168 Dr. Tyiidall on Diamagnelism and Magnea-ystallic Action, 



diagonally opposite to each other, and through each a helix of 

 copper wire was introduced and wedged fast. Each helix con- 

 tained a core of soft iron, which was pushed so far forward that 

 a line parallel to the sides of the box through which the helices 

 entered, and bisecting the other two sides, was a quai-ter of an 

 inch distant from the interior end of each core. The distance 

 between the two interior ends was six inches, and in this space 

 a little beam of light wood was suspended. At the ends of the 

 beam two spoon-shaped hollows were worked out, in which a 

 pair of small balls could be conveniently laid. The beam rested 

 in a paper loop, which was attached to one end of a fine silver 

 wire. The wire passed upward through a glass tube nearly 

 three feet in length, and was connected at the top with a torsion 

 head. The tube was made fast in a stout j)late of glass, which 

 was laid upon the box like a lid, and thus protected the beam 

 from currents of air. A floor of Bristol board was fixed a little 

 below the level of the axes of the cores, the ' board ' being so cut 

 as to fit close to the helices : the two comers of the floor adja- 

 cent to the respective cores and diagonally opposite to each other 

 bore each a graduated quadrant. When the instrument was to 

 be used, two balls of the substance to be experimented with were 

 placed upon the spoon-shaped hollows of the beam and there 

 exactly balanced. The balance was established by pushing the 

 beam a little in the required direction through the paper loop in 

 which it loosely rested ; and to accomplish this with greater ease, 

 two square pieces were sawed out of the sides of the box, and two 

 others were exactly fitted into the space thus opened; these 

 pieces could be taken out at pleasure, and the hand introduced 

 without raising the lid. The torsion-head was arranged so that 

 when the beam bearing the balls came to rest, a thin glass fibre 

 attached to the beam pointed to zero on the graduated quadrant 

 underneath, while the index of the head pointed also to the zero 

 of the graduated circle above. A current was sent through the 

 helices in such a direction, that the poles which operated on the 

 balls were of opposite names — the balls were repelled. Pre- 

 serving the current constant, the index above was turned in a 

 direction opposed to the repulsion until the beam stood again at 

 zero. The torsion necessary to effect this is evidently the ex- 

 pression of the repulsive force exeited at this particular distance. 



Fig. 1. represents the appearance of the beam and helices 

 when looked down upon through the glass lid. Fig. 2. repre- 

 sents the beam and balls attached to the suspending wire. 



When the fibre pointed to zero, an interval of about y^th of an 

 inch usually separated the diamagnetic balls from the core ends.; 

 The intensity of the current was measured by a galvanometer of 

 tangents, and was varied by means of a rheostat. Always before 



