A working electroscope needs a clean dry glass jar, a metal rod passing through a good insulating stopper, and two very light foil leaves hanging from the rod’s end; charge the rod and the leaves repel apart, then fall together as the charge leaks away. It shows how fast charge leaks, which radiation can speed up, but damp insulation and humid air speed it up too, so it can never give a dose rate or prove an area safe.
Part of Radiation Safety
A foil-leaf electroscope detects electric charge by the repulsion between two identically charged leaves, and the rate at which that charge disappears is the only thing it can report about the air around it.
What You Need
The parent article, Nuclear Safety, lists the materials:
- A clear, dry glass jar. Glass is a good insulator and lets you see the leaves.
- A metal rod or thick wire that will pass through the stopper.
- An insulating stopper: dry wood, cork or rubber.
- Thin aluminium foil or gold leaf for the two leaves.
- A charged object: amber, glass or plastic rubbed on fur or silk.
Additions this guide recommends, from general practice rather than the parent: beeswax or paraffin to seal the stopper, a little desiccant to keep the air dry, and a second identical instrument as a control. Sizes below are starting points to adjust by trial, not specifications.
Method: Build It To Hold Its Charge
Step 1: Dry and clean the jar. Wipe it, and warm it gently in the sun or beside a stove, never over a flame, then let it cool before closing it. A film of moisture on glass or stopper carries charge away as quickly as ionised air does.
Step 2: Fit the rod through the stopper. Push the rod through a stopper of dry wood, cork or rubber. Seal the gap and the stopper surface with wax so no moisture path runs from rod to jar. The rod must not touch the glass.
Step 3: Make the leaves. Cut two narrow strips of foil, a few centimetres long. Use the thinnest foil you have; a heavy leaf stays down. Hang both from a small loop or fold at the lower end of the rod so that they touch when uncharged and swing freely.
Step 4: Close the jar. Put the stopper in so that the leaves hang well clear of the glass. A small dish of dry material, such as dried clay or salt, inside the jar helps keep the air dry.
Step 5: Charge it. Rub the plastic, glass or amber on fur or silk and touch it to the top of the rod. The leaves repel and spread, as the parent article describes.
Establish A Baseline Before You Trust Anything
Charge the instrument the same way each time and time how long the leaves take to fall together in a place and weather you can repeat. The parent article says that in normal conditions the leaves stay apart for minutes to hours. Do this several times; the spread of the results tells you how repeatable the instrument is. Keep a second identical instrument at base on the same days: if both fall quickly in damp weather, humidity is the cause; if only the one carried to a location falls quickly, the difference is local, though still not proven to be radiation.
| Observation | What you can conclude | What you cannot conclude |
|---|---|---|
| Falls at the baseline rate | Nothing unusual was detected by this device | The place is safe |
| Falls faster than baseline in a dry place | Something is discharging the air; radiation is one possibility | A dose rate, or that radiation is the cause |
| Falls in seconds | Unusual; retreat and reassess | How dangerous, or why |
| Falls fast on damp days both at base and away | Humidity | Anything about radiation |
Why It Stays An Indicator
Without a source of known strength you cannot give the leaf fall rate a value in any dose unit, and no scale should be painted on the jar. The sensitivity of a small air-filled jar to gamma rays is unknown, so a slow fall does not rule out a field that matters over hours. See Improvised Detectors.
Common Mistakes
- Wet or dirty stopper surface. The most common cause of rapid false collapse.
- Heavy leaves. The leaves do not spread, and the instrument seems dead.
- Charging in a damp room, then testing outdoors. The comparison is meaningless.
- Reading one trial. Repeat tests, with the same charge and timing.
Safety
Three real hazards in building it, and one in trusting it
Do not rub plastic on fur in an atmosphere with flammable vapour or dust, because it can spark. Handle broken glass carefully. Do not heat the jar over a flame. The larger hazard is trusting the result: an instrument whose sensitivity you cannot know cannot declare any place safe, and fast collapse can be humidity.
What’s Next
- Learn next: Survey Procedures — using indicators in a systematic survey
- Combine with: Detection Methods — where the electroscope sits among the options