30 REPORT— 1891. 



of another wire wound close to the former, and form a circuit 

 in which a baUistic galvanometer is placed. This will be called 

 the secondary circuit. 



If now, when a current, C, is flowing through the primary 

 current, it be suddenly reversed, the needle of the ballistic gal- 

 vanometer in the secondary circuit will suffer a momentary 

 deflection, which informs us that a certain quantity, Q, of elec- 

 tricity has passed as an instantaneous current through it. This 

 is due to the fact that, when the current is flowing in one direc- 

 tion through the primary, a certain number, N, of magnetic lines 

 of force pierce the single loop of the secondary, and that when the 

 primary current is reversed the same number of magnetic lines 

 pierce the secondary loop in the opposite direction, so that the 

 total change of lines of force due to reversing the primary cur- 

 rent is 2N, and the swing of the needle of the ballistic galva- 

 nometer is proportional to this change ; or, more exactly, if R 

 be the resistance of the secondary circuit, the total immber N 

 of lines of induction that passed through the single loop of it 

 when C was flowing in the primary may be defined as iEQ 

 (or if, as is necessary in practice, instead of having a single loop 

 we have a number iii, say then N = 2^^EQ), where Q is the total 

 current measured by the ballistic galvanometer. 



The induction B, in its modern signification, may be defined 

 as the number of these lines that pass through every square 

 centimetre of the loop, and the magnetizing force H of the 

 primary current as 47r x C X n, where n is the number of turns 

 of the primary wire per cm. length of the ring. The ratio of 

 the induction B to the magnetizing force H is called the mag- 

 netic permeability of the material of the ring, and is usually 

 represented by /x. 



So, then, H, the magnetizing force, is proportional to the 

 current in the primary, and B, the induction, to the momentary 

 deflection of the ballistic galvanometer in the secondary circuit 

 when the current in the primary is suddenly reversed. These 

 terms and their relations must be thoroughly understood before 

 any knowledge of any value about the modern theory of mag- 

 netic induction caii be at all grasped. 



If, with apparatus of the form I have described, we make 

 a series of experiments in the following way, with rings 

 of different materials, some very striking results will be 

 obtained. 



Begin with a very small current in the primary; reverse it, 

 and take the deflection of the galvanometer in the secondary ; 

 increase the primary current ; reverse it, and take again the 

 deflection of the galvanometer in the secondary ; and so on, 

 increasing the magnetizing current by steps. Plot out the 

 results on a diagram, as in Fig. I. fPl. I.), where the abscissae 



