288 



ELECTRICITY. 



ployed being found sufficient for the purpose. 

 The mechanical contrivance best suited for the 

 production of these currents, consists of a 

 cylindrical keeper hollowed at two sides for the 

 reception of insulated wire wound longitudi- 

 nally, which is made to rotate between the 

 Eoles of a series of permanent magnets, which 

 itter are at present replaced by electro-mag- 

 nets. On imparting rotation to the armature 

 of such an arrangement, the mechanical resist- 

 ance 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 so as to ignite their silk cover- 

 ing. It is thus possible to produce mechan- 

 ically the most powerful electrical or calorific 

 efforts without the aid of steel magnets, which 

 latter are open to the practical objection of 

 losing their permanent magnetism in use. 



Self -augmentation of the Power of a Mag- 

 net. Mr. Wheatstone has published illustra- 

 tions of the property of an electro-magnet to 

 increase its magnetic power by the gradually 

 augmenting currents which itself originates. 

 The construction of the electro-magnet used 

 by Mr. "Wheatstone is the same as that em- 

 ployed in Mr. Wilde's machine (see ANNUAL 

 CYCLOPAEDIA, 1866). The core of the electro- 

 magnet with which Mr. Wheatstone experi- 

 mented was formed of a plate of soft iron fif- 

 teen inches long and one-half inch broad, bent 

 at the middle of its length into a horse-shoe 

 form. Eound it was coiled, in the direction of 

 its breadth, 640 feet of insulated copper wire 

 one-twelfth of an inch in diameter. The arma- 

 ture (upon the plan of Siemens) consisted of a 

 rotating cylinder of soft iron, eight and a half 

 inches long, grooved at two opposite sides so as 

 to allow the -wire to be coiled up longitudinally ; 

 the length of the wire thus coiled is eighty feet, 

 and its diameter the same as that of the electro- 

 magnet coil. When this electro-magnet is ex- 

 cited 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 semi-revolution which are alter- 

 nately in opposite directions ; these alternate 

 currents may be transmitted unchanged to 

 another part of the circuit. If, now, while the 

 circuit of the armature remains complete, the 

 rheomotor is removed from the electro-magnet, 

 on causing the armature to revolve however rap- 

 idly, it will be found by the interposition of a 

 galvanometer, or any other test,' that but very 

 slight effects take place. But, if the wires of 

 the two circuits be so joined as to form a single 

 circuit, very different results will be obtained. 

 The force required to move the machine will be 

 far greater, showing a great increase of mag- 

 netic power in the horseshoe ; and the exist- 

 ence of an energetic current in the wire is 

 shown by its action on a galvanometer, by its 

 heating four inches of platinum wire, .0067 

 inches in diameter, by its making a powerful 

 electro-magnet, and other tests. A very remark- 

 able increase of all the effects, accompanied by 



a diminution in the resistance of the machine, 

 is observed when a cross wire is placed so as to 

 divert a great portion of the current from the 

 electro-magnet. The platinum wire, instead of 

 flashing to redness and then disappearing, re- 

 mains permanently ignited; the inductorium, 

 which before gave no spark, now gives one one- 

 quarter inch in length, and all the other effects 

 are singularly increased. But these effects are 

 far inferior to those obtained by causing them 

 to take place upon the cross wire itself. With 

 the same application of motive power (the 

 power of two men), seven inches of platinum 

 wire were made red hot, and sparks two and a 

 half inches long were elicited in the inductori- 

 um. The phenomenon of the augmentation of 

 power is thus explained: The electro-magnet 

 always retains a slight residual magnetism, and 

 is therefore in the condition of a weak perma- 

 nent magnet ; the motion of the armature oc- 

 casions feeble currents in alternate directions 

 in the coils thereof, which, after being reduced to 

 the same direction, pass into the coil of the elec- 

 tro-magnet in such manner as to increase the 

 magnetism of the iron core; the magnet hav- 

 ing thus received an accession of strength, pro- 

 duces in its turn more energetic currents in the 

 coil of the armature ; and these alternate ac- 

 tions continue until a maximum is attained, 

 depending on the rapidity of the motion and 

 the capacity of the electro-magnet. In conclu- 

 sion, the author remarks that there is an evi- 

 dent analogy between the augmentation of the 

 power of a weak magnet by means of an induc- 

 tive action produced by itself, and that accumu- 

 lation of power shown in the static electric 

 machines of Holtz and others, in which a very 

 small quantity of electricity directly excited is, 

 by a series of inductive actions, augmented so 

 as to equal, and even exceed, the effects of the 

 most powerful machines of the ordinary con- 

 struction. 



Another Form of the Dynamo-Magnetic Ma- 

 chine. Mr. W. Ladd read before the British 

 Association, at its summer session, a paper 

 describing a new electro-magnetic machine, of 

 his own construction. It consists, he says, 

 chiefly of two plates of iron ; to both ends of 

 each plate is fixed a portion of a hollow cylin- 

 der ; these plates are then placed a certain dis- 

 tance apart, and insulated from each other in 

 such a manner that the cylindrical pieces will 

 form two hollow circular passages ; into these 

 spaces two armatures (Siemens's) are placed. 

 The plates are surrounded by a quantity of 

 stout copper wire connected together, the two 

 terminals of which are brought into connection 

 with the commutator of the smaller armature, 

 so that each change of polarity in the armature 

 will augment the magnetism. When the ma- 

 chine is first constructed it is only necessary to 

 pass a current near a small cell of Smee's or any 

 other element, after which it will retain a suffi- 

 cient amount of magnetism for all future work. 



If the armature in connection with the 

 electro-magnet is made to rotate, there will bo 



