QO 



NA TURE 



\yuite I, 1875 



MA GNE TO-ELECTRIC MA CHINES * 



Tj^EW discoveries in physical science have been more 

 -■- important in themselves, or richer in practical 

 results, than Faraday's discovery of the induction of 

 electrical currents ; and with the exception of the im- 

 mortal work of Newton on the properties of Light, it 

 would be difficult to mention any other experimental 

 nvcstigation, as it first issued from the hands of the 



Pacinotti's Machine. 



author, so complete in all its details, or so full of new 

 and original facts. CErsted's grand discovery, which 

 linked together electricity and magnetism, had already 

 yielded a scientific harvest of uncommon richness. It 

 led immediately to the construction of electro-magnets 

 vastly exceeding in power any permanent magnets which 

 were then known or have since been made. The mul- 

 tiplier or galvanometer of Schweigger supplied a new 

 and important instrument for measuring electrical cur- 

 rents, which, with a little modification, becan^e the electric 

 telegraph. Faraday discovered the rotatory character of 



currents by means of a steel magnet— was in 1831 com- 

 pletely solved in the exhaustive memoir by Faraday, in 

 which he announced the discovery of the induction of 

 electrical currents. It may be interesting to describe, 

 nearly in his own words, Faraday's original experiments. 

 Two helices of insulated copper wire were passed 

 round a wooden block, the ends of the wire of one helix 

 being connected with a voltaic battery, and those of the 

 other with a galvanometer. So long as the current from 

 the battery passed through the first helix 

 the needle of the galvanometer remained 

 motionless, but on breaking the connec- 

 tion with the battery, a momentary cur- 

 rent, as indicated by the galvanometer, 

 traversed the wire of the second helix. 

 The direction of this current was the 

 same as that of the primary current of 

 the battery. When the first helix was 

 connected with the battery, another mo- 

 mentary current traversed the second 

 helix, but in this case it was in the oppo- 

 site direction to the primary current. 

 Substituting for the first hehx and the 

 voltaic battery a permanent steel magnet 

 or an electro-magnet, Faraday found that 

 on introducing one end of the magnet 

 into a hollow helix a temporary current 

 was produced in the wire of the helix in 

 one direction, and on withdrawing it an- 

 other temporary current occurred in the 

 opposite direction. For artificial magnets 

 the magnetism of the earth may be sub- 

 stituted, and thus electrical currents can 

 be obtained by induction from the mag- 

 netic conditions which everywhere prevail on the sur- 

 face of this globe. The singular phenomenon first 

 described by Arago, and afterwards elaborately inves- 

 tigated by Babbage and Herschel, that when a copper 

 plate is rotated below a freely suspended magnet the 

 latter tends to follow the motion of the plate, was shown 

 by Faraday to arise from electrical currents induced by 

 the magnet in the rotating metallic disc. 



Soon after the announcement of these important 

 results, Pixii constructed in Paris the first magneto- 

 electric machine. I have still a vivid recollection of this 

 machine as I saw it in Pixii's workshop. The currents 

 were obtained by the rotation of a powerful horse-shoe 

 magnet in front of an armature composed of two short 



Fig. 2.— Pacinotti's Machine (Plan). 



the reciprocal action of magnets and electrical currents ; 

 and Ampere showed that all the properties of a per- 

 marent magnet could be explained on the hypothesis of 

 electrical currents in a fixed direction circulating around 

 the magnet. A problem which proved to be one of 

 surpassing difficulty, and long baffled many of the most 

 distmguished physicists of Europe— to obtain electrical i 



T.ir:iI!!^v"VQ "" °^T Lecture with additions, delivered .at the Belfast i 

 Philosophical Society, March 17, by Dr. Andrews, F.R.S., L. & E, 



bars of soft iron with a connecting crossbar, the latter 

 being surrounded by a long coil of copper wire covered 

 with silk. The armature had, in short, nearly the form 

 of a horse-shoe electro-magnet. With this machine 

 electrical sparks were obtained, and water was freely 

 decomposed. In the rotation of the magnet the faces 

 of the armature or electro-magnet became successively 

 north and south poles with intermediate conditions of 

 neutrality, and the direction of the current changed at 

 every semi-revolution of the magnet. Hence, in the 

 decomposition of water and other dectrolytes, the ele- 



