The Electroscope

An electroscope is a device that is used to detect the presence of an electric charge. This device works on the principle of Coulomb’s law and detects the presence of charge because of the motion of an object(particles).

Who invented the electroscope?

During the 17^th century, a British scientist named William Gilbert was working as a physician for the Queen of England. During this period, he was very keen and observant of his surroundings and discovered electroscopes. The electroscope discovered by William Gilbert is known to be one of the earliest electrical devices discovered in human history.

The first electroscope discovered is also referred to as Versorium due to the pivot on which the needle is balanced. The working of this Versonuim is very simple, and the rotation or the movement of the needle in this mechanism confirms the presence of the charge in the object. When a charged object is brought near the Versonium, the end of the needle closer to the object takes the charge opposite to that present in the object, while the other end takes the same charge as the object and repels it.

The working of an electroscope

The basic working of an electroscope is entirely dependent on Coulomb’s law, which states that the force between any two charged particles is inversely proportional to the square of the distance between the two charges and directly proportional to the product of the magnitude of the two charges. These can be represented as,

Where F is the force between the charges, ε₀ is the vacuum permittivity, Q₁ and Q₂ are the magnitude of the two charges, respectively, and r is the distance between the two charged particles.

The working of the electroscope can be seen when any charged object is brought within the range of an electroscope. The charge present produces an electrostatic force, which causes the molecules present in the electroscope to polarise. The movement of the electron present in the knob of the electroscope tells the nature of the charge in the object. If the charge is positive, the electron will attract, while if the charge is negative, the electron will repel.

Types of Electroscope

Typically, there are two types of electroscopes:

• The pith ball electroscope

The pith ball electroscope was invented in the year 1754 by John Canton. This method of detecting electric charge is straightforward and consists of a stand, a string, and a non-conductive lightweight ball. The ball is tied to the stand with the help of the string, as shown in the image below. When there is a need to check whether an object has an electric charge or not, the electroscope is taken near to it. If the non-charged ball is attracted to the object, the object has some electric charge.

• The gold leaf electroscope

The gold leaf electroscope was developed in the year 1787 by Abraham Bennet. This method of detecting electric charge consists of a rod whose one end has two gold leaf strips attached to it. The rod is kept vertically, with one of the ends with the leaves inside a glass to avoid deflection due to wind, while the other end is kept open, as shown below. When an object with a charge is brought near the device, the leaf of the device deflects and forms a V shape. The gold leaf electroscope is more sensitive than a pith ball electroscope.

Uses of electroscope

There are multiple uses of an electroscope; a few of them are:

• To know the presence of a charge or ionising radiation.

• To compare two different charges.

• To calculate the force between two charges.

• To determine the type of charge and the magnitude of the charge.

Conclusion

An electroscope is an electronic device that confirms the presence of an electric charge in an object. It was invented by William Gilbert in the seventeenth century. Classically, there are two types of electroscopes, namely, the pith ball and the gold leaf electroscopes.