To achieve success in demonstrating electrostatics principles and in conveying the practical significance of many phenomena, it is often important to have the ability to detect the sign and magnitude of the electrical charge on conductive or insulative objects. A convenient tool for making these measurements is the hand-held, non-contacting electrostatic voltmeter instrument (ESV). Such instruments, though somewhat expensive, are now widely available. Unfortunately, the liquid crystal displays with which these units are equipped are small and rather dim, making them difficult to read by either the demonstrator or by the individuals in the audience watching a presentation. An excellent way to provide continuous and direct indication of ESV readings in even a large lecture hall is to combine this instrument with a tonal voltmeter (TVM). The TVM puts out an audible tone of constant amplitude, with the frequency related to the voltage reading. To integrate the TSV with the ESV, the ESV must have bipolar analog output. Some intermediate frequency in the middle of the audio range is chosen arbitrarily as the zero reference for uncharged objects. Then, an upward frequency shift of the audible tone indicates positive charge while a downward shift indicates negative charge. The larger the positive or negative frequency shift, the larger the magnitude of the positive or negative charge being detected. Experience with the TESV in many lecture presentations indicates that even a large audience is quickly trained to distinguish between positive and negative charges.
The TESV shown above consists of a commercially made, hand-held electrostatic voltmeter and a self-contained tonal voltmeter module. Many ESV instruments, including the one shown, come standard with ±2 volt analog output plugs on the bottom. Two jacks mounted to the top of the tonal voltmeter module plug directly into these plugs to make a compact system for detecting electrostatic charge. A volume-control knob and on/off switch are evident on opposite sides of the TVM module and the audio speaker opening is on the front.
A very convenient way to use the
TESV is with the unit mounted on a short camera tripod. The tripod makes it easy to measure
charge quickly and repeatably, and it frees a demonstrator's hands for other tasks. For
example, a great variety of tribocharging phenomena can
be demonstrated easily using this arrangement. The tripod-mounted TESV is also convenient in
demonstrations such as the coaxial shielding and field collapse apparatuses and also the powder charging demonstration.
The TVM circuit [Bender, 1996] uses the ±2 volt analog output from the
hand-held ESV (plus a DC offset) as input. The circuit schematic provided below shows Bender's
original TVM circuit integrated with the resistive voltage divider network needed to permit
tonal indication of both positive and negative polarity charges. A miniature loudspeaker driven
by a single-stage audio amplifier provides adequate volume for use in a lecture hall.
The tonal voltmeter module shown in the
photograph at the top of this page was assembled by L. Begy of the Electrical Engineering
Department at the University of Rochester. He also prepared the circuit schematic shown.
M.O. Bender, "Tonal Voltmeter," Electronics Now,
September, 1996, p. 61.