Electroscope

The **electroscope** is an early scientific tool used to detect electric charge on an object.  It detects charge by the movement of a test object due to the Coulomb electrostatic force (the force of electricity pulling or pushing it).  The amount of charge on an object depends on its size and shape, and on the voltage (electrical "pressure") between it and its surroundings. Electroscopes need a lot of voltage to work, usually between a few hundred to a few thousand volts, so they are used with strong sources of static electricity, such as electrostatic machines. An electroscope can only give a general idea of the amount of charge an object has; a tool that measures the exact electric charge is called an electrometer.

The electroscope was the first tool for measuring electricity to be invented. William Gilbert, a British doctor, invented the first electroscope (called the *versorium*) around the year 1600, using a metal needle that could turn. Two other classic types of electroscopes are still used today to teach about static electricity: the **pith-ball electroscope** and the **gold-leaf electroscope.**

In 1754, British teacher John Canton invented the pith-ball electroscope. It has one or two lightweight balls, originally made of a plant material called pith, hanging from a hook by a silk or linen string. In 1770, Tiberius Cavallo made an electroscope like Canton's but with silver wires instead of string.  Atoms, the building blocks of all matter, are made of electrically charged particles called protons, neutrons, and electrons. Each atom has a positively charged nucleus made of protons and neutrons, with a cloud of negatively charged electrons surrounding it. The pith is a nonconductor, which means the electrons in the ball can't move around and make a current, but they can move a little within the atoms, making a charge. As shown in the diagram, a positively charged object *(B)* is brought near the pith ball *(A)*, the negative electrons *(blue minus signs)* in each atom ''* will get pulled to the side of the atom closer to the object. The positively charged nuclei *(red plus signs)* will be pushed to the side farther away.  Since the electrons in the pith ball are closer to the object *(C)'', they are pulled more than the nuclei are pushed, meaning the pulling force wins and the atom moves towards the object. It has a straight metal rod, usually made of brass, with two thin, flexible strips of gold leaf hanging from the bottom.  A disk- or ball-shaped metal piece, where you put the charged object, sits at the top of the rod. The gold strips are stuck inside a glass jar, usually with a metal bottom that conducts electricity, to protect them from getting pushed by moving air. Often there are grounded metal plates in the jar next to the leaves.  These plates protect the delicate gold strips from too much charge; if they touch the plates, the charge goes away before the leaves can tear.  They also help the instrument work better by capturing any electricity that might escape. In very precise tools, the air was sometimes removed from the jar completely to stop the charge from leaking away.

When a charged object touches the metal top piece, the gold leaves push away from each other in an upside-down 'V'. This is because the charge from the object goes through the top piece and the metal rod to the gold strips, charging them. Since the strips both get the same kind of charge, they push each other away. If you touch the top piece with your finger, the charge leaves through your body into the ground and the gold strips stop pushing each other away.

You can also charge an electroscope *without* touching it with a charged object. This is called charging by electrostatic induction. When a charged object is held near the top of the electroscope, the strips spread apart, even though the charged object isn't touching the top piece. The electric field from the object forces a charge of the same kind the object has into the rod and strips. The strips end up with the same type of charge, so they push each other away.  The opposite charge collects in the top piece, near the charged object.

If you touch the electroscope with your finger while the charged object is nearby, the same-kind charge in the strips moves through your body into the ground, but the opposite-kind charge in the top piece stays where it is. When you take your finger away, the electroscope is left with an opposite overall charge. The strips hang down at first because the charge is stuck at the top, near the charged object. When you move the object away, the charge spreads out to the strips, causing them to push each other apart again. 

File:Gold-leaf electroscope-2.jpg|Condensing electroscope, Rome University physics dept.
File:Gold leaf electroscope with ground strips.png|Electroscope from about 1910 with grounding electrodes inside jar, as described above
File:Gold leaf electroscope homemade.jpg|Homemade electroscope, 1900

## Related pages 
* Electrical measurements
* Electrostatic fieldmeter
* Faraday cup electrometer
* Radiation

## Footnotes
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