It makes appropriate sounds and it vibrates to let you know what’s happening. No, it’s not a mobile phone, writes David Herres, but a handy little solenoid voltmeter.

Way back in 1918, George P. Wigginton invented the solenoid voltmeter.

After that he seemingly disappeared from the history books, leaving scarcely a trace of what must have been an inquiring mind and lively intellect.

What does remain is an elegant and inexpensive little meter whimsically named after him – the ‘Wiggy’.

A conventional multimeter, in digital form, has an alphanumeric readout. Its analogue cousin has a D’Arsonval movement, incorporating a needle that points to the correct reading.

In contrast, the Wiggy employs a spring-loaded solenoid with an attached pointer. As the voltage measured at the probes increases, the solenoid’s core is pulled in further, in opposition to the spring, and the pointer moves to indicate the voltage.

Typically, there are two scales, calibrated for AC and DC. There is a single range, from 0V to 600V.

At the top of the meter is a permanent magnet rotor that shows DC polarity by loudly clicking one way or the other. For AC it vibrates, again quite audibly.

The voltage levels are approximate, so the instrument is used only when exact amounts are not an issue. This is the case in most house wiring situations, which is where this instrument excels.

Apprentice electricians like this tool because it is inexpensive and incredibly rugged. In the volt function, it is line powered, so there is no battery to worry about. (Some solenoid voltmeters include an ohm function, which requires a battery if resistance is to be measured.)

The more advanced electricians also appreciate the Wiggy, and not just because of the low price.

Solenoid voltmeters exhibit low impedance at the probes. Digital multimeters, with solid-state front ends, are high impedance. This difference is all important. Each has advantages and disadvantages, so it is a question of choosing the correct meter for each application.

Impedance, at DC and 50Hz or 60Hz line frequency, is not much different from resistance. At higher frequencies, the effect of capacitive and inductive reactance emerges as an important consideration and it must be factored in to calculate impedance.

For the electrician working at DC or 50Hz, impedance and resistance may be considered the same. Impedance is the more inclusive term, so it is more accurate and we can let it go at that.

In taking a measurement, the meter becomes the load. If it has high impedance, like a digital multimeter, it is a very light load.

The amount of energy required to move the needle of an analogue multimeter or to form the characters in a digital readout is very slight. Moreover, this energy comes from the internal battery.

The current drawn from the circuit that is measured, in the volt mode, is still less so we might say that the meter is invisible to the circuit being measured and does not distort the signal.

Solenoid voltmeters, on the other hand, have quite low impedance, meaning that they have a tendency to load the circuit. This is because, having no battery in the volts mode, they are line powered, drawing energy from the circuit under measurement.

If the output to the meter of the circuit of interest is high impedance and the low-impedance Wiggy is connected, measurement will be distorted and components in the circuit possibly damaged.

Utility power is an extremely low-impedance source, so any meter connected when working on ordinary premises wiring will not affect it.

You can use the solenoid voltmeter in this environment, but for sensitive electronic equipment – including the circuitry in newer appliances with solid-state components – stay with the digital multimeter.

However, this same high-impedance instrument presents a difficulty under certain circumstances.

If you turn on your digital multimeter and connect it to two terminals not connected to each other and do not have a low-impedance voltage differential, the meter may, depending upon internal circuitry, exhibit a ‘phantom’ voltage.

It will read perhaps 10V and the readout will appear to wander in a random fashion. This is because there is high-impedance voltage everywhere – in the probes, your instrument’s plastic case, the air and the solid-state components of the multimeter.

Most of the time this is not a serious problem, because as soon as you clamp onto a lower-impedance voltage source, the phantom voltage will disappear and the readout will stabilise. However, the phenomenon can make for difficulties when attempting to measure low-level voltages as found on a printed circuit board.

The low-impedance Wiggy is a very stable meter when working with stiff utility power. And, as mentioned, it has some distinct advantages that make it particularly useful doing fieldwork.

The solenoid voltmeter emits a loud click when it is connected to a DC source, and a buzzing sound when connected to AC. This makes it useful if you have to get the probes into a tight place and it’s difficult to watch the display at the same time.

These audible indicators are accompanied by a distinct vibrating motion that can be felt if you are holding the meter, which is very helpful in a noisy environment. You can carry the Wiggy in your shirt pocket and interpret readings strictly on the basis of sound and the feel of vibration.

One or both probes can be clipped into brackets on the back of the case, and this simplifies operation if you are trying to touch the probes in difficult locations and hold the meter at the same time.

As mentioned, solenoid voltmeters draw current from the circuit under test. Because of this property, the instrument may be used for testing a residual current device. If it is good, the device will trip when the solenoid voltmeter is connected between live and earth conductors.

Because it draws appreciable current, the solenoid voltmeter should not be connected to live terminals for any length of time or the instrument will overheat and suffer damage. In contrast, a multimeter or clamp-on ammeter can remain connected indefinitely.

If you have an older instrument, it will not be Cat rated. Newer voltmeters, including solenoid voltmeters, are. All categories apply to low voltage only, which in this context means under 1000V.

However, especially when working in an industrial setting, there is always the possibility that an over-voltage transient will infiltrate the circuit and become hazardous to the individual taking a live voltage reading.

This possibility becomes greater the farther upstream you are working. Meters are rated in terms of their ability to withstand this sort of event as follows:

Cat I – Used at protected electronic circuits only.

Cat II – Used at receptacles for plug-in loads.

Cat III – Used for distribution wiring including main bus, feeders, branch circuits and loads.

Cat IV – Secondary utility level and outside cable runs.

The working electrician should be aware of these issues and choose the correct category accordingly.

With experience, the electrician will discover additional advantages in using the handy, if somewhat limited, Wiggy.