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For the most part, many meters are now termed multimeters. This indicates that they may be used for volts, amperes, and also sometimes for ohms. This is accomplished by a switch that connects the meter movement internally to the desired connections. Caution: Never connect an ammeter across the line, nor connect an ohmme-ter to an energized circuit. Always start taking your readings on the highest scale; then move the switch to lower scales as needed.
Edison Pendulum Ammeter
The Edison pendulum ammeter, illustrated in Figure 14-4, consists of a solenoid A and a soft iron core B pivoted at C, with a counterweight D to hold the core at the entrance to the solenoid. You may
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observe that this meter has to be level so the pointer would zero at no current. The iron core will be drawn into the solenoid in proportion to the value of current in A.
The battery guage is similar to the Edison pendulum ammeter just described, except a phosphor-bronze spring is used instead of the counter weight, and the solenoid has two windings: one of heavy
conductor connected to A and B for the ammeter, and another of fine wire connected to A and C for the voltam-meter (Figure 14-5).
A voltage tester (this is not a voltmeter), the type of which is standard equipment to the electrician, is shown in Figures 14-6 and 14-7.
The Amprobe voltage tester provides an indication of the voltage level.
Figure 14-5 Battery gauge. Lightweight, compact units withstand
rough usage. Units use neon lamps to indicate voltage level, based on the old thermometer style.
As previously stated, most voltmeters and ammeters are of the D’Arsonval movement type. See Figure 14-8. A permanent magnet M has two soft iron poles P and P. Between these poles is a smaller soft iron core C, which is supported by brass plates attached to the poles. This core C is mounted on a shaft supported by jewel bearings in these brass plates. A coil of fine copper wire W is wound around the soft iron core C horizontally. There are phosphor-bronze coiled springs, as were shown in Figure 14-1, at each end of the core. These springs control the movement of the core and also serve as flexible leads to supply the electrical current to the moving coil. Maxwell’s rule again prevails, and the coil and core tend to turn to embrace the most magnetic flux. A pointer attached to the
Instruments and Measurements 165
Figure 14-6 Voltage
tester. CourtesyAmprobe Instrument Division of SOS Consolidated, Inc.
core moves and indicates on a scale the amount of current or voltage. This instrument as illustrated is strictly for DC use. Circuit modification within the meter enclosure is required when used on AC.
The electrodynamometer type of meter utilizes the same basic principle as the D’Arsonval movement, except that the permanent magnet is replaced by fixed coils. This type of movement may be used on either AC or DC without damage since there are no permanent magnets. The DC scale for the same voltage or amperage will be different from the AC scale.
Indicating wattmeters are of the electrodynamometer type. They are a combination of a voltmeter and ammeter. The fixed coils are usually the ammeter portion and the moving coil the voltmeter portion. The current coil, being of heavy wire, is the stationary coil. The voltage coil is the movable coil of fine wire and is mounted as in a D’Arsonval movement. At zero watts the voltage coil is at right angles to the current coil. As current and voltage are applied,
Figure 14-7 Voltage tester in
use. Courtesy Amprobe Instrument Division of SOS Consolidated, Inc.
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the voltage coil turns (Maxwell’s rule) to embrace as many lines of force as possible. The scale reads the product of the current times the amperes, or watts.
A discussion of AC clamp-on ammeters and voltmeters may be a little premature due to the fact that current transformers have not been covered. In order to restrict our discussion of meters to this chapter, they will be shown, however.
Figures 14-6 and 14-7 illustrate voltage testers that use neon lights for voltage indications and may be used on AC or DC voltages. Figure 14-9 shows a clamp-on voltmeter-ammeter. Leads are used to connect the instrument to AC voltage. To test AC amperes, the clamp opens up and encompasses the conductor. It uses the current transformer principle, which will be discussed later with AC. This instrument can’t be used on DC.
Figure 14-10 shows a recording meter. Voltmeters, ammeters, and wattmeters come in recording types. These small, compact units provide permanent records of functions being monitored. This frees personnel to do other work while the recorder provides
Clamp-on ammeter and voltmeter. Courtesy Amprobe Instrument Division of SOS Consolidated, Inc.
Instruments and Measurements 167
Figure 14-10 Recording meter. CourtesyAmprobe Instrument Division
of SOS Consolidated, Inc.
written records of fluctuation, loads, and on/off operating time, which helps in troubleshooting systems and installations.