ELECTRO-MAGNETISM. 



25 



effecting: the communication with the 

 voltaic battery by wires, in the usual 

 manner. A small wire, pointed and 

 amalgamated at its end, is affixed to the 

 middle of the magnet, immediately above 

 the cistern, and is bent so as just to dip 

 into the mercury contained in the cistern. 

 A similar wire, proceeding from the 

 lower end of the magnet, is made to dip 

 into the mercury contained in the lower 

 cistern. The lower half only of the mag- 

 net beinir thus made to form part of the 

 galvanic circuit which is continuous 

 from one cup through the cistern of 

 mercury, the wire belonging to the 

 magnet, the magnet itself, the other 

 wire, the other cistern of mercury, and 

 the wire terminating in the other cup 

 receives the exclusive influence of the 

 electric current which passes through 

 it, and begins to rotate with considerable 

 velocity round the axis, which is con- 

 stituted by its upper and lower points 

 of support. The degree of rotatory 

 effect will depend very much on the 

 delicacy of the suspension of the mag- 

 net, so that the friction at the points 

 may be as small as possible. 



(72.) When the magnet is large, it has 

 been proposed to gain additional rota- 

 tory power by directing another electri- 

 cal current to be supplied from a second 

 battery along the upper half of the 

 magnet, but in a direction contrary to 

 that which passes through the lower 

 pole. This might certainly be effected 

 by removing the milled head of the ver- 

 tical screw, and supplying its place by a 

 small cup to hold mercury, and by care- 

 fully amalgamating the lower end of the 

 screw where it touches the magnet. But 

 since the rotatory force is proportional to 

 the power of the voltaic battery used, it 

 is very doubtful whether the second 

 battery required in this latter method 

 might not be equally efficacious if it 

 were employed in increasing the strength 

 of the lirst battery, by being joined to 

 it, in the former mode of conducting the 

 experiment. 



(73.) Having thus succeeded in mak- 

 ing the magnet revolve on its own axis, 

 it next became an object to effect, in 

 like manner, the rotation of a conduct- 

 ing body round its axis. As in the for- 

 mer case it was necessary to apply the 

 electric agency in the interior of the 

 magnet, so in the present instance some 

 means were to be devised for procuring 

 the action of the magnet from the in- 

 terior of the conducting body : hence it 

 was necessary to discard the wire, and 



employ in its place a hollow cylinder of 

 metal, capable of receiving the pole of 

 a magnet in its axis. Such an arrange- 

 ment, which was devised by Mr. Barlow, 

 is exhibited by fig. 45, which repre- 

 sents a section of the apparatus. A 

 bar magnet is fixed upright in a solid 

 stand, which has a cavity adapted to 

 receive it, and which also supports a 

 circular trough of mercury, surrounding 



the magnet as in the former instances. 

 C C is a light hollow copper cylinder, 

 the lower edge of which dips into the 

 mercury in the trough; and the upper 

 part is supported by an arch of the same 

 metal, from the middle of which there 

 proceeds a steel-pointed wire, passing 

 downwards so as to rest in an agate 

 cup fixed to the top of the magnet, and 

 also passing upwards and terminating 

 in a small cup P, holding a little mer- 

 cury, for the purpose of effecting a 

 communication with the voltaic battery. 

 A wire proceeds from the inside of the 

 trough, and passing out, is bent upwards, 

 so as to terminate in another cup with 

 mercury N, for establishing the con- 

 nexion with the other pole of the battery. 

 It is evident that, in this arrangement, 

 the electric current, which we may sup- 

 pose to descend from the positive wire 

 of the battery introduced into the, cup 

 P, being prevented from passing into the 

 magnet by the interposition of the agate 

 cup, can find no other channel than the 

 copper cylinder, down the sides of which 



