ELECTRO-MAGNETISM. 



on the one side.and attraction of the ^ame 

 pole on the other side. This arrange- 

 ment is shewn in Jig. 82, where M is the 

 magnet suspended by the thread T, be- 

 tween the four spiral discs, composed of 

 the convolutions of the wire proceeding 

 from the cup P, and terminating in the 

 cup N. In each disc, the force acting 

 perpendicularly to the plane of the discs, 

 is multiplied in proportion to the num- 

 ber of the circumvolutions of the wire ; 

 and the spiral turns being made in the 

 same directions in all the discs, their 

 actions will concur in producing in the 

 needle a deviation in the same, direction ; 

 and the total force will be four times 

 that of a single disc. This arrangement 

 allows also of a very close approxima- 

 tion of the needle to the discs. 



(127.) The lightness and extreme 

 flexibility of gold leaf have enabled 

 electricians to employ this material for 

 the construction of a very sensible elec- 

 trometer. (See Electricity, 73.) The 

 same properties may be applied with 

 great advantage to the purposes of a 

 Galvanoscope, the electro-magnetic force 

 of the current being estimated, not by 

 the movements of a magnet on which it 

 is made to act, but by those of a move- 

 able conductor through which it is trans- 

 mitted, under the influence of a power- 

 ful magnet. The construction of the 

 Gold-leaf Galvanoscope is similar to that 

 of Bennet's electrometer, excepting that 

 the leaf is single, and there is added a 

 forceps to retain the lower end of the 

 gold-leaf, and complete the galvanic 

 circuit. The slip of gold-leaf g,fig. 83, 

 is suspended loosely from the forceps/", 



while the lower end is laid hold of by 

 another forceps h ; each forceps termi- 

 nating in a cup, the one, P, being above, 

 and the other, N. below, for establish- 

 ing the communications by which the 

 current is transmitted through the gold- 

 leaf. The whole is enclosed in a cylin- 

 drical glass case, the middle of which is 

 placed between the poles of a strong 

 horse-shoe magnet M m, so that the gold- 

 leaf may be nearly equidistant from them. 

 When the circuit is completed through 

 the gold-leaf, the latter will be attracted 

 or repelled laterally by the poles of 

 the magnet, according as the current 

 is ascending or descending ; the broad 

 surface of the leaf becoming convex 

 towards the magnet in the one case, 

 and concave in the other. The curva- 

 ture of the gold-leaf may be viewed 

 through a lens in a direction at right 

 angles to the line of its motion, and may 

 be referred to a fine line drawn upon the 

 tube in the direction of its axis. This 

 instrument is, perhaps, the most delicate 

 test possible of the existence and direc- 

 tion of a weak galvanic current *. 



CHAPTER IX. 



Electro-magnetic Effects of Terrestrial 

 Magnetism. 



(128.) Since the earth acts as if it 

 were endowed with a magnetic power, 

 or rather as if it contained a powerful 

 magnet in its centre, it naturally oc- 

 curred to those who explored the new 

 realms of science which the discovery 

 of Oersted had laid open, that a cur- 

 rent of voltaic electricity would itself be 

 influenced by the magnetism of the 

 earth. It was at first found extremely 

 difficult, however, to devise means of 

 rendering this action visible, in con- 

 sequence of the great feebleness of the 

 earth's action, compared with that of 

 such artificial magnets as we are in the 

 habit of employing. Ampere at length 

 succeeded in obtaining decisive evidence 

 that the conducting wire possessed a 

 directive power by the following con- 

 trivance. Two wires A and B,}lg. 84, 

 bent at right-angles, are made to pass 

 through a cylindrical piece of wood C, 

 fixed at the end of an arm proceeding 

 from the basis of the apparatus. They 

 are made to terminate at both their 

 extremities in small cups, designed to 

 hold mercury, the cups P and N being 

 intended to receive the wires communi- 



* Curaraing's Manual of Electro-Dynamics, p. 178, 



