Basic Radio is a free introductory textbook on electronics based on tubes. See the editorial for more information....

# The Charge of the Electron

Author: J.B. Hoag

Consider two metal plates placed parallel to each other but not in contact, with their inner faces ground very smooth. Imagine also that several small holes have been drilled in the center of the top plate and that an electrical battery has been connected, one side to the upper plate and the other side to the lower plate, as in Fig. 1 A.

 Fig. 1 A. Measurement of the charge of the electron.

Let small drops of oil sprayed from an atomizer fall through the holes in the upper plate; then let us examine the droplets of oil between the plates by means of a long focus microscope. If it were not for the battery the droplets would fall under the force of gravity, but when the battery is connected, it is found that the droplets can be suspended and even moved upwards, according to the strength of the battery. This means but one thing — that the droplets are electrified. Their electric charge was produced at the time the oil was broken up into droplets at the nozzle of the atomizer.

By properly adjusting the battery, one of the oil drops can be precisely suspended in a fixed position midway between the upper and lower plates. Then, of course, the downward pull of gravity is exactly equal to the upward electrical force. When the expressions for these two forces are written down and equated to each other, an equation is obtained in which all quantities can be measured except the electrical charge on the drop of oil. This quantity can then be calculated. Indeed, there are many ramifications to a precise calculation of this electrical charge, but when all of the details have been followed, a most interesting fact is found, namely, that the electrical charge on the drop of oil is always a certain definite amount, or two, or three times, or any integral multiple of this small unit charge. The oil drop never has a charge of 1/2 or 1 3/4 or some other non-integral multiple of the unit charge. In other words, Nature has built her electrical world out of units or "building blocks", like the bricks in a building; not in a continuum like a concrete wall. It can be shown that the individual charges given off by a hot filament in a radio tube are always of a definite amount, equal to the least charge on the oil drop. This is true regardless of the material of which the filament is constructed, or its temperature. Also, certain electrical particles spontaneously emitted from radioactive substances have this same identical electrical charge.

This small "bit" or "grain" of electricity is the electron, first found in 1874 by C. J. Stoney and first convincingly established to the scientific world in the years between 1909 and 1913 by R. A. Millikan. The amount of this quantity of electricity, the smallest known to exist, is exceedingly small: 4.802·10-10 electrostatic unit. This is equal to 1.602·10-19 coulombs. This amount of electricity is so small that a prodigious number of electrons is needed to make up the total that flows through an ordinary light bulb each second. If three million people were to count for eight hours a day at the rate of 200 per minute, they would have had to count from the time of the Trojan War down to the present date in order to count all of the electrons passing through an ordinary Mazda lamp in one second.

There are two kinds of electricity — positive and negative. The electron is negatively charged. As such it is repelled by any other negative charge of electricity and is attracted to a positive charge.

Since the discovery of the granular nature of electricity and the naming of this small quantity of electricity (the "electron"), an additional property of the electron has been found.

Last Update: 2009-11-01