
Heating Effect of the Radium Emanation. 205 
constant-temperature water bath, and remained very steady 
throughout the work. Readings were taken of the position 
of the xylene in the manometer-tube with the radium or 
emanation-tube in one flask, and then again when transferred 
to the other flask, allowing in each case ample time for the 
attainment of steady conditions. To calibrate the readings 
of the micrometer-scale directly in gram-calories per hour, 
two coils of manganin wire were constructed of about 
50 ohms each. One was made ina small compact shape of 
about the same volume as the 30 milligrams of radium bromide 
introduced, while the other was made of many turns of wire 
ona frame of approximately the same shape as the emanation- 
tube. Currents of known strength were sent through the 
heating-coils, and the corresponding differences of pressure 
observed on the manometer. Thus two calibration curves 
were obtained, which differed a little but represented as nearly 
as we could arrange the two sources of heat. Although the 
differential air-calorimeter did very well for the first obser- 
vations over the whole range of several weeks, and was very 
suitable for measuring the quantity of heat emitted, on 
account of its large mass and the volume of air affected it 
did not respond quickly enough to obtain the important 
initial changes. Two pairs of sensitive inner-coil platinum 
thermometers were therefore constructed, having a lag of not 
more than 6 or 7 minutes. The two pairs enabled us to work 
with the radium and the emanation-tube at the same time, 
which was important for the first changes. 
Hach thermometer consisted of 35 cms. of fine platinum 
wire wound carefully on the inside of a-thin glass tube 5 mms. 
in diameter. To secure the wire, the tube was gently warmed 
to a low red heat until the wire stuck fast to the glass, forming 
a coil about 3cms. long. The ends of the coil were gold 
soldered to heavy platinum-wire leads. The tube containing 
the radium, as well as the tube containing the emanation, was 
selected so as to slide easily into the interior of the coil, the 
wire thus being in direct contact with the glass envelope 
containing the source of heat. 
Each thermometer was fitted centrally in a larger glass 
tube, passing through a long narrow water-bath (fig. 2). 
A stirrer in the water-bath kept the temperature always 
uniform. The changes in the resistance of the thermometers 
when the radium or emanation-tube was transferred from one 
to the other were measured on a Callendar type of compen- 
sated resistance-box constructed with great care. This box 
. vo) . 
has already been described by one of us* in another place. 
* H.'T. Barnes, Phil. Trans. vol. excix. p. 185 (1902). 
