The Discovery of the Electron

Determining who discovered the electron is a little fuzzy, like the electron itself. As our introduction to Quantum Mechanics, let's look at some of the history and the people involved with the early experiments. In the later part of the 19th century, physicists were beginning to experiment with cathode ray tubes - glass tubes with 2 high voltage plates (an anode (+) and cathode (-) plate) and a phosphorous coated screen on the end. It was noticed that, when electricity was applied to the tube, a dark stream could sometimes be seen in the tube and the end of the tube would glow. It was also shown that electric and magnetic fields could deflect the position of the glowing spot.

J. J. Thomson used a special cathode ray tube fitted with electric plates and magnetic coils (so that he could measure the strength of the E field and the B field) and thus measure directly the charge-to-mass ratio (e/m) of the particles that were producing the glow. Thomson was the first to do in-depth studies and measurements of these cathode particles, and thus he is generally credited with having discovered the electron. He determined the charge-to-mass ratio, e/m, to be 1.76x1011 Coulombs/kilogram. At the time, there was no accurate way to determine either the mass or the charge separately. But Thomson did believe that the particles he was measuring were part of the structure of the atom. (For a great exhibit by the American Institute of Physics on the
discovery of the electron, click here.)

Enter Robert Millikan. Millikan was the first to precisely measure the charge of an electron using his oil-drop experiment. He determined the value of q for an electron was 1.6x10-19 C. From this, and the e/m ratio, the mass of an electron could be determined: 9.1x10-31 kg.  These experiments help determined a couple of important
facts:

  • The mass of the electron was significantly less than the known mass of the smallest atom. This confirmed that the electron was a part of the atom.
  • The charge due to the electron was observed only in integer multiples of the value of the charge - that is to say, an electron always had a charge of 1.6x10-19 C and that if there were multiple electrons, the total charge was an integer multiple of this number. This led to the idea that charge exists in quantized packets - discrete, finite and predictable amounts. This is an idea we will find repeated elsewhere.

Where do we go from here? Before we examine the structure of the atom further, let's take a look at some fundamentals of quantum mechanics in our next lesson.

For more on this topic, try: http://webserver.lemoyne.edu/faculty/giunta/thomson1897.html

For Practice Problems, Try: Giancoli Multiple Choice Practice Questions (Don't worry if you can't solve all of them just yet!)