68 Royal Society. 



of an inch in thickness, were fixed, side by side, a little apart, with 

 like ends in the same direction, on and parallel to an axis, so that 

 they might act as one bar magnet and be revolved at pleasure about 

 the common axial line. A wire, which entering at one pole was 

 carried along the axis of the magnetic arrangement, was at the 

 centre turned outwards at the equatorial part, and then made to 

 return at a distance outside the magnet to the place from whence it 

 commenced. At times this wire was in three parts ; the axial part 

 being one, a radial part extending from the centre to the surface at 

 the equator and there connected with a copper ring surrounding the 

 magnet, being another, and the part from this ring on the outside of 

 the magnet, back to the place of commencement, being the third ; 

 and each of these could revolve either separately or in conjunction 

 with the other parts, the electric contact being complete in all the 

 cases, whilst the wire was insulated from the magnet by the cover- 

 ing of silk. The ends of this loop, as it may be called, were con- 

 nected with a galvanometer, and thus the presence or absence of 

 electric currents ascertained, and their amount measured. Two 

 galvanometers were used ; one by Rheinkorf, containing fine wire, 

 and very delicate in its action ; the other, constructed by the author, 

 of copper wire 02 of an inch thick, passing only once round each 

 needle ; this, for abundant currents of low intensity, such as those 

 generated in the moving wire, was found many-fold more delicate 

 in its indications than the former. 



The general relations of a moving wire to the magnetic lines of 

 force are then specified, and a reference is made to their discovery and 

 description by the author in the First Series of these Experimental 

 Researches ; and the law of the evolution of the induced electrical 

 current is given. Referring to an easy natural standard, it may be 

 said, that if a person in these latitudes, where the lines of force dip 

 69 degrees, as shown by the dipping-needle, move forward with 

 arms extended, then the direction of an electric current which would 

 tend to be produced in a wire represented by the arms, would be 

 from the right hand through the arms and body to the left. 



It will be seen, upon a little consideration, that a wire which 

 touches a regular bar magnet at one end, and is then continued 

 through the air until it touches it again at the equator, if moved 

 once round the magnet, slipping at the equator contact so as to 

 resume its first position at the end of the revolution, will have in- 

 tersected, once, all the lines of force external to the magnet, and 

 neither more nor less, whatever its course through the air, or 

 distance in parts from the magnet, may be. Now when the external 

 part of the loop above described is moved in this manner a certain 

 number of degrees round the axis of the magnet, the latter being 

 still, a current of electricity in a given direction is shown by the 

 galvanometer; and the proper precautions (which are described) 

 being taken, the current is of the same amount for the same number 

 of degrees of revolution, whether the motion be quicker or slower, 

 or whether the wire be at a greater or a less distance in its course 

 from the magnet. 



