People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
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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.
Build an Electroscope
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.
Charged sheet of paper
DC power supply or battery
Charged 0.1 μfd. capacitor
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.
Fig. 1. In assembling the electroscope, the
author used an empty peanut jar. The cardboard insert in the
lid is removed and a hole is cut in the center of the metal
lid. A small square of polystyrene is then mounted under this
hole and a 2"-long bolt fastened through the center. At one
end. mount the metal angle bracket shown in (C). Glue the gold
leaf to the bracket just beyond the bend as shown in (A) and
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.
Assemble the electrode bolt, insulating plate, and metal
bracket on the jar top. Tighten all of the nuts securely. After
assembly, polish the long surface of the bracket with fine sandpaper
until it is bright. Then scrub the surface with soap and hot
water to remove all dirt and grease. Wash off the soap, rinse
the bracket several times in clear water, and dry thoroughly.
From this point on, do not touch the bracket with the fingers;
do not allow it to come into contact with any oil or grease;
and do not breathe on it. The reason for this cleanliness is
that the gold leaf, which will be attached next, will stick
to the bracket if the latter is the least bit dirty, and the
electroscope will not work.
Now attach the gold
leaf. Gold leaf is exceedingly light and thin and must be handled
in a draft-free room. During this operation, close the windows
and doors and tie a handkerchief or mask around nose and mouth
to prevent blowing the leaf out of shape. Gold leaf comes in
a book of sheets with separating pages of tissue paper. It can
be bought from a sign painters' supply house. Pull out a leaf
with a tissue page on each side and lay this "sandwich" on a
sheet of flat cardboard. Then, with a sharp razor blade or Exacto
knife, cut through the tissue paper and gold leaf to obtain
a strip 3/4," long and 3/8" wide. With a clean wooden toothpick,
remove the top layer of tissue paper, exposing the gold leaf.
Then, spread a thin strip of Duco cement (about 1/16" wide)
along the upper bend of the bracket; this area is shown by the
shading lines in Fig. 1C. Before the cement has chance to dry,
lift the assembly by the jar top and quickly press the cemented
edge of the bracket against the tip of the gold leaf strip.
This procedure will attach the leaf to the bracket. Screw the
top on the jar, and the electroscope is completed.
When the jar rests on its bottom, the gold leaf should hang
against the bracket. As the jar is tilted, the leaf should swing
away from the bracket. If it sticks to the bracket, the latter
has not been thoroughly cleaned, and must be cleaned again.
An electroscope indicates
the presence of an electric charge just as a magnetic needle
will show the presence of a magnetic field. When the electrode
of the electroscope is touched to an electrically charged object
(or, in some cases, simply brought near it), the gold leaf will
move away from the metal bracket. High voltages will swing the
leaf over the greatest distances.
Try combing the hair
briskly and then bringing the charged comb near the pickup
screw. See how the gold leaf rises to show the presence of static
electricity. A glass rod rubbed with a silk cloth will give
the same effect. Static electricity can also be generated by
laying a dry sheet of typewriter paper on a highly polished
desk or table, or on the cover of a magazine having a glazed
surface (like POPULAR ELECTRONICS), and rubbing the paper vigorously
with a wooden pencil. When the paper is lifted, it will be highly
charged; and when brought to the electroscope, it will deflect
the gold leaf. Shuffling feet across a rug, and then touching
the pickup screw with a finger will result in a high voltage
swing of the leaf. A hairy brush or broom after stroking a floor
or polished table top likewise acquires a considerable charge
of electricity. In some localities, however, it is hard to generate
static electricity in any of the ways just described unless
the weather is cool and dry. Humid days often tend to ruin the
When the electroscope pickup is touched
to one terminal of a power supply or high voltage battery, the
leaf will be deflected. Three hundred volts d.c. will move the
size of leaf shown in this article through 45°. Similar leaf
movement is obtained when the screw is touched to one terminal
of a charged capacitor.
Electroscopes sometimes are
used in industry to detect static charges on paper, photographic
film, cloth, plastic sheets, machinery belts, etc. They have
been used in surgical operating rooms and around oil trucks
for the same purpose.
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.