Forces by means of Hydrostatic Pressure. 449 



of Verdet*, with a similar electromagnet, on the electro- 

 magnetic rotation of the plane of polarization of light, the 

 constant magnetic field did not exceed 3500 C.G.S. units. 

 When the poles were brought nearer together, the magnetic 

 force of the magnetic field increased only slightly. 



The residual magnetism — the magnetic force of the mag- 

 netic field between the poles after interruption of the electric 

 current — was also measured and taken into account. This 

 residual magnetism is nearly independent of the strength of 

 current, and depends only on the distances between the pole- 

 surfaces, being greater the nearer the poles stand to each 

 other. The residual magnetism varied in the case of the 

 Ruhmkorff magnet between 15 and 540 CcGr.S. units; and in 

 the case of the Berlin electromagnet between 400 and 500 

 C.G.S. units. 



From the deflection of a declination-needle in the neigh- 

 bourhood of the electromagnet, we can determine only the 

 magnetic moment of the whole electromagnet, not the 

 magnetic force of different points of the space in the neigh- 

 bourhood of the pole-surfaces. Y\ r ith the Berlin electromagnet, 

 the squares of the magnetic forces of the magnetic field 

 between the pole-surfaces were nearly proportional to the 

 power | of the magnetic moments of the whole electromagnet. 

 The pole-surfaces of the Ruhmkorff electromagnet could 

 be placed either vertical or horizontal, so that the lines of 

 magnetic force of the magnetic field between the pole- 

 surfaces were either horizontal or vertical. 



A square trough of nickel-plated brass-plate, with vertical 

 sides of plate glass, and a round iron base of 150 millim. dia- 

 meter, was placed upon the lower horizontal pole-surface of 

 the Ruhmkorff electromagnet, the upper pole-plate brought 

 within a distance of 1*7 millim. or 3*3 millim., and the trough 

 was then filled with different magnetic fluids (sulphates and 

 chlorides of manganese, iron, cobalt, and nickel in water). Con- 

 tact of the pole-surfaces upon magnetization was prevented by 

 means of three pieces of plate glass or of brass of equal thickness. 

 A central opening in the upper pole-surface was connected 

 by means of a thin brass tube with a salphide-of-carbon 

 manometer, and a long india-rubber tube provided with a 

 tap, as in the experiments upon pressures produced by elec- 

 tricity f. A flat bubble of air was introduced by means of 

 the tap into the space between the horizontal pole-surfaces, so 

 as to be in contact w 7 ith both ; the tap was closed, and the 



* Verdet, (Euvres, i. pp. 119 & 120; Ann. de Chim. (3) lii. p. 129 &c. 

 t Quincke, Wied. Ann. xix. p. 719 and Taf. viii. %. 24 (1883) ; Phil. 

 Mag. xvi. p. 3. 



