Wiggy is the registered trademark for a common solenoid voltmeter used in North America and manufactured by Square D, derived from a device patented in 1918.
Rather than using a D'Arsonval movement or digital electronics, the solenoid voltmeter simply uses a spring-loaded solenoid carrying a pointer (it might also be described as a form of moving iron meter). Greater voltage creates more magnetism pulling the solenoid's core in further against the spring loading, moving the pointer. A short scale converts the pointer's movement into the voltage reading. Solenoid voltmeters usually have a scale on each side of the pointer; one is calibrated for alternating current and one is calibrated for direct current. Only one "range" is provided and it usually extends from zero to about 600 volts.
A small permanent magnet rotor is usually mounted at the top of the meter. For DC, this magnet flips one way or the other, indicating by the revealed color (red or black) which lead of the voltmeter (the red or the black lead) is positive. For AC, the rotor simply vibrates, indicating that the meter is connected to an AC circuit. Another form of tester uses a miniature neon lamp; the negative electrode glows, idicating polarity on DC circuits, or both electrodes glow, indicating AC.
Models made by some manufacturers include continuity test lights, which are energized by a battery within the tester. This is particularly advantageous when testing , for example, fuses in live circuits, since no switching is required to change from continuity mode to voltage detecting mode.
Solenoid voltmeters are extremely rugged and not very susceptible to damage through either rough handling or electrical overload.
The probes (test prods) are very sturdy and they can be stored within the meter's body. A probe may also be extended from the meter's body, allowing one hand to both probe and hold the meter in a useful position. Probes are usually securely or permanently attached to the unit, so leads are unlikely to slip out if the tester is dropped.
The solenoid voltmeter has no switches that can be set incorrectly, simplifying operation.
For "go/no go" testing, there is no need to read the scale as application of AC power creates a perceivable vibration and sound within the meter.
Solenoid voltmeters draw appreciable current when operating. This makes them useful for testing residual-current devices (GFCIs) because the current drawn will trip most RCDs when the solenoid voltmeter is connected between the live and earth conductors. Also, when testing power supply circuits, a high-impedance connection (that is, a nearly open-circuit fault such as a burned switch contact or wire joint) in the power path might still allow enough voltage/current through to register on a high-impedance digital voltmeter, but it probably will not actuate the solenoid voltmeter.
Some manufacturers include a continuity test lamp function in a solenoid meter; these use the same probes as the voltage test function. This feature is useful when testing the status of contacts in energized circuits - the continuity light will display if the contact is closed, and the solenoid voltmeter will show voltage presence if open (and energized).
In contrast to multimeters, solenoid voltmeters have no other built-in functions (such as the ability to act as an ammeter, ohmmeter, or capacitance meter); they are just simple, easy-to-use power voltmeters. Solenoid voltmeters are useless on low-voltage circuits (for example, 12 volt circuits). The basic range of the voltmeter starts at around 90V (AC or DC).
Solenoid voltmeters are not precise. For example, there would be no reliably perceptible difference in the reading between 220 VAC and 240 VAC.
They draw a moderate amount of power from the circuit under test and are meant for intermittent operation and will overheat if used continuously.
The low impedance and low sensitivity of the tester may not show high-impedance connections to a voltage source.