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A black and white image of scientist J. J. Thomson wearing a coat and oval shaped spectacles.
J. J. Thomson (credit: www.firstworldwar.com, via Wikimedia Commons)
A diagram of the glass apparatus that was used to discover the electron in J. J. Thompson’s experiment.
Diagram of Thomson’s CRT. (credit: Kurzon, Wikimedia Commons)
Image of a cathode ray tube on x axis between two inverted L shaped north and south pole magnets on y axis, with z axis as a wire carrying high voltage supply to the charging plates inside the C R T. Zoomed image of the charging plate area inside the C R T showing the intersection of magnetic field between the poles in red lines towards south pole on the y axis along with an electron beam in green color line with velocity v toward right on the x axis.
This schematic shows the electron beam in a CRT passing through crossed electric and magnetic fields and causing phosphor to glow when striking the end of the tube.

What is so important about q e / m e size 12{q rSub { size 8{e} } /m rSub { size 8{e} } } {} , the ratio of the electron’s charge to its mass? The value obtained is

q e m e = 1 . 76 × 10 11 C/kg (electron). size 12{ { {q rSub { size 8{e} } } over {m rSub { size 8{e} } } } = - 1 "." "76" times "10" rSup { size 8{"11"} } " C/kg"} {}

This is a huge number, as Thomson realized, and it implies that the electron has a very small mass. Thomson went on to do the same experiment for positively charged hydrogen ions (now known to be bare protons) and found a charge per kilogram about 1000 times smaller than that for the electron, implying that the proton is about 1000 times more massive than the electron. Today, we know more precisely that

q p m p = 9.58 × 10 7 C/kg (proton), size 12{ { {q rSub { size 8{p} } } over {m rSub { size 8{p} } } } =9 "." "57" times "10" rSup { size 8{7} } " C/kg"} {}

where q p size 12{q rSub { size 8{p} } } {} is the charge of the proton and m p size 12{m rSub { size 8{p} } } {} is its mass. This ratio (to four significant figures) is 1836 times less charge per kilogram than for the electron. Since the charges of electrons and protons are equal in magnitude, this implies m p = 1836 m e size 12{m rSub { size 8{p} } ="1836"m rSub { size 8{e} } } {} .

Thomson performed a variety of experiments using differing gases in discharge tubes and employing other methods, such as the photoelectric effect, for freeing electrons from atoms. He always found the same properties for the electron, proving it to be an independent particle. For his work, the important pieces of which he began to publish in 1897, Thomson was awarded the 1906 Nobel Prize in Physics. In retrospect, it is difficult to appreciate how astonishing it was to find that the atom has a substructure. Thomson himself said, “It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms.”

Thomson attempted to measure the charge of individual electrons, but his method could determine its charge only to the order of magnitude expected.

An American physicist, Robert Millikan (1868–1953) (see [link] ), decided to improve upon Thomson’s experiment for measuring q e size 12{q rSub { size 8{e} } } {} and was eventually forced to try another approach, which is now a classic experiment performed by students. The Millikan oil drop experiment is shown in [link] .

Black and white image of physicist Robert Millikan wearing a jacket and a bow tie.
Robert Millikan (credit: Unknown Author, via Wikimedia Commons)
Image of the apparatus used in the Millikan oil drop experiment, consisting of a parallel pair of horizontal metal plates with a pin hole opening in the top plate. The top plate has positive charge and the bottom plate has negative charge. Picture of a flashlight as a bright source of light and a beam of light passing in between the plates from left is shown. A telescope is shown at the front and an oil atomizer above the positive plate is also depicted. A zoomed image of metal plates describing the force acting on the oil droplet is also shown. Arrows pointing upwards are forces of electric field while arrows pointing downwards depict the force of gravity.
The Millikan oil drop experiment produced the first accurate direct measurement of the charge on electrons, one of the most fundamental constants in nature. Fine drops of oil become charged when sprayed. Their movement is observed between metal plates with a potential applied to oppose the gravitational force. The balance of gravitational and electric forces allows the calculation of the charge on a drop. The charge is found to be quantized in units of −1.6 × 10 −19 C , thus determining directly the charge of the excess and missing electrons on the oil drops.

By 1913 Millikan had measured the charge of the electron q e size 12{q rSub { size 8{e} } } {} to an accuracy of 1%, and he improved this by a factor of 10 within a few years to a value of 1 . 60 × 10 19 C size 12{ - 1 "." "60" times "10" rSup { size 8{ - "19"} } " C"} {} . He also observed that all charges were multiples of the basic electron charge and that sudden changes could occur in which electrons were added or removed from the drops. For this very fundamental direct measurement of q e size 12{q rSub { size 8{e} } } {} and for his studies of the photoelectric effect, Millikan was awarded the 1923 Nobel Prize in Physics.

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Source:  OpenStax, Concepts of physics with linear momentum. OpenStax CNX. Aug 11, 2016 Download for free at http://legacy.cnx.org/content/col11960/1.9
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