286 Mr. W. Daily on a Mode of 



11. Transformation from one form to the other, 



12. Further subdivision of the forms of energy 



13. Classification table. 



14. Distinction between energy and what was once called 



entropy. 



15. Distinction between available and unavailable energy; 



and between useful and useless work. 

 1(3. Reason why the energy of ordinary masses is available. 



17. .Reason why planetary energy is almost unavailable. 



18. Reasons why molecular energy is much of it unavailable ; 



and second law of thermodynamics. 

 10. Extent of availability of atomic and of electrical energy. 

 20. Dissipation of energy. 



XXXIV. A Mode of producing AragVs Rotation. 



By Walter Baily,' M.A* 



[Plates LX. & X.] 



A PAGO'S method of producing rotation in a copper disk 

 consists of suspending it by its centre so as to make it 

 lie horizontally above the poles of a horseshoe magnet, and 

 then rotating the magnet about a vertical axis. The rotation 

 of the disk is due to that of the magnetic field in which it is 

 suspended; and we should expect that if a similar motion of 

 the field could be produced by any other means, the result 

 would be a similar motion of the disk. 



Possibly the rotation of the magnet may be the only prac- 

 ticable way of producing a uniform rotation of the field ; but 

 it will be shown in this paper that the disk can be made to 

 rotate by an intermittent rotation of the field effected by means 

 of electromagnets. 



Suppose two magnetic poles to be below a plane sheet of a 

 conducting substance capable of moving in its own plane. 

 Each pole may be regarded as a small circular current parallel 

 to the disk. The currents will be in the same or different di- 

 rections according as the poles are of the same or different 

 names. We will examine the effect of a change in the strength 

 of either of the poles, in giving the sheet a tendency to move. 



There are four cases ; viz. — 



1. Poles alike. One increasing. 



2. „ „ One diminishing. 



3. Poles unlike. One increasing. 



4. „ „ One diminishing. 



* Communicated by the Physical Society, having been read at the 

 Meeting on June 28. 



