168 



Sir W. Siemens. 



[Apr, 26, 



tricians," vol. i, p. 123, and vol. iii, p. 297), and has since been verified 

 by Professor A. Weinhold in the case of platinnm from 100° to 

 1000° C. (" Annalen der Physik und Chemie," 1873, p. 225). 



The apparatus which I propose for determining the dependence of 

 radiation on temperature consists of a platinum or other wire, 0*76 

 millim. in diameter, suspended between two binding screws, marked 

 (A) and (B) on the diagram, carried on two suitable wooden stands. 

 The binding screws are connected through an electro-dynamometer 

 (D), for the purpose of measuring the current, to a secondary battery, 



Diagram showing arrangement of experiment. 



the number of cells in which can be varied. A high resistance 

 galvanometer (G) is also inserted between the binding screws as a 

 shunt to the platinum wire. 



The electro-dynamometer is of the ordinary form, in which the 

 current passes through a fixed coil, and a movable coil consisting of a 

 single twist, hung by a torsion spring in a vertical plane at right 

 angles to the plane of the fixed coil. The couple due to the current 

 is balanced by the torsion of the spring, hence the angle of torsion is 

 proportional to the square of the current. The current through the 

 high resistance galvanometer being a measure of the difference of 

 potential between the extremities of the platinum wire, the reading 

 of the galvanometer, divided by the main current as determined by 



