DISCOURSE OF W. B. TAYLOR: — NOTES. 393 



Stimulated by Joule's successes, several attempts were mjulc 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, makuig 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 AnnaU of Electricity, Feb. 1841, vol. vl. pp. 167, 168. 

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



