Augmentation of the Power of a Magnet by reaction. 471 



strap commences to slip or the insulated wires constituting the coils 

 are heated to the extent of igniting their insulating silk covering. 



It is thus possible to produce mechanically the most powerful elec- 

 trical or calorific effects without the aid of steel magnets, which latter 

 are open to the practical objection of losing their permanent magne- 

 tism in use. 



" On the Augmentation of the Power of a Magnet by the reaction 

 thereon of Currents induced by the Magnet itself." By Charles 

 Wheatstone, F.R.S. 



The magneto-electric machines which have been hitherto described 

 are actuated either by a permanent magnet or by an electromagnet 

 deriving its power from a rheomotor placed in the circuit of its coil. 

 In the present note I intend to show that an electromagnet, if it pos- 

 sess at the commencement the slightest polarity, may become a pow- 

 erful magnet by the gradually augmenting currents which itself origi- 

 nates. 



The following is a description of the form and dimensions of the 

 electromagnet I have employed. The construction, it will be seen, is 

 the same as that of the electromagnetic part of Mr. Wilde's machine. 



The core of the electromagnet is formed of a plate of soft iron 15 

 inches in length and ^ an inch in breadth, bent at the middle of its 

 length into a horseshoe form. Round it is coiled in the direction 

 of its breadth, 640 feet of insulated copper wire -^ of an inch in 

 diameter. The armature, which is according to Siemens's ingenious 

 construction, consists of a rotating cylinder of soft iron 8J 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 

 80 feet, and its diameter is the same as that of the electromagnet 

 coil. 



When this electromagnet is excited by any rheomotor the current 

 from which is in a constant direction, during the rotation of the 

 armature currents are generated in its coil during each semirevolution 

 which are alternately in opposite directions ; these alternate currents 

 may be transmitted unchanged to another part of the circuit, or by 

 means of a rheotrope be converted to the same direction. 



If now, while the circuit of the armature remains completed, the 

 rheomotor be removed from the electromagnet, on causing the arma- 

 ture to revolve, howeyer rapidly, it will be found, by the interposition 

 of a galvanometer, or any other test, that but very slight effects 

 take place. Though these effects become stronger in proportion to 

 the residual magnetism left in the electromagnet from the previous 

 action of a current, they never attain any considerable amount. 



But if the wires of the two circuits be so joined as to form a sin- 

 gle 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 electromagnet, very different results will be obtained. 

 The force required to move the machine will be far greater, showing 

 a great increase of magnetic power in the horseshoe ; and the exist- 

 ence of an energetic current in the wire is shown by its action on a 



