SIX LECTURES ON LIGHT. 



19 



needle, while all the upper half, from E to 

 N, attracts the opposite end. This double- 

 ness of the magnetic force is called polarity, 

 and the points near the ends of the magnet in 

 which the forces seem concentrated are called 

 its poles. 



What, then, will occur if we break this 

 magnet in two at the cent:e E > Will each 

 of the separate halves act as it did when it 

 formed part of the whole magnet? No; 

 each half is in itself a perfect magnet, pos 

 sessing two poles. This may be proved by 

 breaking something of less value than the 

 magnet the steel of a lady's stays, for ex- 

 t xample, hardened and magnetized. It acts 

 like the magnet. When broken, each half 

 acts like the whole ; and when these parts 

 are again broken, we have still the perfect 

 magnet, possessing, as in the first instance, 

 two poles. Push your breaking to its utmost 

 limit ; you will be driven to prolong your 



rection of the needle, and no other. A needle 

 of iron will answer as well as the magnetic 

 needle ; for the need e of iron is magnetized 

 by the magnet, and acts exactly like a needle 

 independently magnetized. 



If we place two or more needles of iron 

 near the magnet, the action becomes more 

 complex, for the the iron needles are not only;' 

 acted on by the magnet, but they act upon 

 each other. And if we pass to smaller masses 

 of iron to iron filings, for example we find 

 that they act substantially as the needles, ar- 

 ranging themselves in definite forms, in obe- 

 dience to the magnetic action. 



Placing a sheet of paper or glass over this 

 bar magnet and showering iron filings upon 

 the paper, I notice a tendency of the filings 

 to arrange themselves in determinate lines. 

 They cannot freely follow this tendency, for 

 they are hampered by the friction against che 

 paper, They are helped by tapping the 



FIG. 6. 



K is the nozzle of the lamp ; M a plane mirror, reflecting: the beam upwards. At P, the magnets and 

 iron filings are placed ; L is a lens which forms an image of the magnets and filings ; and R is a total' 

 ly- reflecting prism which casts the image, G, upon the screen. 



vision beyond that limit, and to contemplate 

 this thing that we call magnetic polarity as 

 resident in t)ie ^lltimate particles of the mag- 

 net. Each atom is endowed with this polar 

 force. 



Like all other forces, this force of magnet- 

 ism is amenable to mechanical laws ; and 

 knowing the direction and magnitude of the 

 force, we can predict its action. Placing a 

 small magnetic needle near a bar magnet, it 

 takes up a determinate position. That posi- 

 tion might be deduced theoretically from the 

 mutual action of the poles. Moving the 

 needle round the magnet, for each point of 

 the surrounding space there is a definite di- 



paper: each tap releases them for a moment j. 

 and enables them to follow their bias. Bu 

 this is an experiment which can only be seen 

 by myself. To enable you to see it, I take a 

 pair of small magnets and by a simple optical* 

 arrangement throw the images of the mag- 

 net i upon the screen. Scattering iron filings 

 over the glass plate to which the small magnets 

 are attached, and tapping the |>late, you see 

 the arrangement of the iron filings in those 

 magnetic curves which have been so long 

 familiar to scientific men.* 



*Very beautiful specimens of these curves have 

 been recently obtained, and fixed, by Prof. Mayer, 

 of Hoboken. 



