542 PARTICULAR CASES OF INDUCTION. 



annular coil, we can at a given moment suddenly establish or 

 suppress a current of known strength I in the magnetising coil. 



The expression , which in the induced current corresponds 

 n 



to the make or break of the principal current, represents the total 

 flow LI of magnetic induction which traverses one of the turns. 

 We could then determine experimentally the value of L, and 

 from it deduce the coefficient of magnetisation k, particularly by 

 formulae (54) and (57). This is the principle of the method 

 recently employed by Prof. Rowland. 



560. WEBER'S HYPOTHESIS ON MAGNETISM AND DIAMAGNETISM. 

 We have seen above how Ampere explains magnetism by 

 molecular currents; we may now examine the physical properties 

 of these currents. 



Consider one of the currents defined by the values L, I, and R, 

 and let Q be the flow of external force in its contour; as the 

 electromotive force is null, equation (10) of (518) becomes 



- 

 dt 



We must also assume that the resistance is zero ; it follows that 

 (64) LI + Q = const = LI . 



The strength I is that of the current which would traverse 

 the circuit in question, if the external flow of force were zero. If 

 we suppose I = (that is, the molecular current originally null), 

 which would correspond to the case of a magnetic medium in 

 the neutral state, we have finally 



LI--Q. 



The current induced in the molecule by an external field 

 produces therefore a flow of force contrary in sign to Q. In 

 other words, the magnetisation equivalent to the current is of 

 opposite sign to the magnetising force; the magnetisation of 

 magnetic bodies cannot therefore be explained solely by currents 

 induced in the molecules of the medium. 



561. Such currents can, on the contrary, account for diamagnetic 

 phenomena. Weber's hypothesis assumes that in each molecule of 

 a diamagnetic medium there are channels along which currents may 

 circulate without resistance. If these channels were in all directions, 

 the molecule would be a perfect conductor. With a linear current 



