The elements of natural philosophy : copiously illustrated by familiar experiments and containing descriptions of instruments, with directions for using : designed for the use of schools and academies (1856)
Sprague, A. W. (Alfred White), 1821-1891 · Boston : Phillips, Sampson & Company · leaf 265
Fig. 212. … * The magnetic power of the electro-magnet depends on the number of turnsthe wire takes around the bar ; thus, a small magnet, closely wound with finecopper wire, may be rendered far more powerful than a larger bar loosely woundwith a coarse wire. The former magnets are used for telegraphs,…
Physics
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