Modern practice of the electric telegraph. A handbook for electricians and operators (1872)
Pope, Frank L. (Franklin Leonard), 1840-1895 · New York : D. Van Nostrand · leaf 73
FIG. 34. Fio. 35. 94. FARMERS HARD RUBBER INSULATOR.—This isshown in Fig. 34. It is a good insulator when new, INSULATION. 61 but by exposure to the weather its surface becomesrough and spongy, and retentive of moisture. It isscrewed to the under side of the cross-arm or woodenblock, which is…
Telegraph
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