Mr. W. Sturgeon on Electro-Magnets. 203 



magnet and cross piece now operate on each other M'ith a 

 gradually increasing vigour; the poles of the one becoming 

 more and more energetic as the power of the opposing poles of 

 the other become further exalted. And this reciprocal increase 

 of action will be exercised on each other till a maximum of 

 mutual attraction is obtained, at which time the magnet will 

 support its greatest load. If more weight be added, the mutual 

 attractive forces of the magnet and cross piece will be over- 

 powei-ed, and the two will separate. The magnetism of the 

 iron no longer displays polarity, and that of the steel partially 

 recedes from its extremities ; but in consequence of the charac- 

 ter of the metal, a considerable polarity is still displayed by the 

 magnet : its energies, however, are very much diminished ; and 

 when the ci'oss piece is replaced at the poles, the effort to stimu- 

 late it to polarity is proportionably diminished. The reciprocal 

 attractions now operate with impaired forces, and consequently 

 the load which can be supported is much less than before. 



A compound steel magnet in my possession weighs 9 pounds, 

 which, when well magnetized will support 120 pounds; but 

 if weights be added till the cross piece falls off, the power is 

 reduced to about 75 pounds. 



Fig. 4. is a representation of an electro-magnetic sphere, 

 mounted on a mahogany frame, consisting of a stout base board 

 and two upright pillars, to the upper extremities of which is 

 fixed a cross piece or stage. On the centre of the stage, and 

 directly over the sphere, is placed a dipping needle ; and near 

 to the extremities are inserted the lower ends of two wires bent 

 twice at right angles, as seen in the figure; the upper extremities 

 of these wires are finely pointed, and support two horizontal 

 needles. 



The sphere NS isacast-ironshell, about 8 inches in diameter, 

 and weighs about 68 pounds. Its surface is divided into three 

 parts, which for convenience may be called tropical and polar, 

 having one of the former and two of the latter. Thetropical re- 

 gion is covered with four coils of copper wire in separate strata, 

 insulated from each other, and also from the metallic sphere, 

 by means of slips of silk; the exterior coil, being uncovered, is 

 seen in the figure. The coils are prevented from slipping to- 

 wards the poles by means of stout iron rings soldered to the 

 sphere, one at each tropical circle. 



'i'he two sets of extremities of the conducting wires are sol- 

 dered, one to each of the two copper discs or wheels bb, through 

 the centres of which the extremities of the horizontal axis of 

 the sphere pass. The lower edges of the wheels dip into por- 

 tions of mercury placed in two semicircular vessels cr, which 

 slide into the side of the pillars a little below the holes which 

 receive the axis of the sphere. 



2 D 2 By 



