. A text-book of electrical engineering;. B^ centimetre of the field in air is equal to g- ergs. Hence OTT B^energy stored in air-gaps = ^ .A .dl ergs, therefore f .dl= —^—. dl, OTT and /= —o^— dynes (56). If F be the force in kilogrammes, F=^ ^\^ =4g^^.io-«kgs* (57). 8?r . 981,000 In a horse-shoe magnet both poles are effective, and the double cross-section must therefore be included in A. If, for example, the cross-sectionof each pole is 10 sq. cm and the induction B = 18,000, we have F = ,000^.^ = 260 kilogrammes. The lifting power of a magnet is often largely affected by magneti
. A text-book of electrical engineering;. B^ centimetre of the field in air is equal to g- ergs. Hence OTT B^energy stored in air-gaps = ^ .A .dl ergs, therefore f .dl= —^—. dl, OTT and /= —o^— dynes (56). If F be the force in kilogrammes, F=^ ^\^ =4g^^.io-«kgs* (57). 8?r . 981,000 In a horse-shoe magnet both poles are effective, and the double cross-section must therefore be included in A. If, for example, the cross-sectionof each pole is 10 sq. cm and the induction B = 18,000, we have F = ,000^.^ = 260 kilogrammes. The lifting power of a magnet is often largely affected by magneticleakage, which we have here neglected. We shall now consider what happens when an electromagnet attractsits armature. We shall assume that the magnetisation curves of the magneticcircuit, both before and after the attraction of the armature, are representedby straight lines through the origin. In Fig. 65 OD and OC represent themagnetisation curves before and after attraction, the length of each air-gap 74 Electrical Engineering. being l-^jz before and IJ2 after attraction. The travel of the armature is then(^1 — ^2)/2- If the winding is connected to mains of constant the current-turns will have the same valueafter as before. In Fig. 65 OErepresents the current-turns andOA and OB the original and sub-sequent values of the magneticflux. It has been shown that theadditional energy stored in amagnetic field when the flux in-creases is given by the formula dW = TI. d<^ ergs, where I is the current in ab-^S- 65 solute units. Hence the total energy stored in the magnetic field when = OEand the flux O^ = OD will be represented by the area of the triangle ODAwhich is equal to that of the triangle OED. When the armature isattracted and the flux has increased to Og = OC the store of energy isincreased to OEC. Hence the energy stored in the magnetic field has in-creased by an amount represented by the area ODC. Hence the mechanical energy required to raise the armat
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