Prof. Faraday on some Points of Magnetic Philosophy. 105 



perhaps appear at the bottom of the cavity, the sum or amount, 

 as compared to the sum of power at the end of the magnet, will 

 show how complete the analogy between this space and the in- 

 terior of a metallic vessel charged with positive or negative elec- 

 tricity is. A cylinder of soft iron, 9 inches in length and 1*6 

 in diameter, had a chamber 09 in diameter and 1 inch in 

 depth formed in one extremity concentric with the cylinder; 

 and being placed in a powerful helix of thick copper wire, and 

 associated with a Grove's battery of ten pair of plates, was ready 

 for experiment : — a like chambered magnet can be prepared by 

 putting a proper iron ring against the end of any electro- or 

 ordinary magnet, and will show the phenomena I am about to 

 describe. A piece of soft iron, not more than 0*3 of an inch in 

 length or thickness, held at the end of a copper wire and brought 

 near the outer edge of the excited magnet pole, will be very 

 strongly attracted ; but if it be applied to the bottom of the 

 chamber it will present no such effect, but be quite indifferent. 

 If applied about the sides of the chamber, it will indicate no 

 effect until it appx-oaches the mouth. If the magnet be placed 

 horizontally, and a piece of card-board be cut, so that it can 

 enter the chamber and represent a horizontal section of its 

 cavity ; and, being sprinkled over with clean iron filings, is then 

 put into its position and the magnet excited for a moment that 

 it may develope its power over the chamber and filings and give 

 them their indicative position ; it will be found that only those 

 near the mouth have been driven into a new position (about the 

 outside angles of the pole), and that four-fifths of those upon 

 the surface of the card within the chamber have been left un- 

 affected, unmoved. If the chamber be filled with iron filings, 

 closed with a card, placed in a vertical position with the aperture 

 downward, and the magnet be then excited and the card re- 

 moved, the filings will fall out ; as they come out they will be 

 caught away, and form a fine fringe round the external angles 

 of the pole, but not one will remain at the bottom of the chamber, 

 or even anywhere within the chamber, except near to its external 

 edge. Yet, if a piece of iron long enough to reach out of the 

 chamber, as a nail 2, 3, or 4 inches long, touch the bottom of 

 the chamber, it is strongly attracted and held there, and will 

 support a weight of several ounces, though prevented from 

 touching the chamber anywhere else by a card with a hole in it 

 placed over the mouth. 



3345. If a small magnetic needle, about Ol of an inch long, 

 be brought towards this excited magnet, it is almost unmanage- 

 able by reason of the force exerted upon it ; but, as soon as it 

 baa entered the chamber, the power rapidly diminishes, and at 

 the bottom the needle is scarcely, if at all, affected. 



