550 yds. of 25,000 cir. mil. = 880 yds. of 40,000 cir. mil. 500 " " 40,000 " " = 500 " " 40,000 " " 1,050 " " 30,000 " " = 1,400 " " 40,000 " " This makes the total resistance of the loop equivalent to 2,780 yards of 40,000 cir. mil. If the contact show a balance for a reading of 372.5, this indicates that the fault is at a distance of 372.5/1,000 of 2,780 = 1,035.5 equivalent yards. Of this, 880 are in the stretch A E. Consequently the fault is: 1,035.5 - 880 = 155.5 yards from E. [Illustration: Fig. 618.--The Fischer portable cable testing set, designed for locating crosses, grounds and breaks in cables, also for conductor and liquid resistance measure. The distinguishing feature of the set is the _master switch_. By means of this switch, connections can be made for the various tests by a single movement, thus avoiding the labor and time which have to be expended in interchanging the connections and memorizing the rather complicated scheme of connections.] CHAPTER XXVIII AMMETERS, VOLTMETERS AND WATTMETERS. An ammeter or ampere meter is simply a commercial form of galvanometer so constructed that the deflection of the needle indicates directly the strength of current _in amperes_. A good ammeter should have a very low resistance so that very little of the energy of the current will be absorbed; the needle should be dead beat, and sufficiently sensitive to respond to minute variations of current. According to the principle of operation, ammeters and voltmeters are classified as: 1. Moving iron; 2. Moving coil; 3. Solenoid or plunger; 4. Magnetic vane; 5. Hot wire; 6. Electrostatic; 7. Astatic; 8. Inclined coil; 9. Fixed and movable coil. Again, they are divided according to their use into two classes: 1. Portable type; 2. Switchboard type. _Milli-ammeters_ or milli-voltmeters are instruments in which the scale is graduated to read directly in thousandths of an ampere or thousandths of a volt respectively. Ques. Describe the moving iron type instrument. Ans. The arrangement of the working parts are shown in fig. 620. A soft iron needle N, is pivoted inside of a coil C, and is held out of line with the axis of the coil by means of a permanent magnet M, when the instrument is idle. In this position, a pointer P, which is attached to the needle, stands at the zero mark of the scale S. If a current be passed through the coil, magnetic lines of force are set up in its center, which tend to pull the needle into line with them, and therefore with the axis of the coil. This pull is resisted by the permanent magnet M, and the amount of deflection of the needle from the zero position depends upon the strength of the current or the voltage according as the coil is wound to indicate amperes or volts.