506 PARTICULAR CASES OF INDUCTION. 



This quantity of electricity may be measured in absolute value 

 (506) by the throw of the needle of the galvanometer, which 

 would enable us to determine the intensity of the magnetic field. 



The direction of the current on Lenz's law, is that which ought 

 to traverse the current in its original position, in order that it may 

 be in stable equilibrium. 



529. DETERMINATION OF THE INCLINATION BY INDUCTION 

 CURRENTS. We have seen (487) that the tangent of the magnetic 

 inclination is equal to the ratio of the works which for the same 

 strength of current correspond to a rotation of 180 of the circuit, 

 starting from a position at right angles to the magnetic meridian : 

 ist, about a horizontal axis perpendicular to the meridian; 2nd, 

 about a vertical axis. This ratio is that of the electromotive 

 forces of induction for the same displacements, and therefore that 

 of the corresponding quantities of induced electricity (506) ; a 

 measurement of this latter ratio will therefore give the inclination. 



530. FARADAY'S Disc. A metal disc, movable in a uniform 

 field about an axis parallel to the direction of the field, forms part 

 of a circuit which communicates on the one hand with the axis of 

 rotation, and on the other with a spring which presses on a point 

 of the circumference. When the disc is put in uniform rotation, 

 a uniform current is also produced in the circuit. It will be seen 

 that the arrangement of this experiment is, as it were, the inverse 

 of that of Barlow (483). -If the plane of the disc is vertical, and 

 the force of the field F traverses it from front to back, and if 

 the direction of the rotation is that of the hands of a watch, the 

 induced current traverses the disc from the centre to the edge. 



If a is the radius of the disc, and w the angular velocity, the 

 electromotive force is 



,-^-- 2 F 



~ dt 2 



An analogous result is obtained, though with a less simple 

 calculation, by placing the disc between the poles of a horse-shoe 

 magnet or between the armatures of two electromagnets. In this 

 latter form, M. Le Roux obtained currents so strong that bright 

 sparks passed between the disc and the spring. 



Moreover, all the experiments, particularly those which were 

 examined in Chap. III., in which the motion of a conductor is 

 produced by electromagnetic or electrodynamic actions, would pro- 

 duce an inverse induction current if the motion was kept up by 

 an extraneous cause. 



