[Figure 16] shows diagrammatically the arrangement of the apparatus for testing a reel of cable. As a rule the instruments should be so placed that one person may manipulate the key and the shunt while at the same time observing the galvanometer.
The 100,000-ohm box, as a protection to the galvanometer in testing, is always kept in the circuit and its value should be subtracted from the resistance determined, except in the case of high insulation resistance when it will not be necessary to make the subtraction.
The universal shunt is always employed with the galvanometer and is used both to vary the current through the latter and to protect it from a violent throw at the instant of making or breaking the circuit at the testing key. This last is accomplished by having the shunt on zero at such times.
The galvanometer being a very sensitive instrument must be solidly supported so as to be free from jars or vibrations.
The special testing key, shown diagrammatically in the figure, has its binding posts plainly marked. It is a double-throw key and has two positions upon each side. When completely closed to the right, the cable is charged through the galvanometer from the positive pole; when to the left, from the negative pole of the battery. In each case the deflection of the galvanometer is in the same direction. When partly closed on either side, the cable is discharged to earth through the galvanometer. (Note: It will be observed that the connections are such that the galvanometer is always connected to the cable core and never to the ground. With this connection, so long as the lead PX is free from leaks or grounds, the galvanometer measures only the current actually passing through the core and not that leaking through any imperfect insulation in the battery and leads.)
Cable testing is a very simple operation, but extreme care is necessary in all operations.
FIG. 16.—CABLE TESTING.
The following is a detailed description:
I. Preparing the cable for testing.—1. Closely examine each conductor end. Look particularly for unusually hard or brittle insulation and for torn, pinched, or punctured insulation, especially near the ends of the armor wires. If any of the ends are not in perfect condition, cut off enough cable to secure good ends. (Caution.—Do not cut off more than enough to secure good ends, for after three or four tests it may be necessary to unreel the whole cable to secure enough of the inner end above water.)