Prof. Tyndall on the Polymagnet. 427 



and found very convenient. It is very similar to an instrument 

 devised by Prof. Reusch, but simpler and more easily constructed. 



Fig. 7 is a plan of tbe top of the glass case which surrounds 

 the polymagnet. At C a brass tube is cemented to the glass, 

 which is here perforated, and through the tube a rod passes fur- 

 nished at its lower extremity with a hook, to which is attached 

 a suspending fibre, ab is a horizontal brass cylinder capable of 

 being turned on its axis by the milled head at a, and thus coil- 

 ing a suspending fibre around a groove marked at d : the cylinder 

 is also capable of sliding right and left, so that the body sus- 

 pended from tbe fibre may be moved laterally, and the amount 

 of motion measured on the graduated bar above*, a'b' is another 

 horizontal suspension rod, in every respect similar to the former. 



The whole instrument, surrounded by its glass case, is shown 

 in perspective in fig. 8. The magnets are visible, with the move- 

 able masses of soft iron resting upon them ; in the centre is seen 

 the helix sketched in fig. 2, and within the helix a bismuth bar 

 supported by several fibres of unspun silk attached to the central 

 rod which passes through the top of the glass case. The manner 

 of suspension of the bismuth will be understood from the draw- 

 ing, certain practical artifices which suggest themselves when 

 the drawing is attentively inspected, being introduced to facili- 

 tate the placing of the axis of the bar along the axis of the sur- 

 rounding helix. The current reversers are seen without the case ; 

 two opposite sides of the latter can be opened by the handles h 

 and hi, so that free and easy access to the interior is always 

 secured. 



• Experiments to be made with the Polymagnet. 



1. All the experiments that are usually made with an upright 

 electro-magnet. 



2. The various portions of the instrument may with great 

 facility be lifted separately out of the case. One of the electro- 

 magnets being thus removed, a rope can be passed through a 

 ring /•, introduced for this purpose into the cross-piece, fig. 1 : 

 adjacent to the screws g and h two plates of brass are seen ; 

 these are attached to the brass reels of the helices, and by pass- 

 ing a pin through the holes shown in the figure, the helices are 

 prevented from falling when the magnet is turned upside down. 

 Attaching the rope to a hook in the ceiling, or to a strong frame 

 made for the purpose, experiments on the lifting power of the 

 magnet may be made. 



3. While one of the magnets is suspended as last described, 

 the other, which is of exactly the same size, can be brought up 



* This arrangement, though very convenient for private research, is not 

 necessary for lecture experiments. 



