Electro-Magnetic Apparatus and Experiments. 265 



the mechanical appUcation of electro-magnetism has been turned 

 to profitable account. This engine is to undergo considerable 

 alteration and improvement, when a description and drawing of 

 it will be published. 



That much remains yet to be determined concerning the most 

 advantageous form and size of magnets and armatures, will appear 

 from the following observations made during last October, while 

 on a visit in Boston, 



First : it is possible to present a piece of soft iron to the most 

 powerful magnet in such a manner that it will not be attracted 

 in the least by the magnet. 



Experiment. — Drill a hole in the center of the pole of an elec- 

 tro or permanent magnet, to admit a small sliding rod of brass. 

 To one end of this sliding rod, fasten a small disc of soft iron. 

 The diameter of the disc must be less than that of the pole of 

 the magnet, and the thickness or axis of the disc, must be con- 

 siderably less than its own diameter. Put the sliding rod in its 

 place, and if the disc of soft iron be exactly parallel to the face 

 of the magnetic pole, it will not be attracted by it, be the magnet 

 never so strong. If the disc is in the least inclined from paral- 

 lelism, it will be attracted by the magnet. The experiment will 

 appear more satisfactory if varied in the following manner. Place 

 the disc of soft iron, with its sliding rod, in a frame, and place 

 the magnet on a rest, so that its position can be varied; the same 

 results will follow as before. Again : put the disc, without its 

 sliding rod, on the center of a large magnetic pole, and it will 

 slip down to the edge of the pole, and there adhere. Again : 

 sprinkle iron filings on a piece of paper laid over the end of a 

 bar magnet ; the filings will cluster over the pole around a va- 

 cant space at its center. Again: drill out the disc of iron so as 

 to make a ring, whose width is greater than its thickness, and 

 present it to the magnet in the same manner as the disc, and the 

 ring will be attracted by the magnet. It appears from this, that 

 the disc, though magnetized by induction is polarized in a radial 

 direction, and the forces counteract, or disguise each other's in- 

 fluence upon the magnetic pole. When the diameter of the disc 

 is greater than that of the magnetic pole, there cannot be this 

 counterpoise of forces. When the disc is inclined to the face of 

 the magnetic pole, it becomes polarized in the direction of an 

 oblique line, joining that part of the disc in contact with the 



Vol. XXXV.— No. 2. 34 



