J. J. Thomson’s cathode ray experiment

Mass spectrometry is an analytical technique that makes use of an instrument called a mass spectrometer to identify and quantify ions based on their mass-to-charge ratios. J. J. Thomson, an English physicist, constructed one of the earliest mass spectrometers (see diagram below) and demonstrated in an experiment in 1897 that atoms are made of subatomic particles called electrons. Using the spectrometer, he determined the charge-to-mass ratio of an electron.

The ioniser on the left of the spectrometer is not a complete vacuum but filled with a trace amount of air molecules, which are always in equilibrium between their neutral and ionic forms due to natural occurring processes like photoionisation. Consequently, free electrons are present amongst the trace amount of air molecules in the ioniser.


Instead of producing electrons from the cathode via a heated filament, the ioniser works on the principle of a ‘cold cathode’, where a high potential difference maintained between the cathode and anode accelerates the free electrons present in the trace amount of air. These electrons collide with other gas molecules and knock more electrons off them, creating a cascade of ions and electrons called a Townsend discharge. The electrons are attracted towards the anode where they are sharpened to a beam called a cathode ray and continue their way to the electric field plates and electromagnet where they are deflected.

When the electrons eventually strike the atoms in the glass at the right end of the spectrometer, they excite electrons intrinsic to the atoms to a higher energy state. When these excited electrons relax back to the ground state, they emit light in the form of fluorescence and the angle of deflection, θ, can be read off the fluorescent spot on the scale.

Working out the mechanics of the trajectory of the cathode ray, Thomson managed to calculate the charge-to-mass ratio of an electron and concluded that the negatively charged ‘corpuscles’ (the term he used back then for electrons) must have come from the dissociation of the trace amount of gas molecules, since the charge-to-mass ratio of a corpuscle is about 1,700 times that for a charged hydrogen atom (the charge-to-mass ratio of a hydrogen ion had been investigated earlier on).


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