OJT THE EFFECTIVE TEMPERATURE OF THE SUN. 
3G9 
Melting substance. 
Temperature. 
Deflection from ;^ero. 
° C. 
1st strip. 
Ag.Cl. 
451 
18T 1 
t 
1st strip. 
Gold 
1041 
42-0 
r 
2nd strip. 
Ag.Cl. 
451 
26-41 
i 
2nd strip. 
Gold 
1041 
62-1 j 
r 
3rd strip. 
Ag.Cl. 
451 
‘24-2 
i 
3rd strip. 
Gold 
1041 
56-8 
r 
Note.—Violle gives the melting-point of gold as 1045° 0. Cali.endae, ‘Phil. Mag.,’ vol. 33, 1892, 
gives 1037° C. The mean, 1041° C., of these modern determinations cannot he far from the truth. 
The three lines thus given are shown in fig. 3. 
In the case of the 1st strip, a piece of palladium was also tried, the melting-point 
of which is given by Violle as 1500° C. ; a deflection of 61 was obtained on the 
scale, which falls exactly on the line given by the other two substances. * 
By means of the straight line, corresponding to the particular strip of platinum, 
therefore, the temperature of the latter may be known with a high degree of 
accuracy by reading the position of the spot of light on the tliermometer scale, 
on which 1 millim. corresponds to about 2° C. 
JoLY,* in his paper, refers to the possibility of a viscous extension of the platinum 
after being raised to high temperatures; we have proved that this does not take 
place in our experiments, by noticing that the spot of light returns exactly to zero 
very soon after the current is cut off, when the platinum has been for some 
15 seconds at a tempeiut ure of over 1500° C. 
The Differential Radio-micrometer. 
This instrument is a modification of the single form described by Professor 
BoYs.t The chief difference consists in a duplication of the circuits, both circuits 
being supported by the same fibre. The remaining changes consist in an alteration 
of the position of the magnets, &c., which for our purpose are more conveniently 
placed vertically instead of horizontally. It was constructed by Messrs. Yeates and 
Sons, Dublin, and the double circuit by Mr. W. Watson, B.Sc., of the Royal College 
of Science, London. 
The instrument is shown in elevation in fig. 4, on a scale of about while the 
circuit is shovm about ^ size on the right of the figure, where R, R! are the two receiving 
surfaces of blackened copper foil, attached to which are the bars of the alloys. 
The two pairs of bars are connected by a circuit of fine cojiper wire, and the whole 
system is supported by a hoop [IT] of similar wire (from which, of course, it is 
* Jolt, ‘ Proc. Roy. Irish Acad.,’ 1891, 3rd series, vol. 2, p. 61. 
t C. V. Rots, ‘Phil. Trans.,’ vol. 180, 1889, A., p. 159. 
MDCCCXCIV.—A. 3 B 
