RADIATION IN ABSOLUTE MEASURE. 
439 
This method has been but little used recently. It was convenient at first on 
account of the battery power at my disposal, and on account of the current galvano¬ 
meter I was using. It also gave me useful preliminary information. 
2. A second method, and that which I have mainly employed, is to take the wire 
at a definite air-pressure—a vacuum—and to maintain this with the help of the pump 
if need be : # then to apply a measured current, increasing step by step from the 
lowest to the highest attainable with the battery at my disposal, or at high vacuums 
safe to use with the wire, and, as before, to find the temperature of the wire. A 
complete series of currents having been taken at one vacuum, a higher exhaustion was 
produced and a new series taken, and so on. The curves appended represent this kind 
of experimenting. 
3. Keeping the temperature of the wire constant (that is, its resistance constant), 
the Sprengel pump was gradually worked, and the current required to maintain the 
given temperature was measured. This method of experimenting has only become 
practicable to me recently. It requires a specially sensitive and readable current 
galvanometer. I propose to make considerable use of it in future, and meantime have 
employed it in the asymptotic experiment described below, p. 446. 
The curves, Plate 25, show emission of heat in gramme-water-degree-Centigrade 
units, per square centimetre of surface of the heated wire at different temperatures. 
The temperature of the outer case was always within 1° of 15° C. Each curve shows 
the loss at the air-pressure marked on it. On the axis of abscissas the temperature 
is given in degrees Centigrade ; while on the axis of ordinates, which shows 
emission, the scale unit is gramme-water-degree unit per squai’e centimetre per 
second. The pressures range from 50 mm. to about 00000675 mm. (about 009 M, 
where M stands for a pressure equal to the one-millionth of the atmospheric pressure). 
I cannot, however, say whether the whole or any part of the most outlying curve on 
the right corresponds precisely to this pressure. It is here that the difficulty enters 
to which I have already referred, viz., the giving off of gas by the wire. When gas 
is being given off by the wire, however small the quantity, it becomes impossible to 
interpret the observations of the McLeod gauge on account of the long time that is 
required for the pressure to equalise itself through the connecting tubes. For the 
same reason I have not been able to push the method of experimenting, in which a 
series of radiations at the same pressure but at different temperatures is determined, 
higher than about *09 M. 
In order, if possible, to get rid of this exhalation of air from the platinum wire—for 
such I take it to be—I have made many experiments in keeping the wire at a high 
temperature and under extreme vacuum for lengthened periods, but all to no purpose. 
Previous to the series of experiments represented by the most outlying two curves shown 
in Plate 25, the wire was kept for five days and nights with a current flowing through 
* This was necessary at high vacuums, when the wire became very hot, as will be explained further 
on, and required experience and caution. 
