Electroscope
How can you tell whether an object is electrically charged just by looking, without touching it or risking a shock? Physicists solved this problem long before modern instruments existed with a simple, elegant device called the electroscope, which uses the very idea of like charges repelling to reveal the presence — and roughly the amount — of charge on an object. A gold-leaf electroscope needs no batteries, no digital display and no calibration scale; two thin strips of metal foil, free to swing apart, are enough. This article builds up the construction, explains why the leaves diverge, shows how the same device can judge the sign of an unknown charge without ever touching it, and works through the reasoning a Class 8 exam question expects.
A gold-leaf electroscope detects and roughly compares electric charge because charge spreading onto its two leaves makes them carry the same sign and repel, so the leaves diverge by an amount that grows with how much charge is present.
The core idea
Construction. A gold-leaf electroscope has a metal rod passing through an insulating stopper into a glass container, which shields the leaves from air currents that would disturb them. A metal disc or cap sits at the top of the rod, outside the container, ready to be touched by a charged object. Two thin leaves of gold (or aluminium) foil hang from the bottom of the rod, inside the glass container, where they can be watched through the glass without being blown about.
Detecting charge. When a charged object touches the disc, charge flows down the metal rod and spreads onto both leaves. Because the two leaves now carry the same type of charge, they repel each other and diverge — they swing apart. This divergence is the whole trick: no divergence means (essentially) no charge was transferred, while a clear divergence confirms the object was charged. Nothing about attraction to the outside object is involved; the leaves move because they push each other apart.
Comparing amounts of charge. The electroscope cannot give an exact number of coulombs, but it gives a useful comparison. More charge on the leaves means a stronger repulsive force between them, so they diverge wider; less charge means a smaller angle of divergence. Touch the same electroscope with two different charged objects, one after another (discharging it in between), and the object that produces the wider divergence carried the larger amount of charge.
Judging the sign of a nearby charge. The electroscope can also reveal the type of an unknown charge held near it, without touching. Suppose the electroscope's leaves are already diverged because it carries a known charge. Bring a second, charged object close to the disc — not touching it. If the divergence increases, the approaching object carries the same type of charge as the electroscope; if the divergence decreases, the approaching object carries the opposite type. This works because a nearby charge only redistributes the charge already sitting on the rod and leaves — it does not add or remove any charge overall.
Why it works
Every part of the electroscope is a plain metal conductor: the disc, rod and leaves are all joined, so charge placed anywhere on this conducting system is free to move and spread over the whole surface. When a charged object touches the disc, charge does not stay bunched at the top — it flows down the rod and distributes itself across both leaves, because like charges repel one another and spread out as far as the conductor allows. Since both leaves receive charge of the same sign, the two leaves now repel each other exactly as any two like-charged objects would, and the only way they can move apart is to swing outward against gravity, which is what we see as divergence. The width of that divergence is a direct readout of the repulsive force: more charge packed onto the same two leaves means a stronger sideways push, so the leaves swing to a wider angle before the pull of gravity (trying to bring them back to hang straight down) balances the electric repulsion. A weakly charged object transfers less charge, produces a weaker repulsion, and so the leaves settle at a smaller angle. This is why the electroscope, though it cannot read out an exact charge in coulombs, still lets a student rank objects by how strongly charged they are, purely by comparing how far the leaves spread. The "bring it close, do not touch" test works for a different reason: it relies on redistribution, not on adding new charge. The electroscope's own existing charge is free to move along its conducting surface. An approaching charged object exerts an electric force on that mobile charge — pulling it toward the disc if the approaching charge is opposite, or pushing it further down toward the leaves if the approaching charge is the same type — without any charge actually leaving or entering the electroscope. If more of the electroscope's own like charge is pushed onto the leaves, they diverge further; if charge is drawn away from the leaves toward the disc, the leaves have less charge left to repel each other with, and the divergence shrinks. Reading which way the divergence changes therefore tells you the sign of the nearby object's charge, even though nothing ever touched.
Worked example
Question: An electroscope is touched with a charged plastic scale and its leaves diverge slightly. A second, more strongly charged plastic scale (charged the same way) is then used to touch the same electroscope. What change, if any, would you expect in the leaves, and why? Afterwards, with the leaves still diverged, a charged rod is brought close to the disc without touching, and the divergence is seen to decrease — what can be concluded about the rod's charge, given the electroscope was known to be negatively charged?
Step 1. Touching the electroscope with the first charged scale transfers some charge onto the rod and leaves, giving them the same type of charge, so they repel and produce a small divergence.
Step 2. Touching the electroscope again with a more strongly charged scale of the same type transfers additional like charge onto the leaves. With more charge now present, the repulsive force between the two leaves increases, so the leaves diverge further apart than before — a greater amount of like charge produces a stronger mutual repulsion.
Step 3. For the second part, the leaves are already diverged because they carry negative charge. Bringing the rod close, without touching, only lets its electric field influence how the electroscope's existing charge is distributed — no charge is added or removed overall.
Step 4. Since the divergence decreases, some of the negative charge on the leaves must be getting drawn back up toward the disc, away from the leaves. This is consistent with the approaching rod being positively charged: unlike charges attract, so the rod's positive charge pulls the electroscope's mobile negative charge upward, leaving less repelling charge on the leaves and reducing the divergence. Answer: the leaves diverge wider after the second touch, and the nearby rod is positively charged.
Common traps
- Thinking the leaves diverge because they are attracted to the charged object — the divergence is due to the two leaves repelling each other, both carrying the same type of charge, not attraction to anything outside.
- Assuming the electroscope gives an exact numerical measurement of charge — it only gives a rough, comparative sense of how much charge is present, based on how far the leaves spread apart.
- Confusing touching the disc (which transfers charge onto the electroscope) with merely bringing a charged object close without touching (which only redistributes the electroscope's own existing charge, and is the method used to judge the type of an unknown nearby charge).
- Believing an electroscope with leaves showing no divergence is definitely uncharged — a very small amount of charge may produce too little repulsion to visibly separate the leaves, so "no visible divergence" is not the same as "completely uncharged."
Ask the tutor: Want to work through a few more "which way will the leaves move" electroscope scenarios together?
Why do the leaves fall back together instead of staying diverged forever?
The leaves stay diverged only while the repelling charge remains on them. Charge can slowly leak away through the air, through moisture on the glass or stopper, or through slight imperfections in the insulation, and as that charge drains off, the repulsive force weakens and the leaves fall back toward each other. If you touch the disc with your hand, you provide a direct path for the charge to flow away through your body to the ground, and the leaves collapse almost immediately — this is exactly how an electroscope is discharged between two tests.
Can the electroscope tell me exactly how much charge an object has, in coulombs?
No. The electroscope only gives a rough, comparative reading: a wider divergence means more charge, a smaller divergence means less, and no divergence usually means little or no charge. It has no numerical scale marked against a known unit of charge, so you can say one object is "more charged" than another by comparing how far the leaves spread, but you cannot read off a value in coulombs directly from the device described here.
Why must the electroscope's leaves and rod be inside a glass container?
The glass container is not there for electrical reasons — the leaves would work the same way in open air, electrically speaking. Its job is to shield the very light, delicate gold leaves from air currents, drafts and accidental knocks, which would otherwise disturb them and make the divergence unreliable or impossible to read. The glass also lets you see the leaves clearly while still protecting them, which is why the electroscope is built as a sealed glass container with just the metal disc exposed at the top.