DISCOURSE OF W. B. TAYLOR: NOTES. 393 



Stimulated by Joule's successes, several attempts were made by 

 others, embodying the same principle of narrow but greatly ex- 

 tended poles. Mr. Richard Roberts constructed what may be called 

 a " disk " magnet, the square plate of iron being nearly two and a 

 half inches thick, with a planed face six and five-eighths inches on 

 the sides, and having a supporting eye formed on its back. Four 

 equidistant parallel grooves each three-eighths of an inch wide and 

 one inch and a quarter deep, divided the square face into five equal 

 oblong "poles." A bundle of 36 copper wires (No. 18) was coiled 

 in and out about these five poles, in three turns. The magnet with 

 its coils weighed 35 pounds. The armature, a similar square plate 

 one inch and a half thick, (without grooves,) weighed 23 pounds. 

 With a battery of eight pairs, (each about 100 square inches, or 

 five-sevenths of a square foot,) the magnet sustained 2,950 pounds; 

 about one ton and a third.* This magnet is obviously equivalent 

 to two or more of Joule's, placed side by side. Mr. Joseph Rad- 

 ford, about the same time, devised another form of "disk" magnet 

 much more novel in construction. In this case a circular plate 9 

 inches in diameter and about an inch thick, (provided with a sup- 

 porting eye at the middle of its back,) had a spiral groove cut in 

 its planed face, one-quarter of an inch wide and three-eighths of 

 an inch deep, making from the center about six turns, and leaving 

 a spiral ridge of metal at the face about half an inch thick. Its 

 weight (without wire) was 16 pounds 2 ounces, or with the wire 

 coil 18 pounds 4 ounces. The armature, a similar smooth disk of 

 about two-thirds the thickness of the magnet, weighed 14 pounds 

 14 ounces. The coil, a bundle of 23 small copper wires entering 

 from the back through a hole at the center of the disk and follow- 

 ing the spiral groove, (which it filled,) passed out at the edge of the 

 disk. By this singular disposition of the coil, the single spiral 

 "pole" or narrow ridge (half an inch in thickness) had a continu- 

 ous north polarity on the one side and a continuous adjacent south 

 polarity on its other side: being in the same condition as a long 

 narrow bar of soft iron having a galvanic current passing longitu- 

 dinally along its opposite sides in the same direction. With a bat- 

 tery of twelve pairs this spiral disk magnet sustained 2,500 pounds ; 

 about one ton and one-eighth, f 



Another variety of the disk magnet devised by Joule, presented 

 an annular face of about 12 inches exterior diameter and about 8 

 inches interior diameter, having 48 radial grooves separating 48 

 radial poles. A bundle of 16 copper wires bent alternately in and 

 out about these 48 lateral ridges or face cogs, produced a series of 



* Sturgeon's Annals of Electricity, Feb. 1841, vol. vi. pp. 167, 168. 

 t Sturgeon's Annals of Electricity, March, 1841, vol. vi. p. 231. 



