168 Professor TyndalVs further Researches 



fig. 8, and thus may study the action of the ends m and p upon 

 the magnets. 



By means of the screw R the magnets can be raised or 

 lowered ; and at the end, t, of the screw is a small torsion circle 

 which can be turned independently of the screw ; by means of 

 the latter the suspending fibre can be twisted or untwisted with- 

 out altering the level of the magnets. 



The front of the box is attached by means of brass hasps, 

 and opposite to the mirror M a small plate of glass is introduced, 

 through which the mirror is observed ; the magnets within the 

 box being thus effectually protected from the disturbances of the 

 external air. A small handle to turn the wheel W accompanied 

 the instrument from its maker ; but in the experiments, I used, 

 instead of it, a key attached to the end of a rod 10 feet long; 

 with this rod in my right hand, and the telescope and scale 

 before me, the experiments were completely under my own con- 

 trol. Finally, the course of the cuiTcnt through the helices was 

 as follows : — Proceeding from the platinum pole of the battery 

 it entered the box along the wire w, fig. 2, which passed through 

 the bottom of the latter ; thence through the helix to H', return- 

 ing to E'; thence to the second helix, returning to E, from 

 which it passed along the wire w' to the zinc pole of the battery. 

 A commutator was introduced in the circuit, so that the direc- 

 tion of the current thus indicated could be reversed at pleasure. 



Experiments. — Deportment of Diamagnetic Bodies. 



A pair of cylinders of chemically pure bismuth, 3 inches long 

 and 0*7 of an inch in diameter, accompanied the instrument from 

 Germany. These were first tested, commencing with a battery 

 of one cell of Grove. Matters being as sketched in fig 2, when 

 the current circulated in the helices and the magnet had come 

 to rest, the cross wire of the telescope cut the number 482 on 

 the scale. Turning the wheel W so as to bring the cylinders 

 into the position fig. 1, the magnet moved promptly, and after 

 some oscillations took up a new position of equilibrium; the 

 cross wire of the telescope then cut the figure 468 on the scale. 

 Reversing the motion so as to place the cylinders again central, 

 the former position 482 was assumed ; and on turning further in 

 the same direction, so as to place the cylinders as in fig. 3, the 

 position of equilibrium of the magnet was at the number 493. 

 Hence by bringing the two ends n and o to bear upon the astatic 

 magnet, the motion was from greater to smaller numbers, the 

 position of rest being then fourteen divisions less than when the 

 bars were central. By bringing the ends m and p to bear upon 

 the magnet, the motion was from smaller to greater numbers, 



