Theory and practice of the electric telegraph; a manual for operators and students (1875)
Churchill, Charles Henry, d. 1904. [from old catalog] · Oberlin, Ohio, Sherman & bros. · leaf 58
the center. Turn the system overwithout separating and operate^ as before. Open the circuit be-fore removing the steel. 7. Power of the Electro-magnet.—1. What apparatus exhibits the power of the electro-magnet? A thick ring of soft iron is cut into semi* circular halves and a small helix is made… … Ring and Helix.8. Form.—l. What form is favorable for strength?
Telegraph
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