More Special-Purpose Meters


The oscilloscope was developed over 50 years ago. As a piece of test equipment, it was one of the most exciting advances in electrical testing at that time and still is. Ordinary analog and digital meters merely give numerical quantities, whereas the oscilloscope does that and in addition, gives a graphic picture of what the test signal looks like. The shapes, frequencies, amplitudes, phases, distortions, interference, and so on, all show up to be examined. One cycle or less of an AC signal, or multiple cycles, or even a stream of cycles can be examined. DC levels and combinations of DC and AC can be viewed.

The horizontal trace of the oscilloscope controls the frequency, and the vertical trace shows the amplitude; together they draw the shape on the screen. Most oscilloscopes are analog devices, relying on grid lines over the screen to show quantities related to the range settings. Very precise measurements can be interpreted with the waveforms. More sophisticated scopes have dual traces so that two waveforms can be compared. But the most sophisticated advancement is the addition of a digital readout of all the waveform data, as each is selected.


Probes are special attachments used with test equipment to increase the meter’s usefulness or accuracy. They are test leads that contain extra parts or circuits. There are high voltage probes to extend the voltage range of the meter; radio-frequency probes to measure RF signals; and oscilloscope probes which help reduce the distortion of the test signal.

The test probe has a firm body, allowing it to be gripped tightly, with a thin test prod which is placed against the test point. Usually, an alligator clip is used for a ground connection. The probe is usually connected to the test equipment through a shielded cable, especially with rf and oscilloscope probes, to prevent the leads from picking up any stray, spurious signals.

The high-voltage probe contains a very large resistor which essentially is added to the meter’s input circuit voltage divider. The high resistance of about 1000 megohms in series with the meter’s 1-megohm tapped voltage divider can divide the input down to about ½ooo of its value on a low scale, to bring it within the meter’s range; i.e., 40,000 volts can be reduced to about 40 volts. These probes are used with dangerous voltages.

The RF probe contains a diode detector circuit to change the radio signal to a corresponding dc voltage level. Some probes, used for small signals, can also contain preamplifier circuits.

The oscilloscope probe contains an RC (resistor-capacitor) circuit, in which the capacitor is adjustable so that the impedance of the probe at the prod end can be matched through the cable to the oscilloscope input. This reduces the distortion of the test signal.

Stray Voltage Meters

Ordinarily, all parts of a circuit which are grounded, or connected to objects that are supposed to be at ground potential, are usually kept at zero volts. No voltages should exist between any two grounded points. Unfortunately, grounds located at different places tend to build up slight resistances between them. Current flow in the area between grounds can produce small voltage drops, raising one ground potential slightly above the other. These are called stray voltages. Metallic objects, such as appliances, metal buildings, fences, etc., could cause shocks because of the stray voltages. This is a particularly annoying problem on dairy farms, where these small shocks to cows can lower milk production. There are stray voltage meters designed specifically for testing stray voltage buildup of around 3 volts. The use of the ground resistance megger can be utilized to locate these imperfect grounds, and ground-fault circuit interrupters can protect against larger ground potential buildups at their locations.


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