208 Mr. K. H. M. Bosanquet on 



A* 



where M is the whole difference of magnetic potential which 

 acts on the induction as it traverses the circuit once. 

 If C be the current in the coils, n the number of coils, 



since the point considered has gone once round each spire of 

 the coil. 



Put M = 1 = 4ttCw. Then, if we put n = 1, 



and the C.G.S. unit of current is 10 amperes; 



.*. C= j- amperes, =*8 ampere nearly. 



Hence the unit of magnetomotive force is that which acts on 

 a circuit singly linked with one spire of a current of 10/(47r) 

 amperes. Thus a soft-iron horseshoe with ends nearly meeting, 

 round which a wire carrying such a current is wound once, 

 would exhibit nearly the unit of magnetomotive force between 

 its poles. (See post, on broken circuits.) 



Example of a ring solenoid. — Let the length of the solenoid 

 round the axis be 100 centim., 



Current = 10/(47r) amperes, number of coils = 1000; 



/. M = 1000; 



— = 10 = «£)= intensity within the ring in air. 



The area of the section of the resistance comes in as a factor 

 on both sides. Strictly the unit resistance would be that of 

 1 centim. length of an air-cylinder whose cross section has an 

 area of 1 square centim. If we suppose the area of the sec- 

 tion of the air-space enclosed in the solenoid to have this value, 

 its radius would be I/n/tt centim. 



In general all the lines of force pass through some one sec- 

 tion, generally the equatorial section of a bar, so that the total 

 magnetic induction is the product of the magnetic induction 

 through unit area and the area of this section. It is usually 

 convenient to express the resistance in terms of the length of 

 a cylinder having the same sectional area. This area appears 

 ou both sides, and may be struck out. 



Suppose the above ring-shaped solenoid to be wound about 



