An electrometer, also known as a potential difference meter or a pointer tester, is a commonly used semi quantitative measuring instrument in high school electrostatic experiments. An electrometer can be used to test the type of charge carried by an object and measure the amount of electricity.

Principle: Connect a charged flat capacitor with Q to an electrometer. At this time, the pointer and metal rod are positively charged, and negative induced charges will appear on the inner surface of the shell.

As a result, an electric field is formed between the metal rod and the shell, and the charge on the surface of the pointer is subjected to the force of the electric field, or in other words, it is repelled by the same type of charge on the rod and attracted by different types of charges on the inner wall of the metal box, causing the pointer to deflect. The more charged it is, the stronger the field strength, and the larger the deviation angle of the pointer.

It can be seen that when the capacitance of the electrometer remains constant, the potential difference U between the two poles of the electrometer is directly proportional to its charged quantity Q. The larger U, the greater Q, and the greater the electric field force on the pointer, resulting in a larger pointer angle. From this, it can be seen that the angle of the pointer can qualitatively reflect the magnitude of the potential difference between the two poles of the electrometer.

Principle of use

When A is charged, the charge is mainly distributed at the four tips of a, b, c, and d. The charges carried by c and d interact with each other through repulsive force, and the pointer is subjected to an electric moment L1 that opens it up. Due to the fact that the center of gravity of the pointer is slightly below the rotation axis O point, when L1 causes the pointer to open, the gravity of the pointer generates a gravity moment L2 that resets the pointer. As the pointer deflects, L1 gradually decreases (because the distance between c and d increases, the Coulomb force decreases, and the arm of force also decreases), while L2 gradually increases (because the arm of gravity increases).

When the charged amount q of A is large, the charged amounts of c and d are also large, and L1 is also large, so α It's just big. Due to q's decision α， therefore α The size of can represent the size of q. This is why an electrometer can be used as a voltage tester.

Due to electrostatic induction, when A is charged, the inner layer of B must carry a charge of a different sign from A. If B is not grounded, the outer surface of B carries a charge of the same number as A. If B is grounded, the outer surface of B is not charged. Due to the symmetry of the electrometer structure, it can be roughly assumed that the charge on B does not produce a torque that causes the pointer to rotate.