Dr, TjTiclall on the Laws of Magnetism. 267 



below by a crosspiece of iron. Round the cylinders were coiled 

 360 feet of copper wire weighing nine pounds^ and upon the top 

 of the cylinders two finely polished parallelepipeds of iron were 

 laid, between which the crystal was suspended. In the present 

 case the crosspiece was removed, and the two cylinders were 

 tightly screwed together — an arrangement pi-ovided for in their 

 construction — and thus converted into a single powerful magnet, 

 9 inches long, 1-3 inch thick, surrounded by a helix containing 

 360 feet of copper wire. The magnet was made fast in a block of 

 wood, and set vertically upright under one end of the beam of a 

 fine balance ; from this end a ball of soft iron was suspended by a 

 copper wire ; the length of the latter was so arranged, that, when 

 stretched full, the ball resting on the end of the magnet, a whale- 

 bone index pointing to a suitably graduated arch showed that 

 the balance-beam was horizontal. From the other end of the 

 beam a scale-pan was suspended which held the weights. Before 

 the experiments, the weight of the ball and its attached wire was 

 exactly counterbalanced ; so that when the magnetism was ex- 

 cited, the attractive force exerted on the ball and measured by 

 the weights was a purely magnetic force. 



After a {ew experiments a slight modification of the above 

 arrangement was found necessaiy. The end of the iron core on 

 which the ball rested had a little cavity in its centre, which re- 

 sulted from its having been turned in a lathe. It being abso- 

 lutely necessary that the ball should rest exactly upon the centre, 

 one of the soft iron parallelepipeds before alluded to was placed 

 upon the end of the magnet. The two diagonals were drawn 

 upon one of its polished faces, and the sphere of soft iron always 

 rested upon the point of intersection. 



3. During the investigation, the battery, the magnet, and the 

 instrument used to measure the intensity of the current (Weber's 

 galvanometer of tangents), were in three different rooms. From 

 the poles of the battery two long bands of sheet copper ran side by 

 side, and passed thus mider the door into the room which con- 

 tained the magnet. After passing the door, they diverged ; one 

 went to the magnet, and the other to the galvanometer of tangents, 

 with one foot of which it was connected ; a similar band went from 

 the other foot direct to the magnet. These two feet were nothing 

 more than the extremities of a ring of copper bent backwards ; 

 the cylindrical bar of copper which composed the ring was |ths 

 of an inch thick, and formed a circle of a foot inner diameter; 

 this was fixed in a steady frame, and always stood vertical. 

 Coincident with the vertical diameter of the ring a thread of 

 coc(^on silk was sus])ended, which supported a small magnetic 

 needle; this swung horizontally in a circular box, the circum- 

 ference of which was divided into degrees, the centre of the 



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