Magneto-Electricity, and Electro-magnetical Machmes. 131 



that subject more particularly, and find the generalization may 

 be extended still further, and include all electro-magnetical 

 motion produced by two magnets, or by a conductor and a mag- 

 net. It may then be stated as follows : If a galvanic current 

 from a battery produces an electro-magnetical motion in any piece 

 of apparatus, and that battery be detached and a galvanometer 

 substituted in its place, and connected with the same wire or 

 poles of the apparatus ; then, on compelling the same motion in 

 the apparatus by hand or otherwise, the galvanometer will be 

 deflected, showing a current of magneto-electricity in a direction 

 opposite to that current from the battery which had produced the 

 same motion. I have tried the experiment with Barlow's "revolv- 

 ing star,"* the "revolving wire" of Mr. Faraday, the "revolving 

 cylinder," with Andrews' revolving magnet, De La Rive's coil, 

 and also with one galvanometer acting upon another. 



The revolving magnet is the most simple magneto-electric 

 machine possible. Take a cylindrical straight bar magnet, and 

 holding two wires from the galvanometer, one in contact with 

 one end of the magnet, and the other in contact with the mid- 

 dle, let an assistant twist the magnet round on its axis, the wires 

 slipping on its surface ; the galvanometer will immediately indi- 

 cate the production of magneto-electricity. Or, the lower end 

 of the magnet may be sharpened into a sort of pivot, and be 

 pressed by the breast down into an indentation in one of the 

 conductors, while the contact at the middle and the rotation of 

 the bar are performed without an assistant. With a magnet eight 

 inches long, and one sixth of an inch in diameter, made of watch- 

 maker's wire, nicely pivoted, and turned by drawing the finger 

 across it, I obtained a deflection of my twelve inch galvanometric 

 needle of sixty six degrees, viz. from N. 45 E. to N. 21 W. It 

 is evident that the galvanometer itself is included in this rule. I 

 connected my large thermoscopic galvanometer, by long wires, 

 with the poles of an elegant and delicate Mellonian galvanometer, 

 and then put the needle of the larger one into rotary motion by 

 hand. The needle of the smaller instrument was deflected quite 

 to the west, while the needle of the larger swept one half of its 



* This experiment is identical in principle with the mode of Mr. Faraday, the 

 apparatus for which is figured in Brande's Chem. 1836, p. 315. 



