Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . Fi£ 143.—Testing the PolaritySingle Turn of Wire. of a The turn is connected to a floating batteryand is free to move. ELECTR OMA GNETISM. 163. Fig. 144. handed. The polarity can be reversed by re-winding in theopposite direction and passing the current, as indicated inFig. 144, or by simply sending the current through the helixin the opposite solenoid is a coil of wire,generally wound on awooden or brass spool, thelength of which is muchgreater than the diameter,Fig 145.
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . Fi£ 143.—Testing the PolaritySingle Turn of Wire. of a The turn is connected to a floating batteryand is free to move. ELECTR OMA GNETISM. 163. Fig. 144. handed. The polarity can be reversed by re-winding in theopposite direction and passing the current, as indicated inFig. 144, or by simply sending the current through the helixin the opposite solenoid is a coil of wire,generally wound on awooden or brass spool, thelength of which is muchgreater than the diameter,Fig 145. The winding isalways in the same direc-tion, layer upon layer, simi-lar to the winding of a spool of thread. The spirals of ahelix or solenoid are equivalent in their magnetic action to asmany circular currents as there are convolutions of wire, sincetheir axes lie in the same straight line. The magnetic whirlsof each turn inside the helix are in the same direction as everyother turn, and the direction of the magnetic field along, andparallel to the axis of the solenoid, is straight and fairly uni-form to within a short distance of the ends. The total fieldis the sum of the magnetic lines of each individual turn asillustrated in the helix, Fig. 1
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