Coal; its history and uses . ly magnetised and demag-netised makes it a useful instrument for imparting motionto other masses of iron. A well-known form of electric engine is represented inFig. 56. It consists of a wheel, to the circumference ofwhich eight bars of soft iron are attached, at equal dis-tances. Four electro-magnets are placed close to thewheel, and matters are so arranged that the currentcirculates in the wire wrapped round each only whenone of the bars of iron is approaching and near to it, andis interrupted as the iron passes it. The bars of iron 282 COAL. CHAP VIII. are thus a
Coal; its history and uses . ly magnetised and demag-netised makes it a useful instrument for imparting motionto other masses of iron. A well-known form of electric engine is represented inFig. 56. It consists of a wheel, to the circumference ofwhich eight bars of soft iron are attached, at equal dis-tances. Four electro-magnets are placed close to thewheel, and matters are so arranged that the currentcirculates in the wire wrapped round each only whenone of the bars of iron is approaching and near to it, andis interrupted as the iron passes it. The bars of iron 282 COAL. CHAP VIII. are thus always pulled by the attraction of the temporarymagnets in the direction in which the wheel is rotating,and never in the direction which would retard its continuous and rapid revolution can thus be main-tained. The passage of the current is regulated by acog-wheel, the projections on which act upon rollers at-tached to metal springs. These when pressed by thecogs touch a fixed metallic button and complete the circuit. Fig. 56.—Electric Engine. through which the current passes, but when the pressureis removed they regain their original position and thecurrent ceases to flow. The electro-magnets are thusalternately active and inactive at the proper times. In order to understand fully the source of the energyof this or any other form of electric engine, it will benecessary to dwell upon the circumstances under which agalvanic current is produced. If some granulated zincbe dropped into dilute sulphuric acid a copious evolution CHAP. viii. COAL AS A SOtTKCE OF POAVEK. 283 of gas takes place, and the zinc and acid are zinc displaces the hydrogen, and unites with thesulphur and oxygen of the sulphuric acid, sulphate ofzinc is formed, and the hydrogen as it is liberated rises tothe surface of the liquid. The heat produced is due tothe molecular motion set up during the substitution ofthe zinc for hydrogen, and amounts to 101891 units foreach pound of zinc dissolved
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Keywords: ., bookcentury1800, bookdecade1870, bookpublisherlo, booksubjectcoal
