70 
IOWA ACADEMY OF SCIENCE Vol. XXVIII, 1921 
a “black-body furnace” ^ maintained at a temperature which can 
be measured with a gas-thermometer. (The gas-scale has been 
extended by Day and Sossman to the melting points of gold and 
palladium which they found to be 1 335.6° K and 1822.5 respec- 
tively.) With the aid of Wien’s Equation, 
Bi^ = C^L—^e 
-~C,/LT, 
where is the energy of wave-length L radiated by a black-body 
at a temperature of T°K, and Cg are constants, and e is the 
base of Neper ian logarithms, the calibration can be extended to 
higher temperatures. 
Suppose some sort of an absorbing screen, which transmits a 
fraction t of the radiated energy, is placed between A and D, and 
that the body is at an unknown temperature vS'°K. Wien’s equa- 
tion for this body is 
= tC^ ^0- 
Also suppose that a black-body at a temperature of 5°K (within 
the range of the gas-thermometer) appears equally bright through 
the pyrometer when it is not viewed through the absorbing screen. 
Wien’s equation is 
Now since B = tBo we have 
Logg ilt = C^ {i/S — i/S^)/L 
in which So is the only unknown ; the present accepted value of Cg 
is 14320 micron degrees.^® Thus with this relation available the 
black-body furnace may .be maintained at a constant temperature 
' — the melting point of gold, for example — and any number of 
points for the current-temperature curve may be obtained by using 
absorbing screens of various transmissions; of course these points 
will all correspond to temperatures below 1 336.5 °K. After this 
part of the curve has been plotted, it can be extended in the man- 
ner described in the first part of this paragraph. 
In general a hot body does not emit as much energy as a black 
body at the same temperature nor is this fractional emission 
the same for all bodies. For example, a stick of carbon emits 
much more energy than a piece of polished platinum at the same 
temperature. In this case, if a deep hole of small diameter be 
present in the carbon stick and a similar one in the piece of plati- 
num, the holes will appear equally bright and will be as bright as 
a black-body at the same temperature. Suppose the platinum at a 
temperture T°K is emitting a fraction r of the energy that a black- 
