C. GENERAL PHYSICS. 139 



this result more or less perfectly, and the latest suggestion is 

 that of Mr. Gloesvenor, member of the Royal Academy of 

 Brussels, who proposes to place the standard compass on a 

 ^prolongation of the bowsprit of the vessel, in this way secur- 

 ino; the advantao-e of havins; all the disturbins' iron on one 

 side of tlie needle, and placed symmetrically with reference 

 to it. The necessary length of tlie j^rolongation Gloesvenor 

 finds by experiment to be twenty-five or thirty feet. To read 

 the comj^ass conveniently, he puts a mirror above it, in which 

 a person standing on the deck may read the needle or com- 

 pass card. A small fixed telescope facilitates the accurate 

 observation. Bull. Acad. 13 elgiqiie, 18V3, 357. 



rEEIPOLAR :NrAGXETO-ELECTKIC IXDUCTIOX, 



Since the discovery of magneto-electric induction by Far- 

 aday, the experiments on the subject have been varied in 

 many ways. One class of these is well explained, viz., the ef- 

 fects of induction in a closed circuit when the distance of some 

 portion of it from a magnetic pole is changed, or the mag- 

 netic intensity itself changed. Another series of induction 

 phenomena those in which a conductor moves in a field of 

 maofnetic force without in the least chang^imj: its distance from 

 the magnetic jooles of which an example is found in the ro- 

 tation of a metal disk about a central maQ-netic axis, has late- 

 ly been made the subject of some excellent experiments by 

 Le Roux. The author judged that the previous experiments 

 on this class of phenomena have been on too small a scale to 

 yield reliable results, and he succeeded in obtaining a rotat- 

 ing disk whose electric phenomena became visible as sparks, 

 the current being nearly as strong as that of one cell of a gal- 

 vanic battery, and whose strength could therefore be studied 

 by the electroscope rather than by the galvanometer. The 

 disk used in these experiments was of fine copper, six inches 

 in diameter and the twentietli of an inch thick, and it made 

 sometimes a hundred and eighty revolutions per second about 

 its axis. At the extremities of one of the diameters of the 

 disk were placed respectively the north and south poles of 

 powerful electro-magnets. 



The magnetic currents induced in the copper disk while it 

 rotates flow from the central axis to the circumference of 

 the disk, or radially, so soon as a thick wire is applied to 



