A balance electroscope consists of a beam, in this case a straw, pivoting on a needle in a horizontal plane. It is used to test if an object is electrically charged. It is not intended for quantitative measurements. It operates on the principle of electrostatic induction and can be used to demonstrate this principle (which I do not show in this instructable). Hope you find it interesting since I really enjoyed making it.
Step 1: Needle Point Housing
Here I closed the thin open end of a glass pipette in a flame.
I also considered using the glass pieces found in incandescent light bulbs, but could not find suitable candidates.
Step 2: Separating the Glass Housing
I used a Dremel tool with a diamond wheel to cut the closed tip off. The glass is very thin and one should apply a very light pressure while rotating the tube against the wheel to cut it off. Do not try to cut through the glass wall in one go, this results in breaking it off irregularly (which is what happened when I cutted it, fortunately the break was not too bad so I just went on and used it anyway). It would probably be better to mount the Dremel and hold the tube in one's hand while cutting it. This way you can control the pressure better. I assume that this should be done without allowing the tube to move laterally while the cut is being done.
Step 3: Fixing the Housing on a Straw
I grinded the cutoff end of the housing to a concave shape with a Dremel tool so that it will fit over the straw. Glued it with silicone sealant over a hole drilled in the middle the plastic straw. Here the beam balances on a needle stuck in an eraser.
Step 4: Mounting Balls Over the Ends of the Beam
Drilled holes in two small polystyrene balls with a cone shaped grinding stone in a Dremel tool and pushed them over the ends of the straw.
Step 5: Fixing the Balls to the Beam
Glued the polystyrene balls to the straw so that they cannot be pulled off by the attractive electrostatic force.
Step 6: Constructing the Balance Point
I glued a sewing needle into the ink tube of a ballpoint pen after washing the ink out of the tube with acetone. Put the ink tube back in the pen. Drill a hole in a bottle top and slid the pen through the hole. Two strain relief grommets fix the pen to the bottle top, one inside and the other one on top of the bottle top.The height of the beam is then adjustable with such a slide fit.
Step 7: The Plastic Bottle Stand
Stuck masking tape around the plastic bottle from which the bottle top was removed. Marked the cut-off line and three legs on the tape. Cut the bottom off and the legs out with a Dremel tool.
Step 8: Assembly
The finished balance electroscope - ready for testing. Covering the beam and polystyrene balls with a conductive coating would probably make it more sensitive, which is what I did.