78 Intelligence and Miscellaneous Articles. 



If these magnitudes x and y are determined, the calculation of 

 the force is best made on the supposition that all magnetism is 

 concentrated in the points in question, and comes out then very 

 simple. 



If the magnetic moment of a magnet is then accurately deter- 

 mined in absolute value, and can any time be accurately controlled, 

 then by means of magnetometrical methods all other magnetic 

 moments and all current-strengths may be easily reduced to them. 

 Magnetic moments alter indeed ; but with steel not quite recently 

 magnetized and well hardened, changes occur such as the earth's 

 magnetism experiences sometimes in five minutes. Hence we get 

 a far more certain absolute determination of current- strengths, 

 when we compare their magnetic strengths with the moment of a 

 well-hardened steel bar, than when we reduce them by means of the 

 tangent galvanometer to the terrestrial magnetism, which, from the 

 concealed masses of iron in our buildings, may show a quite 

 different value at a distance of a few feet from the place of obser- 

 vation. — Sitzungsberichte der AJcademie der Wissenschaften zu Berlin, 

 April 1883. 



ON AN INDUCTION MAGNETIC COMPASS. BY M. MASCAKT. 



Weber's inductometer enables us to determine the magnetic in- 

 clination by measuring the currents induced in a conducting frame, 

 which is first turned through an angle of 180° about a horizontal 

 axis starting from the horizontal plane, and then about a vertical 

 axis starting from a plane perpendicular to the magnetic meridian. 

 Such is the method in use in many observatories. 



If the apparatus is so arranged that the axis of rotation of the 

 frame is in any direction in the magnetic meridian, and if we seek 

 by trial for two directions such that for an angular rotation of 

 180° the needle of the galvanometer experiences equal impulses in 

 opposite directions, the line bisecting these two directions will give 

 the position of equilibrium of a dipping-needle. This second 

 method has recently been proposed by M. Wild. 



It is c^ar that the induced currents must be zero, when the 

 axis of rotation of the frame is exactly parallel to the direction of 

 the magnetic force. 



With the ordinary method of observation, which consists in 

 turning the frame through 180°, we obtain the same degree of 

 exactitude as by measuring the angles of throw ; but the null method 

 enables us to dispense with any measurement of the throw, as well 

 as any regulation of the initial and final positions of the movable 

 frame. We have in this the advantage that, if we are near the 

 wished-for direction, we may, by alternate movements of the frame 

 of the same period as that of the oscillations of the needle, multiply 

 the angles of throw, and thus demonstrate the existence of a deflec- 

 tion which in itself would have been inappreciable. The accuracy 

 of the method is thus considerably increased; we may then greatly 



