Electrostatic Deflection

Electric fields produce forces, just as magnetic fields do. You have noticed this when your hair feels like it’s standing on end in very dry or cold weather. You’ve heard that people’s hair really does stand straight out just before a lightning bolt hits nearby. (This is no myth!)

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 cannot move the foil leaves.

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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 difficult to 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 meter can quantify alternating (or ac) electric charges as well as direct (or dc) charges. This gives electrostatic meters an advantage over electromagnetic meters such as the galvanometers. If you connect a source of ac to the coil of the galvanometer device in Figure :

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or figure:

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the compass needle will not give a clear deflection; current in one direction pulls the meter needle one way, and current in the other direction pushes the needle the opposite way. But if a source of ac is connected to an electrostatic meter, the plates repel whether the charge is positive or negative at any given instant in time.

Most electroscopes aren’t sensitive enough to show much deflection with ordinary 117-V utility ac. Don’t try connecting 117 V to an electroscope anyway. It can present an electrocution hazard if you bring it out to points where you can easily 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 initial current needed to put a charge on the plates. Sometimes, an engineer or experimenter doesn’t want a measuring device to draw current, because this affects the behavior of the circuit under test. Galvanometers, by contrast, always need some current to produce an indication.

If you have access to a laboratory electroscope, try charging it up with a glass rod that has been rubbed against a cloth. When the rod is pulled away from the electroscope, the foil leaves remain standing apart. The charge just sits there! If the electroscope drew any current, the leaves would fall back together again, just as the galvanometer compass needle returns to magnetic north the instant you take the wire from the battery.