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XI. The Problem of the Behaviour of the Magnetic Field about 

 a Revolving Magnet. By S. Tolver Preston*. 



WHEN the earth revolves on its axis, does the magnetic 

 field (the earth being regarded as a magnet) revolve 

 with it? 



With the view to test this question of the behaviour of 

 the magnetic field about a revolving magnet, I have proposed 

 (communicated in an unpublished letter) the following ex- 

 periment whereby some phenomena are in question which 

 (without probably involving any novelty in point of theory) 

 might seem to merit a passing notice. I will briefly indicate 

 this here. 



Let m (fig. 1) represent a cylindrical bar-magnet : the 





Fig. 1. 







Fig. 2. 





Hypothesis (1). 



Hypothesis (2). 



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— 





yj + 



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cylindrical form may be taken for simplicity here. Let the 

 magnet be rotated about its vertical axis below the circular flat 

 plate or disk (of metal) p } seen edgeways in the diagram. 



Then under hypothesis (1), that the magnetic field remains 

 stationary or fixed while the magnet revolves (as Faraday 

 thoughtf he had proved), the magnet will revolve through its 

 own field, and therefore become charged up at its poles and 

 equatorial part by electricity of opposite sign, dependent on 

 the direction of rotation, as fig. 1 indicates. This magnetic 

 field (as it does not rotate) cannot electrodynamically influence 

 the disk p ; but — and here is the point I wish to call attention 

 to — this charge at the upper pole of the magnet can act by 

 static electric induction across the layer of air, and so produce 

 an opposite charge below the disk p. The disk is therefore 



* Communicated by the Author. 



+ Two papers by me in the Philosophical Magazine for February and 

 March 1885 may ba referred to in connexion with this. The present 

 paper, however, is a separate one in itself, or is original and unpublished. 



