180 Transactions of the American Institute. 



tion to what it did when I approached the magnet. I will now rotate 

 the ring around a vertical axis, in the direction in which move the 

 hands of a watch. The needle at the same time moves its spear end 

 towards the ceiling. I reverse the motion of the ring's rotation and 

 the direction of the needle's deflection is reversed also. Thus we 

 have found that whenever the ring moves so as to cut the magnetic 

 lines, an electric current is driven through it ; and on the direction 

 of translation or of rotation of the wing depends the direction of the 

 current. 



We will now gradually recede from the magnet, moving in the 

 direction of its axis, and rotate the ring at various points of our pro- 

 gress. Here the needle, you observe, is not swung over so many 

 divisions of the circle as when we revolved the ring at oar previous 

 station ; therefore the magnetic influence is diminishing. I remove 

 yet further and the needle swings less than before. I do not know that 

 I will get any deflection whatever at this distance ; yes, but very little ; 

 the needle just nodded. Here, however, the needle responds to no 

 motion that I give the ring. So far, then 4 extends the magnetism 

 outward from that iron bar, but no farther. But may it not be that 

 it is really here also, but that our means of detecting it only extend 

 thus far in their indications ? To test this supposition I will lay aside 

 this ring of one turn of wire, and take up another, which is really a coil 

 of nine turns of wire, and has a diameter of about eighteen inches. 

 Nii^e wires will now cut twice the area of magnetic rays. Yes, 

 the needle is deflected, and it replies even here — and here, but at 

 this distance it barely moves ; and here, the needle again refuses to 

 notice the ring's rotation. But have we yet reached the boundary of 

 the magnetic envelope of that bar % I will try again. I replace this 

 coil by another, two and a half feet in diameter, and containing forty 

 turns of 300 feet of -^-th-inch wire. I now hold this in a vertical 

 position, with its plane perpendicular to the axis of the magnet. The 

 ends of its wire are now connected with the galvanometer, and I rotate 

 it about a vertical axis. Look ; the needle moves. I recede to this 

 position ; even here the needle vibrates. I retreat to the extreme end 

 of the stage, and even here the needle swings with the swing of the 

 coil. But 1 can exalt the sensitiveness of the galvanometer a hundred 

 fold by wrapping a coil of thick wire quite close around the needle, 

 and then I can go to the farthest corner of this room ; I can descend 

 into the cellar, or place myself upon the roof; and yet, in all of these 

 positions, the needle will swing to the revolution of the coil. The 

 whole space inclosed by the walls of this house contains — is perme- 



