An account of the Electro- Magnetic Engine. 121 



he required a battery presenting a surface of platina of twenty square 

 feet to obtain one horse-power, but that he hoped to be enabled to at- 

 tain the same power with one from three to ten square feet. Accord- 

 ing to the experiments which Lenz and Jacobi instituted, as to the 

 laws of electro-magnets, it appeared, that the attraction of the electro- 

 magnets was proportional to the products of the magnetising cur- 

 rents ; or, if these be equal, to the squares of the magnetising cur- 

 rents. From the results of the experiments undertaken with a view 

 to confirm the law, that the attraction of two electro-magnets, or an 

 electro-magnet and a bar of soft iron, was as the squares of the 

 strength of the magnetising currents ; it appears that the attraction of 

 two rectilinear electro-magnets, or of an electro-magnet and an anchor, 

 is as the squares of the magnetising streams, provided they both do not 

 touch, but continue at the distance of about a line from one another. 



Jacobi's latest experiments on the chemical and magnetic galvano- 

 meter confirm the law, that the chemical forces are exactly proportional 

 to the magnetical, and that we can deduce a standard for the con- 

 sumption of zinc from the indications of a magnetic needle, submitted 

 to the influence of the magnetic currents employed. But the con- 

 sumption of zinc being proportional to the expenses of maintenance, 

 we can accordingly deduce the cost of working the machine from the 

 magnetic needle. 



The first machine constructed by Jacobi, and set in motion by means 

 of electro-magnetism, was on the following plan. ( See plate.) 



The principal figure of the plate represents a magnetical apparatus, 

 in which eight bars (four bars of soft iron in the shape of horse-shoes) 

 are symmetrically attached to a wooden frame, revolving round a hori- 

 zontal axle, A, and eight other similar, arranged on a sufficiently 

 strong fixed frame. The arrangement of the cylinders admits of every 

 possible variety, it being only necessary that they be symmetrically 

 arranged, and that their poles shall pass each other as closely as 

 possible. Since in all probability the centre of the magnetical gravity 

 is situated at some little distance from the extremity, as is the case 

 with the ordinary magnetic bars, it would be better to arrange it so 

 that the axes of the cylindrical bars should be at right angles to one 

 another, instead of parallel, as in the figure. It must likewise be 

 observed, that there is some difficulty in procuring cylinders of a horse- 

 shoe shape,* so that the axes of the bars be exactly at the same 

 distance, and the bars themselves exactly cylindrical ; all repairs by 



* This relates to Germany. 



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