Introduction: Leaf Electroscope
How to make a sensitive leaf electroscope from common materials, and why it works.
A leaf electroscope is a primitive demonstrator of electrostatic forces.
from wikipedia
An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert around 1600.[1] The pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscope that are still used in physics education to demonstrate the principles of electrostatics.
Step 1: Gather
For this you will need.
A clear container with a plastic lid,and a way to make a 3/8" (9.5mm) hole in the lid.
I have used a spice jar and glass salad dressing jar , but just about anything will work
Tinfoil*, thinner tinfoil makes a more sensitive scope so Dollar store / Poundland tinfoil is perfect.
a ping pong ball, or super ball. you can do without but probably wont get as good of results. Sharp edges cause leakage.
Or instead of making of the electrode you can use a shiny metal drawer knob like the ones listed at the bottom of this step.
A paper clip
Start by drilling, cutting or melting a hole in the center of the lid. All you need is a small hole 1/4-3/8"(6-9.5mm) diameter. something big enough to get some rolled up tinfoil through.
Cut 2 strips of tinfoil about 4X the diameter of the ball, the whole width of the foil.
*A note about tinfoil. It has a shiny and a dull side. This is caused by the mfg process at the final stages it gets so thin they have to double it up so it wont rip. So the shiny side was pressed against a smooth steel roller while the dull side was pressed against another sheet of foil. What this means for us is the dull side has slightly more surface area because it is rougher. This surface area difference *probably* won't make a difference in our usage, but I will be suggesting which direction it should face according to my understanding. I have seen it make a difference in relativity weak acids.
http://www.homedepot.com/p/Liberty-1-1-4-in-Chrome...
Step 2: Make
Start by drilling, cutting or melting a hole in the center of the lid. All you need is a small hole 1/4-3/8"(6-9.5mm) diameter. something big enough to get some rolled up tinfoil through.
Cut 2 strips of tinfoil about 4X the diameter of the ball. make a ball of tinfoil if you need to.
Cover your ball shiny side out and make as smooth a surface as possible. don't fully close the bottom yet.
Bend the paper clip like figure 2 The sharp 90 degree at the top is for a ping pong ball, I made this with a vice but pliers will work. Pinch a tiny bit in the pliers and bend, it doesn't have to be perfect but if you like sharp corners a little tapping with a hammer will make it nice and 90 degrees.
Put the open end of the tinfoil ball on the paper clip. If you are using a rubber ball just pierce it a little with the paperclip. Center the ball on the paper clip like its balancing and pinch the tinfoil tightly around the paper clip.
Pinch 1/2-1" (12-25mm) of tinfoil down the paper clip centering it, this is the support post so make it sturdy.
Using scissors or a knife cut the tinfoil cleanly at the bottom to trim off the excess
The point of this is to make a step in the size of the shaft so when the next layer is applied it will naturally stop at that step.
This can be a little irritating aluminum is a gummy metal that sticks
Take the second sheet of foil and wrap it over the whole thing the same way as the first layer shinty side out
This time pinch it down another 1/2" (12mm) beyond the first layer and cut
Twist the foil at the bottom so it forms a little bit of a point on the paperclip
Thread the paperclip through the lid
Pinching the foil support screw the foil into the lid is far as you can
Flip the lid over and spread foil that has made it through.
Place on the jar unless you reinforce this with something like hot glue it may be a tad delicate
this is the time to make the bar at the bottom of the paper clip triangle centered and level in the jar.
Cut a piece of foil 25% larger than the diameter of your jar by ~1/2" 12mm.
Fold the piece in half bringing the short sides together shiny side out and flatten on something smooth. It may help to intentionally not match the edges a little so you have a pull tab to separate the layers for a later step.
Trim with a paper cutter or scissors to a strip 1/8-1/4" (3-6mm) wide
You now have a tiny strip of foil that you need to carefully separate the halves of. If the strip is too thin it will curl like a Christmas ribbon. A thicker strip is less sensitive.
After separating the leaves fold over the bottom of the paper clip triangle at the crease.
Then smooth again, work the leaves smooth then gently counter bend so they hang closer to parallel but still have the slight outward slope.
Press the leaves together and trim them to length.
You want the longest leaves you can get without them being able to touch the glass, or at least being able to swing up 70 degrees or so. If the leaves touch the glass they can transfer a charge and affect the function. You then have to open the jar and ground the inside by touching it.
You now have a working electroscope but how to test it?
PVC pipe comes in handy, simply rub PVC on your hand and it will pick up a charge.
Step 3: Now What?
A couple of notes on construction. and use;
The instrument needs to be grounded before use, easy to achieve by touching the ball with a finger.
If the leaves touch the glass while sensing, the glass is now charged and the spot the leaves touched needs to be grounded before the instrument will behave predictably.
Rubbing one material with another to get static is the triboelectric effect. An example most people are familiar with is cold dry winter days and being shocked by doorknobs and pets. These days we worry about static electricity destroying our electronics, but without static electricity we never would have discovered the foundations for modern electronics.
Static charges do not build up well on a humid day. The moisture in the air carries charge away. It is pretty common for static demonstrators to carry a hair dryer around with them to dry devices so they function properly in classroom settings.
While the reality of shiny vs dull side probably wont matter much. I chose shiny side out because of the lower surface area and smoother finish. I conjecture a dull side out might suffer from humidity a tiny bit more. I assume the slightly higher surface area will allow more charge to escape, the same way it makes the dull side slightly less resistant to acid.
Any sharp edge will cause static to bleed off through coronal discharge. This is a function of charge distribution. The image with the flat and point show, in a conductor when the shape is flat the charges are even across the surface. Where as at the point there is a much higher charge imbalance. A few lone (+) charges are surrounded by a sea of available opposite charges. This lets charge bleed off much easier.
Static electricity does not behave the same way as current electricity ie from batteries or the wall. This is mostly due to the lack of conductor. Electricity doesn't flow on an insulator, like rubber, pvc, the carpet, or the cats hair. It stays still, ie static. Also because of this static nature it produces very high voltages.
The very high voltage creates a skin effect where the electricity stays on the skin of a conductor, rather than passing through the wire itself. The Skin effect also happens in alternating frequency (AC) wiring. Its just not particularly noticeable to us at 50 or 60 hz hertz where the depth is (~3/8") 8.5mm+. At much higher frequencies this becomes a design consideration.
Like charges repel, and opposites attract. Although its important to note that its much more of a potential difference attracts. Positive and negative is relative to the frame of reference.
Demonstrations:
If a charged rod is brought near an electroscope, its leaves will diverge. If the rod is touched to the electroscope, the leaves will remain diverged, even when the rod is moved away. The electroscope can be charged by induction by grounding it with your finger when the charged rod is near. Remove your finger and then the rod, and the electroscope is now charged with the opposite charge of the rod. This can be checked by bringing the charged rod near again; the leaves now come together.
If you take a balloon and rub it on your hair, as most know it will stick to a wall. This shows the charge imbalance from induction and opposites attracting. Bring that same balloon near your electroscope and the leaves will lift apart.
The reason this is happening is as the balloon gets closer some charge is transferring through electrostatic induction. As the leaves gain a charge the like charges on the inner side exert a force. This force is visualized by the lifting leaves. A later version of this device, an electrometer was used to roughly measure charge.
Or for a more detailed demonstration on electrostatic induction you can re-create Faraday's Ice pail experiment.
I hope this instructable and simple demonstrator are useful.
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