TllE CANTOR LECTURE, 1890. 
51 
Literary and Philosophical Society of Manchester, but their most acces¬ 
sible form is the re-published volume issued five years ago by the 
Physical Society of London. 
In August, 1840, he wrote to the “ Annals of Electricity ” on electro¬ 
magnetic forces, dealing chiefly with some special electro-magnets for 
traction. Both the magnet and the iron keeper were furnished with 
eye-holes for the purpose of suspension and measurement of the force 
requisite to detach the keeper. 
His experiments were followed by some to ascertain the effect of the 
length of the iron of the magnet, which he considered, at least in those 
cases where the degree of magnetisation is considerably below the 
point of saturation, to offer a decidedly proportional resistance to 
magnetisation ; a view, the justice of which is now, after fifty years, 
amply confirmed. 
In November of the same year further experiments in the same 
direction were published. A tube of iron, spirally made and welded, 
was prepared, planed down as in the preceding case, and fitted to a 
similarly prepared armature. The hollow cylinder thus formed was 
2 feet in length, in internal diameter was 1*42 inches, its internal being 
0*5 inch. The least sectional area was lOJ square inches. The excit¬ 
ing coil consisted of a single copper rod, covered with tape, bent into 
a sort of S-shape. 'This was later replaced by a coil of-21 copper 
wires, each 1-25 inches in diameter and 23 feet long, bound together 
by cotton tape. This magnet, excited by a battery of 16 of Sturgeon’s 
cast-iron cells, each 1 foot square and 1^ inch in interior width, 
arranged in a series of four, gave a lifting power of 2775 lbs. 
Joule’s work was well worthy of the master (Sturgeon) from whom 
he had learned his first lesson in electro-magnetism. He showed his 
devotion not only by writing descriptions of them for Sturgeon’s 
“ Annals,” but by exhibiting two of his electro-magnets at the Victoria 
Gallery of Practical Science, of which Sturgeon was director. Others, 
stimulated into activity by Joule’s example, proposed new forms, 
amongst them being two Manchester gentlemen, Mr. Radford and Mr. 
Richard Roberts, the latter being a well-known engineer and inventor. 
Mr. Radford’s electro-magnet consisted of a fiat iron disc, with deep 
spiral grooves cut in its face, in which were laid the insulated copper 
wires. The armature consisted of a plain iron disc of similar size. 
This form is described in Yol. IV. of Sturgeon’s Annals.” Mr. 
Roberts’s form of electro-magnet consisted of a rectangular iron block, 
having straight parallel groves cut across its face. This was described 
in Yol. YI. of Sturgeon’s “ Annals,” page 166. Its face was 6f inches 
square, and its thickness 2 T 7 g inches. It weighed, with the conducting 
wire, 35 lbs.; and the armature, of the same size and inches thick, 
weighed 23 lbs. The load sustained by this magnet was no less than 
2950 lbs. Roberts inferred that a magnet, if made of equal thickness, 
but 5 feet square, would sustain 100 tons weight. Some of Roberts’s 
apparatus is still preserved in the Museum of Peel Park, Manchester. 
On page 431 of the same volume of the “ Annals,” Joule described 
yet another form of electro-magnet, but which, in actual fact, was built 
up of 24 separate flat pieces of iron bolted to a circular brass ring. 
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