Principles of Electricity

The Electroscope

An electroscope is not a difficult device to build. The basic electroscope is simply a glass jar in which two strips of foil (typically gold leaf) which are suspended from an electrically conductive metal rod. The rod comes into the jar through a stopper made of a non-conductive material such as glass, rubber, or (less often) cork. The top of the rod extends out of the jar, and is used as a sort of "antenna".

Our electroscope can be built using a piece of copper wire, a glass jar (mason jar will do just fine), a rubber stopper, a piece of thick copper wire, and some aluminum foil. See the illustration.

First, take the copper wire (aprox 4-8 guage) and bend it into a sort of small coat hanger with a long piece on the top. Take the stopper to the jar and drill a hole in it large enough to TIGHTLY push the copper wire through it. Once the hole is drilled, push the copper wire through the top, bend the top part into a kind of shepherd's hook.

Now cut a strip of aluminum foil (it is better to use "gold leaf foil" if you are fortunate enough to have some)so that it is a wide as the "coat hanger", and fold it in half. For best results - make sure the foil is completely flat, and that there are no rough edges on it (use sharp scissors to cut it). Now droop the foil over the coat hanger, and push the stopper/lid onto the top of the jar. Note that it is best to use as "dry" of air as possible - so baking the jar a bit before you seal it is not a bad idea. You might also use "canned dry air" sprayed into the jar if you choose, but don't spray it too fast, as it may condensate. You are done.

To test your electroscope, comb your hair with a plastic comb, then touch the antenna with it. If the leaves of the foil fly apart - you are done! Alternately, you can rub your feet on the carpet and then touch it with your finger.

If you are using a mayonayse jar or a mason jar with a metal lid, and don't have a rubber stopper big enough - you can substitute by drilling a hole in the jar lid, and using a rubber grommet that tightly fits the copper wire, or silicone sealant, provided that the copper wire does NOT touch the lid of the jar.

The Electroscope is one of the first devices ever used to detect the presence and amount of electricity. The type of Electroscope we are building was developed 1787 by British clergyman and physicist Abraham Benne, however previous versions include a pivoted needle called the versorium, invented by William Gilbert sometime around 1600, and the Pith Ball Electroscope invented by John Canton in 1754.

It is normally not considered to be highly accurate, and is not typically used to directly measure "amounts" of electric current or voltage, but is more of a measurement of "presence" of static electric charge. As a side effect, it can also to some extent measure the presence of X rays, cosmic rays, and even radiation from radioactive material.

When no electrical charge is present, the strips of foil hang straight down. When a charged body is brought close to the "antenna", both strips acquire a like charge (both are positve or both are negative). When this happens, they repel each other and spread apart forming a "V". At this point, the electroscope is said to be "charged". If at this time, an opposingly charged body is brought near the antenna, the charge on the strips neutralizes, and they droop straight down again, discharging the electroscope.

In certain circumstances, a calibrated microscope can be used along with it to measure the exact movement of the strips. By doing so, scientists can more precisely measure the voltage of the charge on the electroscope in microvolts (uV or millionths of a volt). When the nominal distance between the foil "plates" are known, electric currents moving through ionized air can be measured, thus yielding a method of quantitative measurment.


Try touching the antenna (electrode) with various things. Write down the results of each. See if you can find a pattern!
Some examples would be:
  • balloon
  • styrofoam cup
  • plastic comb
  • ping pong ball
  • a coin
  • a wool scarf
  • a silk handkerchief
  • a plastic ruler
  • a wooden dowel
  • a drinking glass
  • a rubber eraser

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