Dr. C. Fromme on the Magnetism of Steel Bars. 191 



menced my experiments by submitting according to this me- 

 thod the ellipsoids to the action of the two components of the 

 terrestrial magnetic force. 



In reality, between the permanent magnetisms ascertained 

 before and after a series of observations no relation was shown 

 which could permit any conclusion to be drawn of any altera- 

 tion produced by the earth's magnetic force. 



I will only sketch the method very generally ; a detailed 

 description, with development of the formulae, will be found in 

 the above-mentioned treatise by Riecke. 



A helix is required, and connected with this a galvanometer 

 the astatic pair of needles of which have the longest possible 

 oscillation-period. The helix can be rotated about an axis per- 

 pendicular to its geometrical axis, and is placed with its axis 

 of rotation in the plane of the magnetic meridian and horizontal 

 or vertical when the vertical or horizontal component is made 

 use of. If the helix (the surface of the coils of which should 

 at first be perpendicular to the inducing component) is turned 

 quickly through 180°, a current is induced in it, which ex- 

 presses itself in the deflection of the needle of the galvanometer. 

 If by the multiplication method the deflection be strengthened, 

 the sine of half the resulting deflection-angle will be a measure 

 for the intensity of the inducing current. 



If now we fix an ellipsoid, with its principal axis coinciding 

 with the axis of the helix, in the centre of the latter, and again 

 rotate it 180°, to the current induced by the direct action of 

 the terrestrial force a second will now be added, arising from 

 the circumstance that the magnetism of position induced by the 

 terrestrial-magnetic component has reversed its relative posi- 

 tion to the ends of the helix. 



Since this does not hold good of the permanent magnetism, 

 if any, present, the latter remains perfectly inactive. In order 

 to obtain only the current induced by the reversal of the tem- 

 porary magnetic moment, it is therefore necessary to operate 

 once with the helix alone and then with the helix and inserted 

 ellipsoid. 



Let J and i denote respectively the currents which are ex- 

 cited mediately by the reversal of the magnetic moment, and 

 by direct induction, and a and /3 the angles of deflection of the 

 needle of the galvanometer ; then we have the relation 



Now 



J : i= sin ^ — sin -^ : sin —\ 



w ' 



