1867.] Augmentation of the Power of a Magnet by reaction. 369 



electro-magnets. On imparting rotation to the armature of such an arrange- 

 ment, the mechanical resistance is found to increase rapidly, to such an 

 extent that either the driving-strap commences to slip or the insulated 

 wires constituting the coils are heated to the extent of igniting their insu- 

 lating silk covering. 



It is thus possible to produce mechanically the most powerful electrical 

 or calorific effects without the aid of steel magnets, which latter are open to 

 the practical objection of losing their permanent magnetism in use. 



III. 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. Received February 14, 1867. 



The magneto-electric machines which have been hitherto described are 

 actuated either by a permanent magnet or by an electro-magnet deriving 

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

 note I intend to show that an electro-magnet, if it possess at the commence- 

 ment the slightest polarity, may become a powerful magnet by the gradu- 

 ally augmenting currents which itself originates. 



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

 magnet I have employed. The construction, it will be seen, is the same as 

 that of the electro-magnetic part of Mr. Wilde's machine. 



The core of the electro-magnet 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 mch in diameter. The armature, 



which is according to Siemens' s ingenious construction, consists of a rota- 

 ting cylinder of soft iron 8| 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 

 electro-magnet coil. 



When this electro-magnet is excited by any rheomotor the current from 

 which is in a constant direction, during the rotation of the armature cur- 

 rents are generated in its coil during each semirevolution, which are alter- 

 nately 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 rheomo- 

 tor be removed from the electro-magnet, on causing the armature to revolve, 

 however 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 

 electro-magnet 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 single cir- 



