BAKUS.] 
CALIBRATION OE ELECTRICAL PYROMETERS. 
125 
is shown. A A is a block of lime into which two capillary holes have 
been drilled, just large enough to receive the wires fi and y of the 
thermo-couple. These are united above by a little button of platinum 
lying at the base of a spherical cavity in the block A A. B is an oxy- 
hydrogen blow-pipe by which the button is fused. A lid of calcined 
lime similar to the block A A in form but having a lateral outlet oppo- 
site B may be added. But the ignition is intense enough without this. 
In the following table are given the values for e 20 obtained by heating 
the button in the open hearth, Fig. 23. The thermo-couple is old No. 
18, the wires of which have been fused and drawn over again. 
Table 28. — Thermo-electric datum for temperatures above the melting point of platinum. 
No. 
, 
020. 
Time. 
Remarks. 
° G. 
Micro- 
volts. 
h. m. 
18 
20 
20400 
1 48 
First experiment. 
20 
20 
20000 
20400 
2 50 
3 00 
^Second experiment. Fresh 
) block of lime. 
20 
20000 
5 33 
yriiird experiment. Fresh 
20 
20400 
50 
) block of lime. 
The curiously constant electrical result for the temperature of the 
oxyhydrogen flame under the given circumstances is remarkable. It 
is interesting to note that when by any accident metallic connection is 
broken, there at once appear violent polarization disturbances. This 
shows that at the temperature of the OH 2 blow-pipe, lime is quite a 
good conductor of electricity, for it is less probable that under the 
given conditions conduction should take place through the hot gases. 
The thermal equivalent of the value of e in hand, however, is only 
1,600°, a datum certainly too small by 200° or more. This small value 
is significant. 1 It is in accordance with the small thermal datum f<»r 
the boiling point of zinc, calculated from thermo-electric data, which 
apply only for the interval 0° to 400° (cf. p. 116). Hence it appears that 
the equation e=ar-\-bz a is an approximation, and that in high-temper- 
ature work the term 
c(T 3 -V) 
can not be neglected. This is equivalent to saying that, apart from 
considerations involving the Thomson effect, the electro-motive force 
at each junction of the thermo-couple has virtually the form 
e=at+bt 2 +ct*+ .... 
and that the real nature of the function e is not known. It is probable, 
however, that for practical thermometry an equation with three con- 
stants will suffice for all attainable ranges of temperature. 
1 Cf. Introduction, pp. 49 and 50. 
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