RADIATION TN ABSOLUTE MEASURE. 
449 
polished, and the other half coated with lampblack. I have also tried a wire 
extremely thinly coated, or rather stained in appearance, with platinum black. The 
same measured current passes through both halves of the wire, or, when I desire it, 
I can vary the amounts in the two halves by means of the electrodes e, e, e. By 
means of these electrodes, also, the differences of potentials at the extremities of the 
two halves are taken; and thus the energy expended in each part can be found, 
and the temperature ascertained at the same time : and with the glass envelope I am 
at present using, the condition of the wire and the light given off hy it, if any, can 
be observed. 
With this apparatus, and using a platinum wire No. 22 S.W.G., Off millim. in 
diameter, one half thinly coated with lampblack and the other half bright and polished, 
I find that, with a current passing through the wire which keeps the polished wire at 
a strong red heat approaching to whiteness, the blackened portion is scarcely rendered 
luminous, I find, also, that an. alteration in the surface of the wire, which, so far as 
appearance goes is very slight indeed, may yet give a very marked alteration in 
the emissive power of the surface. The merest staining of the platinum wire, by 
washing it over with solution of ammonio-chloride of platinum and then heating 
with the current, is sufficient to keep the stained half non-luminous when the polished 
half is heated to redness. 
In the ‘ Proceedings of the Boyal Society,’ No. 243, 1886, p. 207, Mr. Moetimer 
Evans, C.E., has given an account of some remarkable observations on the light¬ 
giving properties of polished and unpolished carbon filaments in incandescent lamps. 
With the help of Mr. Evans I have been enabled to repeat some of his experiments ; 
and the result has been a complete verification of his conclusions. In repeating 
these experiments careful measurement was made of the vacuum used: a point 
which was left doubtful in the original investigation. 
An Edison A-lamp was taken for the purpose of experimenting, being chosen on 
account of the remarkable dulness of the surface of the filaments used in the Edison 
lamps. The filament in these lamps has almost the appearance of a lampblacked 
surface. This lamp was opened and the filament re-mounted in a form more con¬ 
venient for the purpose, being somewhat shortened in the process. The filament 
was then placed in a bulb which was exhausted with the Sprenciel pump down to a 
point measured by the McLeod gauge ; all the necessary precautions for making 
a good incandescent lamp being strictly attended to. The lamp was sealed off 
from the pump, and was tested against a suitable Swan lamp, which was kept at 
moderate incandescence, as it is not desirable to incandesce Edison filaments very 
highly. The lamps were regulated, with resistances and a rheostat, in such a way 
that the brightnesses of the two filaments matched and the energy (current and 
potential) required in each lamp was determined. The Swan lamp using a definite 
amount of energy was taken as the standard to which return was to be made in 
subsequent trials. The candle-power of each lamp was also observed. 
MDCCCLXXXVII.— A. 3 M 
