ON THE EFFECTIVE TEMPERATURE OF THE SUN. 
363 
I. General Method and Instruments. 
The general idea in this investigation was to endeavour to balance the heat of the 
sun by means of an artificial source of heat at a high known temperature, thus 
obtaining both directness and simplicity as far as possible. The artificial source of 
heat was a strip of platinum heated by an electric current; this strip formed part of 
a modified form of Joey’s Meldometer, which is described below, and its temperature 
could be determined at any moment with a high order of accuracy. 
The radiation from a known area of the incandescent strip was balanced against 
that coming from the sun in a differential radio-micrometer—a modified form of 
Professor Boys’s well-known and excessively delicate instrument. 
The essential theory of the method was extremely simple. Knowing the apparent 
areas of the sun and the artificial source of heat (the latter, of course, being much the 
greater), and knowing the law connecting radiation and temperature, we can at once 
find to what point the latter would have to be raised to balance the sun, if these 
apparent areas were made equal. But this would be the required effective tempera¬ 
ture of the sun, if the emissive powers were equal, and both bodies could radiate 
directly and without intervening absorption on to the receiving surface of the radio¬ 
micrometer. 
This extreme simplicity, however, cannot be obtained, and correcting factors have 
to be applied for— 
(а) Emissive power of the platinum strip ; 
( б ) Beflecting power of the glass in the heliostat, which keeps the beam of 
sunshine in the required position ; 
(c) Terrestrial atmospheric absorption. 
Each of these will be discussed in turn, after the instruments used have been 
described. 
The general arrangement of the apparatus is shown in fig. 1 . 
H is the heliostat, which is placed on a window sill outside the laboratory, about 
4 metres from the radio-micrometer B, aiid the meldometer M. The two latter 
instruments are supported on a table which stands on a concrete pier passing through 
the floor of the room. 
is the scale of the meldometer, the distance from to M being about 3 metres. 
S 3 is the scale of the radio-micrometer, and and L 3 are the lamps, corresponding to 
the two instruments. C is a variable carbon resistance ; 7 ' is a platinoid coil; and 
the platinum strip in M are in circuit with 26 Epstein accumulator cells, by means of 
which the strip is heated to any desired temperature. 
In an experiment, a beam of sunlight is reflected on to the receiving surface of one 
circuit—say, the lower—of the radio-micrometer, and the heat from the platinum 
strip on to that of the higher; the two circuits are arranged so that, under these con¬ 
ditions, the two sources of heat produce turning moments in opposite senses, and 
3 A 2 
