Thomson's work in identifying
the electron as a constituent part of atoms was of incredible
importance. Nevertheless, it fell short of a complete triumph
because Thomson's experiments could not determine the charge and
mass of the electron independent of each other. The charge of
the electron was determined by R.A. Millikan after an exhausting
research effort measuring the charge on oil droplets.
The apparatus and experiment can be described something like this. Oil of some known density can be sprayed into a fine mist by an atomizer. During this violent physical separation, the droplets acquire a residual charge caused by the presence of absence of an undetermined number of charge carriers. He built a pair of parallel plates of known separation and drilled a small hole in the top plate.Oil sprayed over the top plate would fall and a few droplets would accidentally fall through the hole and into the space between the plates where they were viewed with a microscope lens. Left alone the droplets fall by the combined action of gravity and air resistance (which is substantial for such a small object). Applying a voltage to the plates created an electric field between the plates and caused an additional force to act on the droplet. By timing the fall (and rise if he reversed the field) of the droplet between the plates, he could calculate the charge on the droplet. By itself, this is of little value because he did not know how many charge carriers were present. He then introduced a source of radiation from a radioisotope (next unit). The radiation ionizes random air molecules in the air which subsequently changes the charge on the droplet whenever the droplet collides with the ion. By measuring the new charge on the droplet he could determine by how much the charge changed. Still, he did not know how many charge carriers were added or removed. He repeated these steps thousands of times, each time getting a number which represented the change in charge on a droplet. He then analyzed the data to find the largest number that could fit into the change in charge. After several years of careful , eye-numbing work, Millikan concluded that the fundamental unit of charge on the charge carrier is no less than 1.6 x 10^-19 coulombs.
Remember that Thompson had measured the charge-to-mass ratio for cathode ray particles to be q/m = 1.76 x 10^11 C/kg. That means theat the mass of the electron can be determined by dividing the unit charge by the charge to mass
He said nothing about this being the limit of how small charges could be. There could be smaller charge carriers; he simply could not find them. And because atoms are electrically neutral, this number must apply to positive and negative charges.
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