METHOD OF INDUCTION. 625 



to 27rI~S. By determining the weight P necessary to tear them 

 asunder, or to prevent them from touching, we have 



or 



If the surface S made an angle with the direction of the 

 magnetisation, we should have 



g? = 271-I2S sin 6 . 



1198. METHOD OF INDUCTION. The flow of induction Q 

 (1194), which traverses a circuit surrounding a magnet, may be 

 written 



denoting by F the mean force on the surface S of the circuit, and 

 by A the mean component of magnetisation in the corresponding 

 section S' of the magnet. The value of Q would be obtained either 

 by the discharge due to the sudden suppression, or the establishment 

 of the magnetisation, or by that produced when the circuit is trans- 

 ferred to a great distance, or to a position for which the flow of 

 induction which traverses it is null. 



The magnetising force F is due solely to the free magnetism of 

 the magnet, or to the free ends of the magnetic filaments which 

 compose it. The value of the term FS is a minimum when the 

 circuit tightly encloses the magnet. If this is in the shape of a 

 very elongated cylinder, and the circuit is placed near the median 

 region, the force F is virtually null, and we have 



This equation is strictly rigorous even for an annular magnet 

 forming a closed solenoid (371). 



1199. The sudden changes of magnetism necessary for the ap- 

 plication of this method can only be easily produced by currents. 



Suppose, for example, that a cylindrical bar of section S', and of 

 great length in comparison with its diameter, is placed in the uniform 

 field produced by a current I ; for example, the field 473-^! of a 

 cylindrical coil (495) which is very long compared with the bar, 

 or more generally in the field GI of a frame of uniform field. If 

 the action of the ends of the bar itself in the central section of the 



VOL. II. S S 



