106 Prof. Faraday on some Points of Magnetic Philosophy. 



3346. If, instead of the core and chamber described, an iron 

 tube of sufficient thickness of metal (as part of a gun-barrel) be 

 employed, tben like effects occur. If the magnetic needle be 

 introduced, it ceases to be acted upon when about 1*5 inches 

 within the tube. If the tube be more or less filled with iron 

 filings, and then be excited and held vertical, they will all pour 

 out and fall away, except those which are retained at the external 

 edges. Yet, if a long nail be introduced, so as to be partly out 

 of the cylinder, then it will be strongly attracted at the internal 

 point, where it touches the iron of the tube core. 



3347. The realization of like effects by grouping together the 

 poles of ordinary magnets gives most interesting results. I have 

 four very hard steel magnets, each 6 inches in length, 1 inch in 

 breadth, and 0*4 nearly in thickness. -n- ,a 



When the four like poles are put 

 together, fig. 10, they form a flat 

 square chamber in the same plane as 

 that of the magnets. If a piece of 

 stiff paper, the size of this chamber, 

 be raised on a block 0*2 of an inch 

 high, then sprinkled over with iron 

 filings, and the magnets afterwards approached regularly until 

 the square chamber is formed, a little tapping on the card will 

 then ai'range the filings in lines from the sides of the square 

 chamber to the centre. The filings show at once the direction 

 of the lines of force in this medium plane, and their greater 

 abundance at the middle of each pole than at the re-entering 

 angles ; and if the filings be then removed and the indication of 

 the course of the lines be followed out by a small magnetic 

 needle, it will be found that the lines rise upwards from this plane 

 above, and descend from it below, and then turn back upon their 

 course in the free space over and beneath the arrangement 

 towards the S poles of the different magnets. The condition 

 will be understood in a moment, by considering the sphondyloids 

 of power belonging to each magnet (3271.), and the manner in 

 which they are associated when the four like poles come together. 



3348. When the magnets are turned edges upwards, they 

 form a vertical chamber 1 inch high and 

 only - 4 of an inch in width, and now 

 phsenomena like those just described occur, 

 but only near the entrances to the chamber; 

 as the little needle proceeds into the en- 

 closed space, the power of the magnets be- 

 comes less and less, and at the middle of 

 the chamber scarcely a trace remains ; that 

 place being,like the closed chamber, formed with six poles (3341.), 



