72 Electro-magnetic Experiinents. # 



velope, if possible, something more of its physical nature. There 

 was an iron bar, of the best Scotch iron, 60 lb. in weight, 

 rounded by the hammer from the square. It was turned into 

 the form of a horse-shoe magnet, the arch at the top being a 

 semicircle, and the sides proceeding from it in right lines, till 

 their extremities approached to 3| inches distance between their 

 inner limbs. The diameter of the semicircular arch is 1 4 inches, 

 that of the iron itself 2 inches. On this was coiled copper-wire, 

 of l^hickness varying from a tenth of an inch, its general diame- 

 ter, to one-twelfth. There were 36 coils put on, each of such 

 a length that rather more than 900 feet of wire embraced the 

 iron. The coils were put on in three tiers ; the first tier en- 

 veloped about 14) inches in length of the iron from each extre- 

 mity, — the second tier covered the interval between the ends of 

 the first and the spring of the arch, — and the third covered the 

 arch. The first tier again consisted of 9 thicknesses of wire, 

 the second of 7, and the third of 4 ; so that the first contained 

 18 coils, the second 14, and the third 4. The total length of 

 wire in each coil was 10 yards, and the average length of each 

 embracing the iron 26 feet. To insulate the wire it was not 

 lapped ; the operator had no assistance, and that would have 

 been an endless labour : the following method was adopted. 



The iron was first lapped with varnished silk : on this was 

 coiled five lengths of naked wire, two forming the first tier, two 

 the second, and the remaining one embracing the arch : thus 

 the iron had one complete envelope of wire, and as the intervals 

 between the turns of the coils averaged one-quarter of an inch, 

 the wire was effectually insulated from contact with itself or with 

 the iron. The copper so put on was then overlaid with a double 

 thickness of varnished ribbon, varnished in every case after it 

 was put on; and a second thickness of copper was coiled on, so 

 that the wire fell into the interstices of the former coils. Affain, 

 a double fold of varnished silk was wrapped on, and the opera- 

 tion was thus conducted until there were 9 thicknesses of wire 

 on the first portion of the iron, 7 on the second, and 4 on the 

 arch. The wire was coiled on with great tightness, by means of 

 an instrument made for the purpose, similar to that with which 

 sailors sheath their ropes ; viz. a block of hard wood, with a 

 longitudinal channel adapted to the curved surface of the iron, 



