112 Mr. Watkins on Magneto-electric Induction. 



Description of the Apparatus. 



(«). A piece of soft iron bent in the form of a horse-shoe 

 magnet, partly surrounded as at bb by copper wire 

 covered with silk in the usual manner. 



(c). A permanent magnetic needle revolving on an axis as re- 

 presented in the figure, which axis has a contrivance 

 of points dipping successively into a divided cup of 

 mercury, one division of which is in connexion with 

 the copper element of a voltaic circuit, and the other 

 in connexion with the zinc element. The cup for the 

 mercury cannot conveniently be shown in this figure, 

 but it is placed so that the points on the axis, which 

 have the effect of changing the current in the copper 

 wire enveloping each arm or branch of the soft iron, 

 may dip into it successively as the axis rotates. 



(d). A cup of mercury connected with one end of the copper 

 wire coiled on the arms of the soft iron, while the 

 other end of the wire is immersed in a similar cup si- 



