ELECTRO-MAGNETISM. 



31 



this purpose the circumference o r the 

 disc should merely touch the mercury in 

 the trough. It is necessary also that it 

 be well amalgamated ; this is best done 

 by removing it from its centres, and 

 cleansing the edge thoroughly by a file, 

 and then dipping a piece of wire into 

 nitrate of mercury, and taking up with 

 it a portion of the mercury contained in 

 the nitrate, transferring it to the edge of 

 the disc, by rubbing the wire, coated 

 with mercury, round it. This substi- 

 tution of a continuous for a divided disc 

 was suggested by Mr. Sturgeon, 



(86.) The same current rr.ay be em- 

 ployed to turn two wheels with radii, by 

 disposing them in the manner shewn in 

 fig. 56, at the extremities of a horizontal 

 wire which is supported on two pillars 

 arising from the stand, and which serve 

 as the common axis of the wheels. 



Fig. 56*. 



energetic, nothing more will be required 

 than a single pair of plates. The most 

 convenient form of a battery of this kind, 

 is that described by Mr. Watkins, and 

 which is represented \nfig. 57. 



It consists of a double cylindrical vessel 

 made of thin copper, with a bottom of 

 the same metal. A plate ofzinc rolled 

 into a cylinder, of a diameter interme- 

 diate between those of the copper cylin- 

 der, is introduced between them, but 

 prevented from touching them in any 

 part, by three wooden feet placed at the 

 bottom of the vessel, and also by pieces 

 of wood interposed as wedges between 

 the sides. 



A copper wire is soldered to the inside of 

 the top of the outer copper cylinder, and 



Fig. 57. 



The lower ends of the rays dip into 

 troughs of mercury, each lying between 

 the poles of horse-shoe magnets. Each 

 trough has its respective wire and cup 

 P and N for making communications 

 with the voltaic battery. The current 

 passing from the one cup to the mercury 

 in the trough on the same side, rises 

 along the radius, which dips into it, and 

 passing along the axis, arrives at the 

 other wheel ; then descending along its 

 radius into the mercury, it makes its 

 exit by the cup on that side. The elec- 

 tric currents, moving in opposite direc- 

 tions in the two wheels, require a con- 

 trary disposition of the poles of the two 

 magnets by which they are to be acted 

 upon : that is, the poles of the two mag- 

 nets that are within the wheels must 

 both be of the same kind ; as must also 

 be those that are exterior to them. The 

 velocity of the wheels thus revolving by 

 the united action of both magnets is 

 very great. 



(86.) The experiments on electro-mag- 

 netic rotation we have described, do not 

 require for their successful performance 

 a voltaic battery of any considerable size 

 or power. If the magnets be sufficiently 



* The engraver has forgotten to insert the horse- 

 shoe magnets in this wood-cut. They should have 

 been placed aa the one in fig. 55. 



has a small cup P, fixed at its extremity; 

 the wire passing through the bottom of 

 the cup in order to come in contact 

 with the mercury placed in it. Another 

 and similar wire N, is also affixed to 

 the upper edge of the zinc cylinder, 

 likewise terminating in a cup which 

 holds mercury. The battery is charged 

 by filling the copper vessel with diluted 

 acid ; and the electric current, which is 

 the effect of the voltaic action thence 

 arising, may be easily transmitted to 

 the apparatus where it is wanted, by 

 means of two bent copper conducting 

 wires, one end of the one being inserted 

 into the mercury contained in the cup 

 proceeding from the zinc cylinder, and 

 the other in the cup fixed to the copper 

 cylinder; while the other ends are im- 

 mersed in the mercury placed in the 

 cups attached to the apparatus. The 

 current may be arrested or renewed at 



