Submarine mine cable is shipped on reels having an outer sheathing for protection in transit, with at least 12 feet of both ends of the cable brought out and coiled on the head of the reel for test purposes. If the cable is not for immediate use, it should be moved to the cable tank, and by means of the overhead trolley and cable tongs put in its position in the tank, the two ends being properly tagged and firmly fixed so as to allow it to be tested. In arranging the multiple cable in the tanks that which is to be used first should be most readily accessible.
The cable tank should be provided with a cover to keep it clean, as well as to lessen as much as possible variations of temperature. Enough clean water to cover by several inches the outer sheathing of the cable reels should be kept in the tanks, but in climates where the water in the cable tanks would normally freeze to a depth exceeding 2 feet, the water should be let out of the tanks before ice begins to form and not again admitted until the following spring. In localities where the tanks may become a breeding place for mosquitoes, as a preventive measure, salt water from the ocean or bay should, when practicable, be used for filling the tanks, or where it is necessary to use fresh water sufficient salt should be added to produce a 3 per cent solution. No oil or kerosene should be used in the tanks.
The methods of recording tests and of classifying and transferring submarine mine cable are prescribed by orders from the War Department. The tests of submarine mine cable at posts will consist in determining the insulation and conductor resistances.
The insulation surrounding the conductor of a cable is supposed to be uniform in regard to quality of material, density, and thickness. The resistance which it offers to the passage of a current through it will then vary inversely with its length. In comparison the insulation resistance of 1 mile of cable is taken as the standard. This insulation has a large negative temperature coefficient; that is, an increase of temperature lowers its resistance. It is customary to reduce all insulation resistance to that at a standard temperature of 60° F., and for this purpose reduction factors applicable to the particular insulation compound should be furnished with the cable. (Note: It has been found that for most compounds, if the logarithms of the resistance are plotted as ordinates against the temperature in degrees F. as abscissæ, the resulting curve will be very nearly a straight line.)
The ordinary methods of measuring resistance—that is, by means of a Wheatstone bridge, or by fall of potential, or by voltmeter—can not be used in measuring resistance as high as that of the insulation of a submarine cable. For this the direct deflection method is employed.
In brief, this consists of the following steps:
First. The deflection produced in a galvanometer by a current from a battery through a known resistance, usually 100,000 ohms, is determined, whence is calculated the resistance through which this same battery would produce a deflection of one point using the unity shunt. This is expressed in megohms and is called the galvanometer “constant” under the conditions.
Second. The deflection produced by the current from the same battery through the insulation of the cable is determined, whence, from “First,” the corresponding number of megohms is calculated.
Third. This multiplied by the length of the cable in miles and corrected for temperature gives the required insulation resistance per mile.
This testing can be made most satisfactorily on dry days, but a close adherence to the instructions herein given relative to the preparation of the cable ends, the insulation of the cable lead and of the battery, and the drying out of the test room and instruments should enable satisfactory work to be done under adverse conditions of weather or climate. The following apparatus is required: Reflecting galvanometer, universal shunt, special testing key, 100,000-ohm resistance box, battery of dry cells giving approximately 100 volts, and stop watch.