276 Mr. W. Sturgeon 07i the Disiribuii07i and Retention 



over-powered by the exciting pole, and the needle obeys the 

 influence of the predominating polar energy. No velocity, how- 

 ever, that I can give the plate will produce a change in the 

 character of the dip. The angle certainly varies by placing 

 the needle in various positions with regard to the magnet, and 

 by varying the course of the plate ; but in no instance has the 

 character of the dip been destroyed. The same thing takes 

 place when another north pole of a magnet is placed over the 

 former and above the plate. 



When the plate is rotated betvveen the poles of a horse- 

 shoe magnet, the distribution of polarity depends upon the 

 nearest pole, and is regulated as if that pole alone were pre- 

 sent. There are, however, some other curious facts to be ob- 

 served in these experiments, which are not easily described. 

 They may perhaps be of the same character as those which 

 appear in copper and zinc, &c,, and which are more easily 

 traced, in consequence of the polarity excited in those metals 

 being distributed in a peculiar manner, less vigorous, and the 

 needle consequently more manageable. 



Experiments on Cojjper, Zinc, &;c. — We are indebted to the 

 successful investigations of M. Arago for directing us to an 

 experimental demonstration of the presence of magnetism in 

 copper, zinc, and all those metallic bodies in which, till then, 

 plausible conjecture alone had supplied all the knowledge we 

 possessed respecting its existence. 



The experiments of that celebrated philosopher were first 

 known in London in the early part of 1825. They consisted 

 of a judicious application of the magnetic momentum generated 

 by the rapid motion given to those metals copper, zinc, &c. 

 in which the magnetic force is too feeble for detection by the 

 simple presentation of the most delicate needle. Plates of 

 those metals were rotated with great velocity, in their own 

 planes, on a vertical axis, while a magnetic needle was deli- 

 cately suspended over them. The magnetic force of the me- 

 tals by this process became exalted in proportion to the ra- 

 pidity of their motion ; and generated to a sufficient degree 

 of intensity to deflect the needle from its natural position, and 

 even to cause it to perform complete revolutions in the direc- 

 tion of those given to the plate beneath it*. 



The experiments of M. Arago were immediately i-epeated 

 with a great deal of interest in this country, particularly by 



* The earlier experiments of M. Arago were made by vibrating a magnetic 

 needle in the centre of a ring or hoop of each of the metals examined. 

 Mr. Barlow had discovered that brass possessed magnetic properties, some 

 years before M. Arago's experiments were known. (See Barlow's Essay on 

 Magnetic Attractions, 2nd ed. 1823. p. 17.) 



Messrs. 



