When transferring to an uncharged electroscope an electric positive or negative charge, it can be noted that the leaves of the electroscope will diverge at a smaller or larger angle.
Using an electrified stick, we touch the rod of the electroscope and remember the angle at which the leaves will diverge. In order to give out a discrepancy between the leaves of the electroscope at a larger angle, it is necessary to transfer a charge to it from a larger area of the charged stick. And, on the contrary, the leaves converge when you touch the rod of the electroscope with your hand.
Thus, we find that the force of the electric charge on the bodies is more or less. Therefore, we can talk about such a concept as the magnitude of the charge, and therefore about its measurement.
This became possible thanks to the discovery at the end of the XVIII century. law on the interaction of electric charges. This law was discovered by the French physicist Coulomb.
Coulomb's law was discovered experimentally: the scientist conducted experiments with torsion scales, with which he measured the force with which electrified objects interact.
The torsion balance consists of a light beam that does not conduct any electric charges of the beam, suspended on the finest metal thread in a glass vessel of cylindrical shape. A gilded cork ball is fixed on one edge of the shaft, and a counterweight on the other. With its upper end, the wire is attached to the center of the head, equipped with a pointer and rotating on a scale having divisions, which serves to determine the size of the angle of twist of the fixed wire.
The lid of the vessel has an opening through which another, exactly the same ball b, is supplied at the insulator, which is equal to the ball a in size. The size of the angular distance between the gilded balls a and b is calculated from the divisions that are on the cylindrical vessel. To do this, turn the scale head by a certain angle, you can change the distance.
After both balls were charged and installed at any distance, Coulomb could determine the force with which these balls interact, measuring the angle of twisting of the thread.
If the device is pre-calibrated, then by measuring the angle of rotation of the head, you can find out with what force the electrified balls interact.
When the distance between the balls changed, Coulomb found that with constant charges the force with which they interact will be inversely proportional to the double distance between their centers.
The solution to the question of measuring the magnitude of the charges on the balls was as follows: if ball b is charged and, having taken out of the device, is in contact with another ball, then exactly half of the charge will go to another ball from ball b. Thus, half the charge will remain on it. Having placed ball b back into the device, Coulomb found that with the same distance between the balls, their interaction force decreases by half - in direct proportion to the reduction in the charge.
Similarly, the charge of a moving ball a changed.
Thanks to these experiments, a law was discovered, which later became known as the Coulomb law, its definition is as follows: the interaction force of two point charges is directly proportional to their values, inversely proportional to the square of the distance between the charges and directed along the line that connects these charges.
The law of the Coulomb of Amonton is expressed by the formula:
F = k (q1q2 / r²),
where q1 and q2 are the values of the point charges that interact, r is the distance between these charges, and k is the proportionality coefficient, which depends on what units of measure the quantities will be included in the formula.
In this case, point charges are those charges that are on bodies of any size and shape that are small enough in comparison with the distances at which their interaction is considered.
Studies have shown that the magnitude of the force F is influenced by the environment, and the formula that expresses the Coulomb law is applied only when charged bodies interact in a vacuum.
Thanks to Coulomb's law, a unit of electrical discharge was established. So, by it is meant a charge acting in a vacuum on an identical charge, which is at a distance of one centimeter, with a force of one dyne. This is an absolute electrostatic unit of charge.