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Article V. 

 Investigations on Radiant Heat. By H. Knoblauch*. 



[From Poggendoi'ff's Annalen der Phyxik, &c. for January and March 1847.] 



Description of the Apparatus. 



IN my investigations I made use of a thermo-multiplier, an 

 instrument which has been brought to such wonderful perfection 

 by Pecquerel, NobiU and Melloni, that in experiments on radia- 

 tion an unquaUfied preference must be conceded to it beyond all 

 other thermoscopes. 



The accuracy of this apparatus, upon which, with its great 

 delicacy, its peculiar value depends, is owing to the following 

 circumstances : — 



1. That, on account of the coating of the pile with lamp- 

 black, it is equally susceptible of every kind of calorific rays. 



2. That, after reduction of the deviations of the needle of the 

 galvanometer to degrees of electric force, its indications may be 

 regarded as measures of the heat received by radiation, because 

 the intensity of the electric current excited in the pile by the 

 difference of temperature, is proportionate, within the limits of 

 these experiments, to this difference of temperature. 



The truth of the first position has been placed beyond doubt 

 by the investigations of Melloni, and that of the second by those 

 of both Becquerel and Melloni. 



The Thermal Pile which I made use of consists of twenty- 

 five pairs of bars of bismuth and antimony, each of which is 35*5 

 millim. long, 2*3 miUim. broad, and 1*5 millim. thick. They are 

 carefully isolated as far as the part where they are soldered, and 

 cemented into a brass ring, beyond which they project 5*5 

 millim., forming five series, each consisting of five pairs. Their 

 extremities are sloped off, so that the anterior surface of each pair 

 forms a right angle, its sides being 2'1 millim. and I'O millim. 

 in length. The surfaces of both sides of the pile are exactly the 

 same, and are coated with lamp-black of uniform thickness. 



The side next the source of heat is furnished with a polished 

 metallic cylinder 30 millim. in diameter and 60'9 millim. in 



* Translated by J. W. Griffith, M.D. 



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