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.