234 MEASUREMENT OF CURRENTS. 



evidently the position which should be chosen as standard ; it may 

 also be observed that it is the only one for which the sensitiveness of 

 the balance is not modified. 



If the magnetisation were constant, the action would be propor- 

 tional to the intensity of the current. This is the case with very 

 weak currents; but in general the magnetisation produces also a 

 temporary magnetisation, which is at first, for mean currents, pro- 

 portional to the intensity, and always gives an attractive action. 



The condition of equilibrium may then be expressed by an 

 equation of the form 



in which p is the weight, A and A' two constants to be determined 

 by experiment. 



This formula was used by Lenz and Jacobi,* who made some 

 modification in Becquerel's balance. Their apparatus comprised two 

 coils, and two magnets suspended to the ends of the same beam. 

 The two wires of suspension are unequal, and the two poles V being 

 downwards, one of the magnets is above the corresponding coil, and 

 the other below ; the action is one of repulsion. 



The proportionality of the temporary magnetisation to the in- 

 tensity of the current would be inadmissible for very powerful 

 currents, and the apparatus should then be graduated experi- 

 mentally. 



If the magnet is replaced by a mass of soft iron, the action, which 

 is always one of attraction, commences by being proportional to the 

 intensity of the current, and then increases less rapidly ; in this case 

 also an empirical graduation is necessary. 



The second arrangement is to be preferred for strong, and the 

 first for weak, currents. 



856. MOVABLE COILS. When a coil of surface S, is traversed 

 by a current I, its magnetic moment is SI. If the coil is attached to 

 a unifilar or bifilar suspension, so that, for the position of equilibrium, 

 its axis is perpendicular to the magnetic meridian, the passage of this 

 current tends to bring the axis into the meridian. We may again 

 either bring the coil to its original position by a convenient torsion of 

 the suspension, as in the method of torsion applied to magnets (830), 

 or, as in ordinary galvanometers, we may leave the apparatus to itself, 

 and observe the deflection which corresponds to the fresh position of 



* LENZ and JACOBI. Pogg. Ann., Vol. XLVII., p. 227. 1839. 



