ELECTRO-MAGNETISM. 



t ached, which ascends, and is inserted 

 into an opening made in the side of a 

 long quill, or a tube formed of portions 

 of quills inserted successively into each 

 other, and about six or seven inches long. 

 The wire, passing along the interior of 

 the quills, comes out at the end, and 

 being then wound round the outside of 

 the tube in a helix, along its whole 

 length, is made again to enter the quill 

 at the other end ; and proceeding back 

 along the axis, is brought out near the 

 middle, and made to descend till it meets 

 the other plate, to which it is soldered. 

 The whole apparatus is suspended at its 

 centre of gravity by a piece of untwisted 

 silk-thread. The plates being dipped 

 into dilute acid, while thus supported, 

 the galvanic action excited in them is 

 sufficient to render the helix magnetical. 



Heliacal Rotations. 



(110.) An ingenious mode of exempli- 

 fying the rotatory action of a magnet on 

 a conducting wire, when coiled into a 

 helix, was contrived by Mr.Watkins, and 

 described in his Popular Sketch of Elec- 

 tro-Magnetism*. The apparatus, re- 

 presented in/o-. 75, consists of a horse- 

 shoe magnet, h'rmly secured to a wooden 

 stand. Each of the poles of the magnet 



Fig. 75. . 



P. 73. 



is encircled by a heliacal coil of copper 

 wire, having a slender bar across its top, 

 with a needle point in its centre, turning 

 in a conical hole drilled in the end of the 

 magnet, with a small platina cup above 

 it, in order to hold a globule of mercury. 

 The lower end of each of these coils ter- 

 minates in slender, pointed wires, which 

 are soldered to them, and which are in- 

 tended to dip into mercury contained 

 in a wooden cistern below it, .fixed by 

 screws to the leg of the magnet. A wire 

 also proceeds from the lower part of 

 each cistern, and, being bent upwards, 

 terminates in a small cup, also capable 

 of holding mercury. A brass standard 

 rises from the basis of the apparatus, 

 having a forked piece attached to its 

 upper end, with two points descending 

 into the two platina cups upon the tops 

 of the coils ; and there is also another 

 cup placed at the top of the forked piece, 

 holding mercury. The voltaic circuit 

 may thus be completed in various ways ; 

 either by placing wires in the mercury 

 contained in the small side cups, and 

 connected with one pole of the battery, 

 while other wires, communicating with 

 the other pole, are placed in the cup on 

 the top of the apparatus ; or else, direct- 

 ing one and the same stream of elec- 

 tricity through the whole of the appara- 

 tus, by joining one of the side cups with 

 the positive, and the other with the ne- 

 gative side of the battery. In the former 

 case, the current, passing in the same 

 direction, whether upwards or down- 

 wards, in the two coils, and being acted 

 upon by the different poles of the magnet, 

 will be urged to revolve in opposite di- 

 rections : in the latter case, the contrary 

 directions of the currents in both wires, 

 ascending in the one, and descending in 

 the other, being respectively acted upon 

 in opposite modes by the contrary poles 

 of the magnet, the combination of these 

 two contrarieties will produce rotations 

 in the same direction in both the wires. 



CHAPTER VIII. 

 Galvanometers. 



(111.) The action of a circular or spi- 

 ral coil has been applied to the construc- 

 tion of an instrument for detecting small 

 quantities of galvanic electricity, or Gal- 

 varioscope ; and also of a Galvanometer, 

 or instrument for measuring the inten- 

 sity of any galvanic current. For this 

 purpose, the diameter of the circle must 

 exceed the length of the needle which it 

 surrounds, in order to allow the latter to 

 place itself in the plane of the circle 



