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July 1955 Popular Electronics[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.
Here is an easy-to-build project that demonstrates how like electrical charges repel. The electroscope was developed by English physicist William Gilbert in 1600. I remember playing with one in physics class in junior high school (one of the few classes that interested me at the time). The electroscope in the article calls for the use of gold leaf, but it can be any type of good electrical conductor. In fact, I found a video (below) on YouTube that shows how to make an electroscope out of commonly available materials. This would make a good conversation piece to sit on your desk.
Early experimenters used this device to detect presence of electricity and static charges.
By Rufus P. Turner
The electroscope is one of the oldest electrical instruments. William Gilbert built one in the year 1600. Nevertheless, it is still a useful device. It is easy to build, and many interesting experiments can be performed with it.
An electroscope consists simply of a small strip of gold leaf, one end of which is attached to a metal plate of about the same size. The leaf hangs parallel to the plate and may even rest against it. This assembly is then attached to a metal rod and enclosed in a glass jar. It can be seen from the outside but will be protected from drafts of air. The metal rod protrudes and permits external connections.
These four drawings illustrate basic experiments that may be performed with the simple electroscope. Many others may be carried out with this device. See text for suggestions.
Ordinarily, the leaf hangs close to the plate or in contact with it. But when an electrically charged body is brought close to the rod, the leaf stands out, away from the plate. The distance it moves depends upon how strong the voltage of the charged object happens to be. The leaf remains lifted as long as the charged object is kept in position, or as long as the object remains charged. It may remain lifted even after the body is removed, if the electroscope has been charged. When the electroscope is discharged, by "shorting" it momentarily to the earth, the leaf will fall.
This action occurs because the electrified body charges the metal plate and the gold leaf by induction. Since the leaf and the plate are attached together, they receive the same sort of charge, i.e., positive or negative. And since a basic law of electricity is that like charges repel each other, the light gold leaf is repelled by the plate.
One of the simplest measuring instruments may be easily constructed
in a few minutes.
The electroscope consumes no current while indicating the presence of electrons. Furthermore, it requires only one connection to a circuit, and often no direct connection at all.
How To Build It
Building an electroscope requires a small glass jar, a small strip of gold leaf, and a few odds and ends from the junk box. Figure 1 shows details of construction.
Make a small metal bracket, as shown in Fig. 1C. Any convenient non-magnetic metal such as aluminum, brass, or phosphor bronze will be satisfactory. To make the bracket, cut a 1 1/8" x 3/8" strip. Drill a clearance hole for a 6-32 bolt (# 28 drill) 3/16" from one end and 3/16" from the side of the strip. Finally, make a right-angle bend to form the bracket.
The next step is to cut a 5/8" -diameter clearance hole in the center of the metal jar top. (See Figs. 1A and 1B). There is no objection to making this hole larger, but it should not be smaller than 5/8". To cut the hole, the author used a radio socket punch. If a plastic jar top is used, the hole will not be required. The pickup electrode of the electroscope consists of a 2" -long 6-32 bolt, and the purpose of the clearance hole is to insulate the bolt from the metal jar top. This bolt is supported by a small square of polystyrene (1 1/4" square) fastened to the inside of the jar top with four small 3-40 bolts and nuts, as shown in Figs. 1A and 1B. A clearance hole (#28 drill) in the center of this square passes the 6-32 bolt.
A "hot" piece of radioactive ore has been observed to discharge a previously charged electroscope. However, don't expect to take this instrument on a prospecting trip in place of the more expensive Geiger counter. It just is not sensitive enough or rugged enough for such an application.
Electroscope Video Demonstration
Posted August 31, 2011