At the end of the 19th century, there was no generally accepted model of the atom. Most physicists believed that the atom was indivisible, although the discovery of radioactivity cast doubt on that in the minds of some physicists. At the same time it was generally believed that electric charge, like mass, was infinitely divisible.
To explain the connection between electricity and matter, some scientists in the late 19th century argued that there had to be a fundamental unit of electricity. In 1891 the Irish physicist, George Stoney, introduced the term “electron” to describe this smallest unit of negative charge.
The charge on electron was first measured by J.J. Thomson and two co-workers (J.S.E. Townsend and H.A. Wilson), starting in 1897. Each used a slightly different method. They conducted a series of experiments on cathode rays and after observing that the beam of light in the cathode ray tube is attracted to a positive charge and repelled by a negative charge he concluded that the rays consist of a stream of small, electrically negatively charged particles which have a mass over a thousand times less than that of a hydrogen atom. Thomson has discovered the electron. From this point onward, it becomes increasingly clear that atoms are not fundamental particles, but in fact are made up of smaller particles.
Townsend's work will be described as an example.
Townsend's work depended on the fact that drops of water will grow around ions in humid air. Under the influence of gravity, the drop would fall, accelerating until it hit a constant speed.
Several items were measured in this experiment.
1. the mass of a water droplet (actually the average mass of many)
2. the total electric charge carried on all the droplets (this was done by absorbing the water into an acid and measuring the charge picked up.)
3. the velocity of the droplet
4. the total mass of all water droplets (found by measuring the acid's increase in weight)
He determined the e/m ratio of the droplets (2 divided by 4), then multiplied by the mass of one droplet to get the value for e. Thomson, Townsend, and Wilson each obtained roughly the same value for the charge on positive and negative ions. It was about 1 x 10¯19 coulombs. This work continued until about 1901 or 1902.
The measurement of the electron's charge independently was achieved by Millikan by his famous experiment from 1909 and with Thomson's results also a value for the electron mass was obtained. This experiment is called the "oil-drop experiment" and it was the first successful scientific attempt to detect and measure the effect of an individual subatomic particle. For this and his work on the photoelectric effect Robert Millikan won the 1923 Nobel Prize in physics.
A schematic of the original Oil Drop Experiment.
Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This caused the air to become ionized, which basically means that the air particles lost electrons. A part of the oil droplets captured one or more of those extra electrons and became negatively charged.
By attaching a battery to the plates of the lower chamber he created an electric field between the plates that would act on the charged oil drops; he adjusted the voltage till the electric field force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. Some drops have captured more electrons than others, so they will require a higher electrical field to stop.
Particles that did not capture any of that extra electrons were not affected by the electrical field and fell to the bottom plate due to gravity. When a drop is suspended, its weight m · g is exactly equal to the electric force applied, the product of the electric field and the charge q · E.
The values of E (the applied electric field), m (the mass of a drop which was already calculated by Millikan), and g (the acceleration due to gravity), are all known values. So it is very easy to obtain the value of q, the charge on the drop, by using the simple formula:
m · g = q · E
Millikan repeated the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for q.
The charge q on a drop was always a multiple of 1.59 x 10-19 Coulombs. This is less than 1% lower than the value accepted today: 1.602 x 10-19 C.
The original Millikan Paper from Physical Review
Millikan oil drop experiment was a pretty good experiment that’s like listening to Vanessa-Mae plays Bach's Partitia in E
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