MEASUREMENT OF PERMEABILITY 



the process through which it has been carried. For if the current 

 in the reverse direction is further increased the curve does not 

 descend as it originally ascended from O, but is much steeper. 



The Ballistic method. This was first devised by Weber 

 and has since been used in many important researches. We 

 shall describe its application to a ring-shaped specimen of the 

 metal to be tested. Round the ring is wrapped a coil of wire A, 

 Fig. 208, and in the circuit is a battery, an amperemeter G x , an 

 adjustable resistance B 1? and a commutator K. If r is the mean 



FIG. 208. 



radius of the ring and if the number of turns of the coil is N 

 per cm. length of the circle of radius r, a current A in the coil 

 gives a field of intensity H = 4?rN A /1 at the distance?-. We 

 shall suppose the ring so thin that H is constant across a 

 section, and of this value. Round one part of the ring a 

 secondary coil I of n turns is wrapped. In the circuit of this coil 

 is a ballistic galvanometer G 2 , an adjustable resistance B 2 , an 

 " earth coil" E, and a small coil D, which can be used to reduce 

 the swings of the needle G 2 . The earth coil is used to find the 

 total quantity of electricity passing through G 2 for a given throw. 

 SuppoM. 1 that E is laid on a horizontal table. If the sum of the 

 areas of its turns is S and the intensity of the earth's vertical 

 field is V, the total fiux of induction through it is VS. If E is 

 suddenly turned over through 180, the change in induction is 

 X \ S, and if R is the total resistance of the circuit the total flow 



2 V S 



of electricity is coulombs. 

 10 R 



