ON" THE EFFECTIVE TEMPERATURE OF THE SUN. 
367 
in tbe experiment is not far from a straight line, if is a concave mirror fixed to the 
axis of rotation; it gives the image of a luminous slit on a straight scale, 3 metres 
away, and thus indicates an expansion of the strip, as already explained. A piece of 
stout copper wire, C. IF., is connected with the forceps, and dips into a mercury cup, 
M.C., by means of which a movable electric connexion is maintained with the 
remainder of the circuit. Sp. is a flat spiral spring, which is necessary to keep a 
slight tension on the strip. A water-jacket of gilded brass (shown in dotted lines) 
rests on the top of the slate block during an experiment; its shape is shown in 
fig. 2 a, which is a cross-section ; its length is a little greater than that of the strip. 
Fig. 2a. 
Section of Watei’-jacket. 
and in the middle of each of its long sides is a circular hole through either of which 
the heat of the incandescent platinum passes, the hole not in use being plugged up 
with a gilt brass cap. The water-jacket serves two purposes : one is that of pro¬ 
tecting the glowing platinum from air currents, which would otherwise tend to 
produce quick variations in its temperature; the other is that of preventing any 
radiation from the platinum except that which passes through the aperture into the 
radio-micrometer. 
Calibration of the Platinum Strip. 
The platinum was obtained from Messrs. Johnson, Matthey, and Co., Hatton 
Garden, London, who reduced it in thickness until a convenient current (25 amperes) 
from the accumulators was able to raise it to full incandescence. 
The calibration experiments were performed as follows :— 
The mirror connected with the strip was turned until the reflected spot of light 
occupied a convenient position on the scale, which stood at a distance of about 
3 metres, and was placed at right angles to the zero position of the index beam of 
light. A very small fragment of silver chloride (approximately of a milligramme 
in weight) was then placed on the platinum strip, near the middle of its length, and 
a low-power microscope was so held in a clamp that the fragment could be plainly 
seen through an aperture in the water-jacket. The melting point of AgCl is taken as 
451° C. (on the authority of Carnelley*), at which point the platinum was under a 
red heat, so that a candle had to be arranged to shine through an open end of the 
water-jacket, the gilt sides of which reflected the light so well on to the silver 
* Carnelley “ Melting and Boiling-points Tables.” 
