A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . expenditure of electricenergy and tends to weaken the electric current that main-tains it will be discussed later (§ 406). Certain experiments will presently be described, whichillustrate the development of electric motors from thestage of mere scientific interest to that of commercialimportance. 400. An Elementary Motor.—Fig. 280 represents a coilof wire, C, mounted in such a way as to revolve freelybetween the poles, N and S, of a horseshoe magnet. Theends of the coil a
A text-book of physics, largely experimentalOn the Harvard college "Descriptive list of elementary physical experiments." . expenditure of electricenergy and tends to weaken the electric current that main-tains it will be discussed later (§ 406). Certain experiments will presently be described, whichillustrate the development of electric motors from thestage of mere scientific interest to that of commercialimportance. 400. An Elementary Motor.—Fig. 280 represents a coilof wire, C, mounted in such a way as to revolve freelybetween the poles, N and S, of a horseshoe magnet. Theends of the coil are soldered to plates of metal m and m,which are fastened to the upright axis a, but are insulatedfrom it, that is, separated from it by some non-conductorof electricity. The arch at the top of the figure is of brass,and supports two binding-posts, p and pf, which are in- ELEC TROMAGNE T1SM. 511 sulated from it. From p and pr elastic strips of metal, sand s, reach to the plates m and mf. A current of electricity entering at p will pass by s to m,then through c to m, and out by way of s and pr. The cur- ai> SZ^L. Fig. 280. rent used with this apparatus may be supplied by one ormore Daniel cells. At the centre of a coil the line of the magnetic force dueto a current in the coil itself is (see Exercise 53) at rightangles with the plane of the windings, and so directed that, 512 PHYSICS. to an observer looking forward along it, the current circu-lates clockwise in the coil. This line of magnetic force willnow be called the axial magnetic line of the coil. The lines of magnetic force of the magnet are directedfrom N to S. EXPERIMENT i. Fasten back the springs s and sf so that they shall not touch mand mf. Setting the coil in various positions, touch the batterywires to in and m, and continue the experiments until the followingfacts are illustrated: 1st. That there are two positions of the coil from which the currentdoes not turn it, namely, those in which the axial magnetic li
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