The discovery of the electron had its origins in a quest to get electricity to pass through a gas. Lightning in the sky is a common example of electricity travelling through air. However scientists in the nineteenth century explored ways to pass electricity through a gas in the sort of controlled way that had been achieved with metals and certain liquids. In their attempts they used high voltages and glass tubes where the gas they contained could be reduced to low pressure.
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Circular metal electrodes containing
slits are held in black rubber seals to allow the air in the
glass tube to be evacuated. The ends of the tube contain a
white fluorescent coating
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Two
developments moved the quest closer to success. In 1836, Nicholas
Callan, working in Maynooth, invented the induction coil (to provide
high voltages). Then in 1855, Henry Geissler, working in Germany
produced the first really efficient vacuum pump.
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Geissler and others observed many curious effects in discharge tubes. These tubes contained a gas and two or more metal plates. The plates were connected to a high voltage and the tube was connected to a vacuum pump so as to extract enough gas for the pressure of the remaining gas to be low. Various patterns of light of different colours were observed in a range of circumstances. (One can think of the modern day neon advertising signs as developing from this discovery).
| Discharge Tube at low pressure and high voltage. |
Geissler noticed that when the pressure went low
enough and the voltage high enough, that the glass behind the anode
glowed. He believed that radiation from the cathode had caused this.
(The cathode was the metal plate connected to the negative terminal of
the voltage source). Hertz was the first to demonstrate that this
radiation was penetrating and Goldstein introduced the name cathode rays
in 1876.
Many scientists began investigating the properties of cathode rays.
William Crookes (England) found that they travelled in straight lines
and when he found they had momentum and energy it suggested that they
were in fact particles (later called electrons, by George Stoney
(Ireland)).
| Crookes Tube |
Crookes placed a distinctive metal shape between the electrodes and found that a shadow was cast on the glowing coating. This suggested that the cause of the glow was rays emitted by the cathode hitting the phosphorescent material. The cathode rays travel towards the anode in straight lines, and continue past it for some distance.
| A modern day version of a Maltese Cross tube. |
Discovery that cathode rays have a negative
charge
In 1895 Jean Perrin (France) showed that the
cathode rays (electrons) had a negative charge. He made his discovery by
deflecting the cathode rays so that they accumulated on an electroscope.
| A modern day version of a Perrin tube. |
Discovery that the cathode ray was
infact a particle
In 1897 J.J. Thomson measured the charge-to-mass ratio (q/m) of cathode
rays, thereby confirming them to be particles and in the process is
creditted with discovering the electron.
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Thomson tried to measure the "charge-to-mass ratio of the
cathode rays. In order to do so he needed to find the
velocity with which they travelled.
He got around the difficulty that this posed by deflecting the rays one way with an electric field and then the opposite way with a magnetic field. His success in obtaining a value for q / m confirmed that cathode rays were particles, henceforth called electrons. |
Amongst Thomson's findings were that;
1. The value of the charge-to-mass ratio (e/m) of the electron was
always the same, irrespective of the metal used as cathode or the
gas in the tube.
2. The value of e/m for the electron was nearly 2000 times greater than
the comparable value for the hydrogen ion.
The work of Thomson represents the discovery of the electron and the
recognition that it was a fundamental constituent of all matter.
Thomson proposed the "plum pudding" model of the atom, where he
visualized the electrons as "plums" embedded in a "pudding" of positive
charge.
While he succeeded in measuring the ratio of the electron's charge to
its mass, niether the charge nor the mass were individually known until
Millikan's experiment of 1909.
The charge on the electron
In 1909, Millikan (an American) conducted his famous "oil-drop"
experiment. He sprayed tiny oil-droplets into a gap between two metal
plates. Those droplets which acquired a charge, came under the control
of the voltage applied to the plates. By measuring the voltage needed to
arest the fall of the droplets in a viscous medium he determined the
charge on the oil-droplets. Repeating the experiment for many droplets,
Millikan showed that the results could be explained as integer multiples
of a common value. His results were all integer multiples of a basic
number which he took to be the smallest possible unit of charge, i.e.
the charge on a single electron.
Thus, cathode rays are streams of electrons observed in vacuum tubes.
When the cathode is heated, it emits some radiation which travels to the
anode. If the inner glass walls behind the anode are coated with a
phosphorescent material, they glow.
| Millikan Oil-Drop Expt. |
