204 Mr. W. Sturgeon on Electro-Magnets. 



By this arrangement the extremities of the wires which 

 encircle the globe will constantly be in connection with the 

 mercury in the two cups, in whatever position the poles may 

 be placed. The connections with the battery ai'e accom- 

 plished by two wires joining its plates, and the two portions of 

 mercury in the vessels c c. 



Fig. 4. 



If this apparatus be so placed, before the battery be attached, 

 that the needles stand at right angles to the vertical plane of 

 the frame, and the dip of the central needle is counterbalanced, 

 they will be arranged nearly parallel to each other, as their 

 distance prevents them being affected by the ball. 



If the axis joining the poles of the sphere be now brought 

 into a horizontal position, by turning the key k, which enters 

 the horizontal axle, the equator will be directly under the dip- 

 ping needle. With this position of the apparatus, make the 

 galvanic connections so that the pole of the sphere which faces 

 the north may assume north polarity, or that species of polarity 



which 



