermal indications are as nearly as possible instantaneous, 
and the discrepancy of the lag error is therefore a minimum or nil. 
(707) 
