With the materials now available, and the skill which has been acquired in their manufacture, both the resistances and the standard cells are obtainable which are remarkably constant, and both can be readily checked for accuracy.
Location of Opens.--These measurements are based on the fact that the capacity of wires in a cable is ordinarily a measurable quantity, which, in wire of uniform diameter, is proportionate to length. In making these tests, a fault finder is used together with a buzzer, dry cells to operate it, small induction coil, and telephone receiver. These instruments are to be found in any telephone exchange. It is best to locate the buzzer at some distance from the fault finder in order that it cannot be heard by the operator.
Fig. 605.--To use galvanometer of Leeds and Northrup fault finder in series with the battery: Set switches as shown, and connect between posts Gr. and 2 (see figs. [587] and [588]). The galvanometer will have the maximum sensibility with the pointer at 1,000 and the minimum at zero.
Fig. 606.--To measure high resistances, such as the resistances of faults with Leeds and Northrup fault finder. First Method.--Arrange the switches as shown in the figure. Connect posts Gr. and 2, turn the handle until the galvanometer needle comes to rest at an even deflection of ten divisions. Call the reading A. Connect in the unknown resistance between Gr. and 2. Now close the switch W, so that the figure 1 appears on the top of the block, and again bring the galvanometer to a deflection of ten divisions and call the reading B. Then X = (10,000 B ÷ A) - 1,000. In case X be a high resistance, it will be found that the galvanometer will not deflect ten divisions for any position of the pointer. In such case, choose a number of divisions which is a factor of ten, such as 5, 2, or 1, and multiply (10,000 B ÷ A) by ten divided by the number chosen, as 10/5, 10/2, 10/1. For example, for a deflection of two divisions: X = (10/2)(10,000 B ÷ A) - 1,000. The satisfactory range of the set for high resistance measurement may be increased by using an outside battery of higher voltage. With the contained battery, satisfactory measurements can be made up to 1 or 2 megohms. When outside battery is used, connect one terminal to the post Ba, and the other to 2 for the reading A. Connect the battery and unknown resistance in series between these posts for the reading B. When an outside pressure of 25 volts or over is used, the switch W should not be closed unless there be a resistance in series with the battery of 10,000 ohms or over. Second Method.--For use as a voltmeter to measure high resistances. (More convenient but not quite as accurate as first method.) Set the switches to RV, M and 10. Turn the knurled nut on the galvanometer so as to set the needle to the extreme right hand side of the scale. Connect the posts 2 and Gr. with a short piece of wire. Turn the rotating pointer on the scale until the galvanometer needle moves over about 20 scale divisions when the battery key is closed. Remove the connection between 2 and Gr. as the voltmeter terminals. This makes a simple way of testing for various kinds and amounts of trouble. On a wet cable a deflection of 10 to 15 divisions indicates heavy enough trouble to locate with the fault finder. With a little care, trouble showing only 5 or 6 divisions can be located.
Before attempting locations for opens it is well to make the following measurements:
1. The insulation of the broken wire and the insulation of the good wire with which it is to be compared.
This may be done as shown in [fig. 606]. It is best that the insulation resistance be fairly good, but experiments indicate that good results can be obtained by the methods which follow, even when the insulation is as low as 100,000 ohms, and fair results when as low as 50,000 to 100,000 ohms.
