Electrostatic deflection

Electric fields produce forces, just as do magnetic fields. You have probably noticed this when your hair feels like it’s standing on end in very dry or cold weather. You’ve probably heard that people’s hair really does stand straight out just before a lightning bolt hits nearby; this is no myth. Maybe you performed experiments in science classes to observe this effect.

  The most common device for demonstrating electrostatic forces is the electroscope. It consists of two foil leaves, attached to a conducting rod, and placed in a sealed container so that air currents will not move the foil leaves (Fig.). When a charged object is brought near, or touched to, the contact at the top of the rod, the leaves stand apart from each other. This is because the two leaves become charged with like electric poles—either an excess or a deficiency of electrons—and like poles always repel.

The extent to which the leaves stand apart depends on the amount of electric charge. It is somewhat difficult to actually measure this deflection and correlate it with charge quantity; electroscopes do not make very good meters. But variations on this theme can be employed, so that electrostatic forces can operate against tension springs or magnets, and in this way, electrostatic meters can be made.

An electrostatic device has the ability to measure alternating electric charges as

well as steady charges. This gives electrostatic meters an advantage over electromagnetic meters (galvanometers). If you connect ac to the coil of the galvanometer device ,the compass needle might vibrate, but will not give a clear deflection. This is because current in one direction pulls the meter needle one way, and current in the other direction will deflect the needle the opposite way. But if an alternating electric field is connected to an electrostatic meter, the plates will repel whether the charge is positive or negative. The deflection will be steady, therefore, with ac as well as with dc.

Most electroscopes aren’t sensitive enough to show much deflection with ordinary

117-V utility voltage. Don’t try connecting 117 V to an electroscope anyway; it might not deflect the foil leaves, but it can certainly present a danger to your body if you bring it out to points where you can readily come into physical contact with it.

   An electrostatic meter has another property that is sometimes an advantage in

electrical or electronic work. This is the fact that the device does not draw any current, except a tiny amount at first, needed to put a charge on the plates. Sometimes, an engineer or experimenter doesn’t want the measuring device to draw current, because this affects the behavior of the circuit under test. Galvanometers, by contrast, always need at least a little bit of current in order to operate. You can observe this effect by charging up a laboratory electroscope, say with a glass rod that has been rubbed against a cloth. When the rod is pulled away from the electroscope, the foil leaves will remain standing apart. The charge just sits there. If the electroscope drew any current, the leaves would immediately fall back together again, just as the galvanometer compass needle returns to magnetic north the instant you take the wire from the battery.