between Optical and Electrical Qualities of Metals. 175 
temperatures above 20° C.—is proportional to the difference 
of temperature between the black body and the thermopile. 
(This fact has also been expressed by M. Planck’s law of 
radiation *.) Consequently the radiation of the hot platinum- 
foil could always be referred to the radiation of an equally 
heated black body; that is tantamount to a direct deter- 
mination of the value (l00—R). The continuity of these 
experiments at high temperature with those at lower was 
secured by observing the galvanometer-deflexions with both 
arrangements of apparatus between 700° and 800° Celsius. 
By forming the ratio of two deflexions, corresponding to 
the two arrangements, the temperature of the source of 
radiation being the same, one obtains a reduction factor 
by which the deflexions observed at higher temperatures 
must be multiplied so that the results may become at once 
comparable with those observed at low temperatures. Table V. 
shows the results of our observations, after this calculation. 



TABLE V. 
| 
Temperature of | : 
the Platinum.-foil | es ae a 
‘Zin Celsius degrees. | ee ae 
prete.! 
ae ) ee | Strip of platinum in front of the 
1320 153 | narrow diaphragm C,. 4 surfaces 
1214 130 of fluorite. Measurement of tem- 
1100 | 109°5 |} perature with the optical pyro- 
976 | 89-7 ! meter. The black body gave at 
858 74:6 170° a (reduced) deflexion of 196 
762 / 63°6 a 
804 / 68:0 \ 
695 532 | 
614 45°4 | | Hollow case of platinum in front of 
493 / 32°2 ‘ the diaphragm ©,. 3 surfaces of 
404 24-4 fluorite. Measurement of tempe- 
oe / 18°2 | rature with the thermoelement. 
261 12-4 
169 6°5 ) 




By the ordinary electrical method of measurement, the 
specific conductivity of our platinum-foil was determined, 
ky=6°9D (specific resistance =0°154), the temperature-co- 
efficient amounting to 2=0°0024, between 18° and 65°. 
These numbers show that the platinum employed was not 
pure; but for our purpose this was of no importance. 
A very good agreement between the computed and the 
* M, Planck, Ann, d. Phys. iv. p. 553 (1901). 
