302 
compensator as shown in Fig I. It consists of a fine 
platinum wire A, which is partially immersed in mer- 
cury B. When the bulb is heated the mercury in the 
capillary tube expands and short circuits the platinum 
loop, thus diminishing the resistance of the circuit. 
This balances a change in e.m.f., due to a rise of 
temperature of the cold junction. 
All the contacts of the different parts of the cir- 
Fig. 1. cuit should be carefully made, and wherever possible 
this should be done by soldering. The hot junction of 
the wires used in the couple should be fused together. For easily fusible 
metals, such as copper, this can be done in the Bunsen flame, but for 
platinum oxygen is required. Platinum may also be fused in the electric 
are. At the cold junction the lead wires should be soldered to the thermo- 
element wires. The wires composing the couple, which are subjected to 
high temperatures, should be insulated throughout their entire length by 
glass tubes or pipe stems. Asbestos thread may also be used for tempera- 
tures below 1300° C. Small fire clay tubes pierced by two holes may also 
be procured and are very convenient. For industrial work the couple 
should be inclosed by an iron or porcelain tube. The former should not 
be used for temperatures over 800° C. 
Radiation Pyrometry.—F rom the fact that the intensity of light emit- 
ted from a body increases very rapidly with rise of temperature the op- 
tical method is well adapted to the measurement of high temperatures. 
lor example, the luminous intensity of the red part of the light emitted 
by a body of 1500° C. is 130 times the intensity of 1000° C., and at 2000° C. 
it is more than 2100 times as great as at 1000° C. It thus appears that a 
comparatively rough measurement of the luminous intensity of an incan- 
descent body would give a pretty accurate measurement of its temperature. 
This conclusion, however, is modified by the fact that different bodies at 
the same temperature emit very different amounts of radiant energy. The 
radiating power of a body depends not only upon the temperature but also 
the composition and nature of the surface. In order that the radiation and 
optical methods can be used for comparison of temperatures it is necessary 
that the effect of differences of surfaces be eliminated. This can be done 
by reducing the radiation from all surfaces to the radiation that would 
occur from some ideal surface arbitrarily taken as a standard of com- 
parison. 
