Torsion Wire Electroscope

I am fascinated by vintage physical instruments. An Electroscope is a very old measurement device. It was invented around 1600 by the british scientist William Gilbert. He called it "versorium". I had the idea to combine it with the torsion wire principle. A torsion wire is just a wire under mechanical tension. It is able to twist around its own axis by torque. Principally this works like a very weak restoring force. Because in a mechanical measurement device the restoring force has the same level as the physical force you want to measure, a weak restoring force makes it possible to measure weak signals.

Don't hesitate to make this project. I'm bad at metal work. So if i can do it, you can do it also!

I used the following materials:

• brass sheet 0.5mm thick
• plexiglass 4mm thick
• plexiglass 5mm thick
• brass ball 50mm diameter with M8 thread
• 2 plexiglass rods 10mm diameter, 250mm length
• "Angel Hair" very thin copper wire
• Nuts and screws M3/M6/M8

Not in picture

• iron wire 1mm diameter
• brass wire 1mm diameter
• brass wire 0,2mm diameter
• 2 brass rods 60mm length, 4mm diameter
• aluminum rod 10mm diameter, 25mm length
• aluminum rod 6mm diameter, 10mm length
• 3 wooden stands

The 10mm aluminum rod was cut on a lathe and a 0.8mm hole was drilled in the middle. A steel needle was fixed in the hole with super glue. A 2nd hole with 2.5mm was drilled at the side and a 3mm thread is cut in. A M3 screw is turned in halfway. A third 2.5mm hole is drilled on the bottom of the rod and another 3mm thread is cut in.

A round Plexiglas sheet with 160mm diameter is cut out of the 5mm thick piece. It will be the base for our construction. Three wooden stands are glued to the bottom of the Plexiglas disk symmetrically. Drill three 3mm holes. One in the middle of the disk and two more in a distance of 12mm from the center hole. Drill the holes slightly over with a 7mm drill to make space in the holes for the screw heads to sink in. Fix the pivot with a 3mm screw in the middle of the disk.

Drill 2.5mm holes at each end of the rods and cut 3mm threads in. Measure 32mm from one end of each rod and drill one 4mm hole perpendicular to the rod axis. Fix the rods on the base disk with M3 screws as shown.

Use a torch or a big soldering iron to solder a piece of 0.2mm brass wire to the head of ech brass rod as shown. Then shift-in the rods into the 4mm holes on the plexi-rods. Turn each of the brass wires one winding around the screw at the pivot, cut the open wire ends and turn tight the screw.

Take the 4mm thick Plexiglas sheet and cut out a rectangular shape of 40mm x 15mm. Drill a 8mm hole in the middle and one 3mm hole at 12mm distance from the center hole to the left and to the right.

Take a piece of 1mm brass wire and bend it to a hanger-like triangle. Solder the wire into the slot of the screw head of the 25mm long M8 screw. Fix a 4mm-piece of shrink sleeve directly under the screws head with a torch. Fix the Plexiglas plate with M3 screws and nuts. Place the M8 screw with nuts as shown in the picture.

I used a small piece from the 6mm aluminum rod and cut it on the lathe so that one end got a small recess of 5mm. I drilled a 0,5mm hole through the recess. This will be one of the eyelets for the torsion wire. Another hole with 1mm diameter is drilled right through the middle of the rod. It will take the pointer. A third hole of 0.8mm is drilled on the bottom end of the rod. It will sit on the pivot needle after assembling all parts.

The pointer is made of anodized iron wire. A 8mm nut is used to put some weight on the pointer because torsion doesn't work without suitable mechanical strength. It#s epoxy-glued to the aluminum rod. I slided the glass tube over the rear part of the pointer. It is used to balance out the pointer so that it is swinging exactly horizontally

I used a 120mm x 35mm piece of brass sheet which i bent slightly to get a roundly shaped scale that matches the radius of the pointer. I soldered M3 brass screws to the edges of the brass plate fitting into respective holes in the base disk fixed with M3 nuts and washers.

I bent the front end of the pointer downwards so it may go across the scale and giving comfortable readings. You might bent the pointer at a position you want. The longer it is the more accurate your readings become. The scale is labeled with a piece of paper lettered with some fantasy units just to have something to read.

Pull out a strain of angel-hair wire with a length about 40cm. Thread one end of the wire through the eyelet on the pointer assembly and twist the wire to get it fixed. Now put the other end through the holder on the top. Adjust the length in a way that the pointer assembly turns slightly around the pivot needle without sitting on its tip. Now twist the upper wire end to fix it. Cut away the loose wire ends with a scissor.

The brass ball has an M8 thread. Screw it on the top assembly and fix it with the upper M8 nut. Now you're electroscope is fully assembled!

Give the wire some minutes to settle, then turn the brass ball slightly to adjust the pointer to scale zero. Now apply some charge by rubbing a piece of plastic against fur and place it near the brass ball. You will see a reasonable deflection of the pointer!

One problem i discovered was that the deflection almost vanished as i pulled the charged object away. I think this is because most of the charges are concentrated where the most electrical mass is and this is the brass ball. Only when the charged object is still near the ball the charges are forced down to the pointer assembly by electrical induction. When the charged object is pulled away the electrical induction also vanishes and the charges go back into the ball. This something i have to fix in my next design.

If you like watch my video. I apologize it's in german. Soon i will make a HD video in english and put the link here!