

[ 425 ] 



LIX. The Polymaynet. By Professor Tyndall, F.R.S. fyc. 

 [With a Plate.] 



THE polymagnet was devised for the purpose of exhibiting 

 before a class of pupils as many as possible of the phe- 

 nomena of electro-magnetism and diamagnetism. 



The instrument consists of an arrangement of two horseshoe 

 electro-magnets, a helix of covered copper wire disposed between 

 them, and suitable means of suspension. 



A section of one of the electro-magnets and its surrounding 

 spirals is given, fig. 1 , Plate IV. ab, cd are two cylindrical cores 

 of soft iron, which are united by a cross-piece of the same material, 

 ef. Through the cross-piece pass the strong screws y and h into 

 the cores, and by them the ends b and d of the cores, which are 

 accurately planed so as to ensure perfect contact with the cross- 

 piece, are attached to the latter. The diameter of the cores is 

 1"125 inch, and their distance apart, from centre to centre, 

 4*85 inches; the cross-piece e/is drawn in proportion. 



Round each core is a helix of copper wire, overspun with cotton, 

 which was afterwards saturated with shell-lac. In winding the 

 helix, two lengths of wire, one covered with red cotton and the 

 other with green, were laid side by side and coiled as a single 

 wire. The diameter of the wire is 0*1 of an inch, and the weight 

 of it which surrounds each limb of the magnet is 12 lbs. For 

 all four limbs, therefore, a weight of 48 lbs. is made use of. 



The second electro-magnet is in every respect similar to the 

 one just described. 



Fig. 2 is a front view of a helix of covered copper wire, in- 

 tended to be placed between the two electro-magnets ; it has an 

 internal diameter, ab, of 1 inch ; an external diameter, cd, of 8 

 inches, and measures along its axis 1*15 inch. The diameter of 

 its wire is 0*065 of an inch, and its weight is 6 lbs. ; it is wound 

 so as to form a double coil, as in the case of the electro-magnets. 

 The radial strips, and central and surrounding ring seen in the 

 figure, are of brass, and hold the coils of the helix compactly 

 together. 



Fig. 3 represents a stout slab of mahogany which supports 

 the apparatus, ab, cd are hollows cut in the slab to receive the 

 cro3s-pieces of the two electro-magnets ; from e to / the slab is 

 cut quite through, the cross-pieces merely resting on the portions 

 between / and b, f and d, &c. The small apertures at x, x' show 

 where the screws enter which attach the cross-piece to the slab 

 of wood. The central aperture at y shows where the pin y of 

 the helix, fig. 2, enters, the helix thus occupying the central 

 portion of the board. Right and left are two projections for the 

 reception of two current reverscrs, which will be described im- 



