ELECTRO-MAGNETISM. 



4G 



eating with the poles of the voltaic bat- 

 tery ; and the others, n and p, which are 

 placed the one immediately above the 



Fig. 84. i 



P TST 



other, receiving the two ends of the wire 

 W W w, which passes through a small 

 piece of wood at S and is bent below 

 into a square or rectangle R R. The 

 upper point of the wire rests on the 

 bottom of the cup n; the lower point 

 being merely made to dip into the mer- 

 cury in the cup p, without touching it, 

 so that the whole of the wire, with its 

 connecting piece S, has perfect freedom 

 of motion round a vertical axis passing 

 through the point of support in the up- 

 permost of the two cups, n. When a 

 connexion is made with the battery by 

 means of the cups P and N, so as to 

 direct an electric current through the 

 wire W W w, it will, from the extreme 

 delicacy of its mode of suspension, obey 

 the magnetic influence of the earth, and 

 arrange itself so that the plane of the 

 rectangle R R shall be perpendicular to 

 the plane of the magnetic meridian ; 

 and it will always return to this position 

 when turned aside from it by the hand, 

 or any other cause. 



(129.) Another arrangement -which 

 exhibits the same effect is the one 

 already described, 100, and represented 

 in fig. 66 ; where the electro-magnetic 

 force is increased by the number of coils 

 composing the spiral wire. This spiral, 

 as in the last case, immediately assumes 

 a position in a plane perpendicular to 

 the magnetic meridian, as soon as it is 

 made the channel for the transmission 

 of a current of voltaic electricity. The 

 mode of suspension here described is 

 that of Professor Van den Boss*. 



(130.) The apparatus invented by M. 

 Be la Rive, and described 96, with the 



* Edinlnirgh Journal of Science, No, XII, 



improvement described in $ 09, is also ex- 

 ceedingly well adapted for the exhibition 

 of the directive power of the galvanic 

 current ; for in consequence of the per- 

 fect freedom of motion allowed it, while 

 floating in a fluid, it very readily as- 

 sumes the position due to the magnetic 

 influence of the earth. \Vhenthe plates 

 are immersed in acidulated water, as in 

 M. De la Rive's original experiment, the 

 gas liberated by the action of the acid 

 on the plates, prevents them from tak- 

 ing a steady position ; but when put 

 into a little floating cell, the whole rea- 

 dily takes the position above mentioned, 

 and even slowly vibrates about it. The 

 same phenomenon is also obtained by the 

 arrangements described in 108 and 

 109, which have also the advantage 

 of exhibiting the strong resemblance 

 which these instruments, actuated solely 

 by electrical currents resulting from gal- 

 vanic action, have to artificial magnets. 

 For in consequence of their lengthened 

 cylindrical form, the magnetic forces are 

 directed along the axes, and the heliacal 

 cylinder places itself, like a magnet, with 

 its axis in the magnetic meridian ; 

 whereas, when a single circle, or com- 

 bination of circles in a single plane is 

 taken, that plane will arrange itself so 

 as to be at right angles to the plane of 

 the meridian, that is, will be in a plane 

 passing east and -west ; the face of the 

 plane only looking to the north and 

 south. 



(131.) In all the cases above de- 

 scribed, the multiplication of the spiral 

 or circular turns of wire is not pro- 

 ductive of the advantage that might be 

 expected, because, as already remarked 

 with regard to the galvanometer of Le- 

 baillif, 123, although the power is in- 

 creased, yet the weight to be moved by 

 that power is increased nearly in the 

 same proportion ; and the resulting 

 motion is therefore nearly the same. 



(132.) The next point of comparison 

 between the action of the earth on the 

 conducting wire, and on the magnetized 

 needle, relates to the dip. Since the 

 direction of the force of terrestrial 

 magnetism 'is in a line situated in the 

 magnetic meridian, and inclined about 

 seventy degrees to the horizon, the ope- 

 ration of this force on a Voltaic wire, 

 bent in a plane so as to describe a 

 circle, square, parallelogram, or any 

 other figure which terminates where it 

 commenced, forming what has been 

 termed a closed circuit, is to bring it 

 into a position where it is perpendicular 



