JANUARY 15, 1914] 
Optical and Radiation Pyrometers, 
The temperatures reached in many modern manu- 
facturing processes are so high that the temperature 
can only be measured by optical or radiation methods. 
It was not, however, until Le Chatelier*® invented 
his optical pyrometer in 1892 that any really satis- 
factory attempt was made to determine the tempera- 
ture of a hot body by measuring the radiations 
emitted by it. 
The intensity of the light emitted by a hot body 
Fic. 3.—Diagram of Féry absorption pyrometer. 
varies immensely with the temperature,'’ and there- 
fore, at the first glance, one would assume that the 
easiest way to measure a temperature would be to 
compare photometrically the light emitted by the hot | 
body with that emitted by a second hot body at a 
definite temperature. This would be the simplest way 
of doing so, if all bodies at the same temperature 
emitted the same amount of light, but unfortunately 
such is not the case, the light, for example, from 
incandescent iron and car- 
hon is much greater than 
that of porcelain’ or 
platinum at the same tem- 
perature. 
Kirchoff first propounded 
the idea of a “ black-body 
as being a body which 
would absorb all radiations 
falling upon it, and would 
neither reflect nor transmit 
any. He also showed that 
the radiation from such a 
black-body is a function of 
the temperature alone, and 
was identical with the 
radiation inside an enclo- 
sure, all parts of which are 
at the same temperature. 
All substances, if they are 
heated inside a black-body, 
will emit the same radia- 
tion, and if looked at 
through a small opening in 
the furnace will appear of 
uniform brightness. Stefan 
was the first to state that 
the energy radiated was 
proportional to the fourth power of the absolute tem- 
perature. Boltzmann later deduced the same law 
from thermodynamic reasoning. This law has since 
become known as the Stefan-Boltzmann radiation 
law, and may be stated as follows :— 
On the Measurement of High Temperatures,”” H. Le Chatelier 
(Comptes rendus, vol. cxiv., pp. 214-216, 1892). 
11 If the intensity of red light A=o"656u emitted by a hot body at rooo* C. 
is called 1, at 2000° C. the incensity wili be 2100 times as great (see C. W. 
Waidner and G. K. Burgess, ** Optical Pyrometry,” Bulletin No. 2 of the 
Bureau of Standards). 
NO. 2307, VOL. 92] 
SECTION ON AC 
Fic. 
NATURE 
= Prece 
Reb GLass 
71 
oi 
The total energy radiated by a black-body is pro- 
portional to the fourth power of the absolute tem- 
perature, or E=o(T*—T,*), where E is the total 
energy radiated by the body at absolute temperature 
T to the surroundings at absolute temperature T, and 
a is a constant depending on the units used.'” 
This law has received ample experimental verifica- 
tion throughout the range over which temperature 
measurements can be made. 
As previously mentioned, the first satisfactory 
radiation pyrometer was that designed by Le Chate- 
lier. 
The instrument is really a form of photometer, in 
that it is arranged to match the luminous radiation 
obtained from an incandescent body against that ob- 
tained from a standard lamp. This instrument in the 
form modified by Féry is illustrated in Fig. 3. It 
consists of a telescope DB, which carries a small 
comparison lamp E attached laterally. The image of 
the flame of this lamp is projected on to a mirror, 
F, placed at 45° to the axis of the telescope, the 
mirror being silvered only over a narrow vertical 
strip. The telescope is focussed on the object the 
temperature of which it is desired to measure, the 
object being viewed on either side of the silvered 
strip. A coloured glass in the eyepiece ensures mono- 
chromatic conditions. A pair of absorbing-glass 
wedges, C and C,, are placed in front of the objective 
of the telescope, and these wedges are moved later- 
ally by means of a screw until the light from the 
object under observation appears of equal brightness 
to that emitted by the standard lamp. A table pro- 
vided with the instruments converts the readings 
obtained by the scale into degrees centigrade. 
The MHolborn-Kurlbaum pyrometer is shown 
diagrammatically in Fig. 4; it is a photometric in- 
= 
concn 
Ls 
45 Mirror 
AsBsSorBINC SCREEN 
Sunt 
Micur Ammeter 
RHEOSTAT 
—The Holborn and Kurlbaum optical pyrometer, 
strument of rather a_ different character. <A 
small incandescent lamp L is mounted in the 
focal plane of the objective and eyepiece of 
a telescope. The lamp circuit is provided with 
a battery, rheostat, and sensitive ammeter. The 
12 The laws of radiation and the various forms of optical and radiation 
pyrometers are fully discussed in Dr. Burgess’ book, foc. cit. Two other 
good résumés will be found —(1) ‘‘ Optical Pyrometry,” C. W. Waidner and 
G. K. Burgers (Bulletin No. 2 of the Bureau of Standards ; and (2) ** The 
Black Body and the Measurement of Extreme Temperatures,’ A. L. Day 
and C. E, van Ostrand (Astrophysical Journal, vol. xix., 1-40). 
