274 Mr. W. Sturgeon on the Distribution and Retentioti 



The opposite polarity is promiscuously scattered in the area 

 of the plate. 



Experiment 5. — In this experiment the magnet is laid upon 

 a round board above and parallel to the plate ; it must also be 

 something shorter than a diameter of the plate. 



Let the magnet be placed over the diameter a c, with its 

 south pole at a. Now cause it to revolve on its centre until 

 each pole has completed one revolution, and remove it quickly 

 from the plate parallel to its last position. 



By this process the iron plate will become decidedly and 

 regularly polar; and when the revolution of the magnet is in 

 the direction of the arrows (fig. 5.), an aggregate 7iorth pole 

 will be determined in the limb d a, and an aggregate south 

 pole in the limb be. The centre of force in those limbs will 

 frequently be found in one and the same diameter of the plate, 

 at a short distance from its edge, and generally between 5° 

 and 10° from the diameter <zc. These poles are easily dis- 

 covered by holding a small dipping-needle over the plate. 

 The equator of the plate which separates its two polar re- 

 gions is also found in this way. The needle remains hori- 

 zontal over this line, which is frequently an exact diameter of 

 the plate, at right angles to its magnetic axis. If the needle 

 be held on either side of this line, one pole or the other will 

 dip, according to the polar character of the iron, beneath it. 

 Fig. 5. will give a pretty exact representation of the distribu- 

 tion of polarity of the plate by one revolution of the magnet. 

 The dotted line e q is the equator, or line over which the 

 needle exhibits no dip. From this equatorial line the dip in- 

 creases towards S. and N., over which points it stands vertical, 

 showing the point s to be the south pole, and n to be the north 

 pole. The plate is, therefore, a very regularly polarized mag- 

 net ; and if means be not employed to disturb it, this distri- 

 bution of polarity will be retained by the iron for some consi- 

 derable time. 



If the magnet were to perform its revolution in the opposite 

 direction, the polarity would be distributed as represented by 

 fig. 6. 



When the plate is of good iron and well annealed, these 

 pha;nomena are very unifoi'mly exhibited ; but if it has been 

 much hammered without subsequent annealing, the equatorial 

 line is found variously curved, and the poles are not exactly in 

 one diameter. In most cases, however, if the experiments be 

 dexterously performed, there will be found a tolerably regu- 

 lar distribution of magnetic polarity. The disc may be placed 

 in any other position either vertical or sloping, whilst the mag- 

 net revolves round it, and the same kind of distribution will 



be 



