connected across a circuit or voltage source to measure voltage. Panel-mounted voltmeters are permanently wired into the circuit in which they are to be used.
Portable voltmeters are designed to measure one or more ranges of voltage. Those intended for measurement of more than one voltage range are provided with range selector switches. The range selector switch internally connects the appropriate multiplier resistor into the meter circuit for the range of voltage to be measured; for example, a voltmeter may be designed to use a O-l milliampere milliammeter as a voltage indicator. For each setting of the selector switch, a different multiplier resistor is connected into the meter circuit. For each selection, a particular resistor value is designed to limit the current through the milliammeter to a maximum of 1/1,000 of an ampere (1 milliampere) for a full-scale reading.
In a similar way, voltmeters designed to use a micrometer, for example, a 50-microampere meter, include multiplier resistors that limit the meter current to a maximum value of 50 microamperes. In this case, 50 microamperes are flowing through the meter for a full-scale deflection of the needle.
Voltmeters that use either a milliammeter or micrometer to indicate voltage have a scale calibrated to read directly in volts. The flow of current in either type of meter represents the electrical pressure (voltage) between two points in an electrical circuit; for example, the two points may be the hot (ungrounded) conductor and the neutral (grounded) conductor of a 125-volt circuit. In this case, the voltmeter is said to be connected across the line.
The line voltage indicator (fig. 7-20) is much more durable than most voltmeters for rough construction work. Its durability is mainly due to its simple design and construction. It has no delicate meter movement inside the case as do the analog meters previously mentioned. The two test leads are permanently connected to a solenoid coil inside the molded case.
CAUTION Do not use the line voltage indicator on voltages exceeding the capabilities of the indicator.
An indicator, attached to the solenoid core, moves along a marked scale when the leads are connected across a voltage source. The movement of the core is
Figure 7-20. - Line voltage indicator.
resisted by a spring. The indicator comes to rest at a point along the scale that is determined by both the strength of the magnetic field around the solenoid and the pressure of the opposing spring. The strength of the magnetic field is in proportion to the amount of voltage being measured.
In the center of the tester is a neon lamp indicator. The lamp is used to indicate whether the circuit being tested is ac or dc.
When the tester is operated on ac, it produces light during a portion of each half-cycle, and both lamp electrodes are alternately surrounded with a glow. The eye cannot follow the rapidly changing alternations so that both electrodes appear to be continually glowing from ac current. Two other indications of ac voltage are an audible hum and a noticeable vibration that can be felt when the instrument is hand-held. When the tester is operated on dc, light is produced continuously, but only the negative electrode glows; therefore, the tester will indicate polarity on dc circuits. Both the test probes and the glow lamp enclosure are colored red and black. If, while you are testing a dc circuit, the electrode of the glow lamp on the side colored black is glowing, this glow indicates the blackContinue Reading