Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1891 edition. Excerpt: ...cable-testing are scarcely applicable to every-day work, but should always be applied when testing cables at a factory for acceptance or rejection, as the various points enumerated above throw considerable light on the specific quality of the dielectric and its prospective durability. It is not to be imagined that an relaxation of careful methods is advocated in every-daj work, but it is not generally necessary in making periodical tests to take reversals or to take more than one minute's reading with the zinc current on each wire, except in the case of new cables or when a fault is suspected. CHAPTER XI. Test for Insulation.--(Continued.) To return to our specific example of the test for insulation resistance. The behavior of the spot should be carefully watched, and at the end of one minute from the time of turning on the current the exact deflection should be noted. This deflection serves for working out the insulation resistance by comparison with the galvanometer constant; the figures for insulation resistance are generally stated to be "after one minute's electrification." We will assume that the cable being tested is 2,640 feet long and that the deflection after one minute is 60 divisions on the scale. The constant of the galvanometer being 30,000, the absolute insulation resistance of the cable is-3 W9 = 500 megohms. As cables vary in length it is obviously useless, for purposes of comparison, to know the insulation resistance of a cable without also knowing its length, and even so it is not convenient to have to refer to two separate quantities; therefore it is usual to speak of the resistance per unit of length, and the insulation resistance per mile is the figure by which we can best judge of the actual condition of a...
