Experimental science. Elementary, practical and experimental physics (1890)
Hopkins, George M. (George Milton), 1842-1902 · New York, Munn & co. · leaf 500
hragm recedes from the magnet the current set upin the bobbin is in the opposite direction. In the telephonethese currents have sufficient power to operate a secondinstrument of the same sort; but owing to the fact that thearmature is very light, and never touches the magnet norrecedes very far… … Another form of apparatus (Fig. 465), operating on thesame principle, generates currents sufficiently powerful towork a polarized bell or annunciator over a line severalmiles long. This magneto key is made by clamping two six-inch horseshoe magnets upon opposite sides of two soft ironpole extension…
Physics · Lantern projection · Electric apparatus and appliances
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