METHOD OF INDUCTION. 62! 



This field may be evaluated by any other method for instance, by 

 oscillations or deflections. 



If the magnet is placed so that its field on a magnet is parallel 

 to the terrestrial field, the oscillations of the needle (1140) are 

 determined in the terrestrial field alone, then in a field formed 

 by the sum or difference of the terrestrial field and of the field 

 of the magnet. 



We may also work by deflection, by making the magnet act on a 

 declinometer placed in a transverse position by a bifilar suspension 

 (1142), or by a combination of oscillations and deflections. 



We shall thus determine either the ratio of the field of a magnet 

 to the terrestrial field or the ratio of the fields of two magnets. 

 Allowing for the distance, and the term of correction if there is 

 one, we shall deduce from it either the ratio of the magnetic mo- 

 ment Mj to the terrestrial field or the ratio of the two magnetic 

 moments M x and M 2 by the ratio of the corresponding fields 

 F l and F 2 . 



If the field of the magnet is perpendicular to the magnetic 

 meridian, it is made in like manner to act on a needle situated 

 just in the meridian, and the deflection is measured as in Gauss' 

 method (1174). We may also observe the oscillations of a de- 

 clinometer situate in a transverse position, and subject first to the 

 resultant field of the earth and the bifilar, then to the sum or the 

 difference of the actions of this field and of a magnet. 



1194. METHOD OF INDUCTION. Suppose that a magnetised 

 body of any given form is surrounded by a closed circuit in the 

 plane perpendicular to the x axis. As the component of the 

 magnetic induction perpendicular to this plane is X 1} the flow 

 of induction across the circuit is (324) 



Q= f (^dydz = - f ^-dydz + 4-rr f \ 



the integral being extended to the entire face S of the circuit. 



If the magnetisation is suddenly suppressed, the circuit will be 

 the seat of an induced discharge which will give the value of Q. 

 The discharge will be the same if the circuit is carried to a great 

 distance from the magnet, or in a position for which the flow 

 across it is null. 



When the circuit closely encloses the surface of the magnet, 

 the discharge gives the internal flow of induction in the corres- 

 ponding section. 



