436 



Prof* Cummwg OH Tlieiinomagiietic Rotation, [Dec. 



Article VIII. 



On Thermomagnetic Rotation. By the Rev. J. Gumming, MA. 

 Professor of Chemistry in the University of Cambridge. 



(To the Editor of the Annals of Philosophi/.) 



MY DEAR SIR, Cambridge, Nov. 18, 1823. 



In the Annals of Philosophi/ for Seiptemher, you did me the 

 favour to insert a notice of two instruments for exhibiting the 

 rotation of wires by thermoelectricity ; the magnet being apphed 

 externally in the one, and internally in the other. 



The parallelogram of silver and platina to which the magnet 

 was applied externally, was attached to an agate cap, and the 

 whole poised on the point of a long needle, in which case a 

 counterpoise was obviously necessary. I have since found it 

 more convenient to bend the parallelogram into the form of a 

 semicircle, having the agate cap nearer to the wire than the 

 centre of the circle. 



A lamp and magnet being placed opposite to each other are 

 sufficient to produce rotation ; but the effect is improved by 

 adding another magnet at 90° from the first, having its poles in 

 the contrary direction, and being connected with it by a bar of 

 soft iron placed beneath them. With this arrangement, the 

 rotation will be from right to left, or from left to right, according 

 to the position of the lamp. 



A B, platina ; B C F D A, silver ; E, 

 agate cap. 



The second magnet is placed near F G, 

 having its N end upwards. 



If the lamp be applied beneath B, the 

 rotation is in the direction B G A ; but if 

 it be opposite to F G, the rotation is 

 A G B. The annexed figure represents the 

 apparatus, which, exclusive of the agate 

 cap, weighs about four grains. 



PS. If six parts of bismuth, and one 

 of antimony in powder, mixed together, 

 and inclosed in a glass tube, be touched 

 by a hot wire connected with the galva- N 

 noscope, the deviation is first positive, 



and then negative, as I have before mentioned to be the case 

 with the alloy of these metals melted together in the same pro- 

 portions. I am, dear Sir, very truly yours, 



J. Gumming. 



