1867.] Physic. 283 



On imparting rotation to the armature of such an arrangement, the 

 mechanical resistance was found to increase rapidly, to such an ex- 

 tent that either the driving strap commenced to slip, or the insul- 

 ated wires constituting the coils were heated to the extent of 

 igniting their insulating silk covering. It was thus shown to be 

 possible to produce, mechanically, the most powerful electrical or 

 calorific effects, without the aid of steel magnets, which latter were 

 open to the practicial objection of losing their permanent magnet- 

 ism in use. 



The reading of this paper was followed by one by Professor 

 Wheatstone, F.K.S., " On the Augmentation of the Power of a 

 Magnet, by the Eeaction thereon of Currents induced by the 

 Magnet itself." In this magneto-electric machine the core of the 

 electro-magnet is formed of a plate of soft iron, 15 inches in length, 

 and -J inch in breadth, bent at the middle of its length into a 

 horseshoe. Eound it are coiled, in the direction of its breadth, 

 640 feet of insulated copper wire, rath inch in diameter. The 

 armature consists of a rotating cylinder of soft iron, 8 -J inches in 

 length, grooved at two opposite sides, so as to allow the wire to be 

 coiled upon it longitudinally ; the length of the wire thus coiled 

 is 86 feet, and its diameter is the same as that of the electro- 

 magnetic coil. 



If now the wires of the two circuits be so joined as to form a 

 single circuit in which the currents generated, by the armature, 

 after being changed to the same direction, act so as to increase the 

 existing polarity of the electro-magnet, the force required to move 

 the machine will be very great, showing a great increase of mag- 

 netic power in the horseshoe ; and the existence of an energetic 

 current in the wire is shown by its action on a galvanometer, by 

 its heating four inches of platinum wire "0067 inch diameter, by 

 its making a powerful electro-magnet, by its decomposing water, 

 and by other tests. 



The explanation of these effects is as follows : — The electro- 

 magnet always retains a slight residual magnetism, and is therefore 

 in the condition of a weak permanent magnet. The motion of the 

 armature occasions feeble currents, in alternate directions, in the 

 coils thereof, which, after being reduced to the same direction, pass 

 into the coil of the electro-magnet in such a manner as to increase 

 the magnetism of the iron core ; the magnet having thus received 

 an accession of strength, produces in its turn more energetic cur- 

 rents in the coil of the armature, and these alternate actions con- 

 tinue until a maximum is obtained, depending on the rapidity of 

 the motion and the capacity of the electro-magnet. 



It is easy to prove that the residual magnetism of the electro- 

 magnet is the determining cause of these powerful effects. For 

 this purpose it is sufficient to pass a current from a voltaic battery, 



