Cathode Rays

Long before anyone knew what an electron was, physicists were watching mysterious glowing beams shoot across sealed glass tubes and arguing over what they could possibly be made of. Cathode rays were the first clue that atoms had something smaller hiding inside them.

Discharge tubes and low-pressure gas

Through the second half of the nineteenth century, physicists experimented with sealed glass tubes containing gas at very low pressure, with a metal electrode fused into each end. William Crookes built and refined the standard version of this apparatus, now known as the Crookes tube: a glass envelope, evacuated by a pump down to a small fraction of atmospheric pressure, with two electrodes connected to a source of high potential difference. As the tube is pumped down further and further, the character of what's seen inside changes — at moderate low pressure the whole tube glows, but at sufficiently low pressure the glow fades away almost entirely, except for a patch of fluorescence on the glass at the end of the tube furthest from the negative electrode.

Applying a large potential difference across the electrodes in this near-vacuum causes a beam to appear, travelling from the negative electrode (the cathode) toward the positive electrode (the anode). This beam was named a cathode ray, and the sealed tubes producing it, discharge tubes or cathode ray tubes. The name reflects only where the beam seemed to originate — at the time nobody knew whether it was a stream of matter, a form of radiation, or something else entirely.

Building up a picture: what the observations showed

Long before anyone could measure the properties of cathode rays precisely, a handful of simple observations built up a consistent picture of how they behaved.

• Straight-line travel. Cathode rays travel in straight lines from the cathode. A solid object placed in the tube between the cathode and the far wall casts a sharp-edged shadow on the fluorescing glass behind it, showing the rays don't bend around obstacles the way sound or water waves visibly do.
• Fluorescence. Wherever the rays strike the glass wall of the tube, or certain specially coated screens, the material glows (fluoresces). This glow is what let early experimenters see the beam's path and position at all, since the rays themselves are invisible.
• Mechanical momentum. A small paddle wheel, mounted on rails inside the tube so it is free to roll, can be made to turn when placed in the beam's path. Since only something carrying momentum can exert a force to turn the wheel, this showed cathode rays were not simply a form of light — they behaved as if they were physical, moving matter.
• Deflection in fields. The beam is deflected by both electric fields (bending toward a positively charged plate) and magnetic fields (bending in the direction given by the motor effect for a negative charge). Both deflections are consistently in the direction expected for a stream of negatively charged particles, never the opposite.

Particles, or a wave in the ether?

Even with all these observations in hand, what cathode rays actually were remained genuinely unsettled for years, and split opinion along roughly national lines. British physicists, including William Crookes himself, mostly argued cathode rays were a stream of charged material particles — fast-moving fragments of matter, small enough to pass essentially unimpeded through the tube. Physicists in continental Europe, including several influential German researchers, mostly argued instead that cathode rays were some kind of wave disturbance travelling through the ether — the hypothetical medium then thought to fill all space and carry light waves, similar in spirit to how sound waves travel through air.

Part of the difficulty in settling the argument was that several of the observed properties — straight-line travel and fluorescence in particular — were, on their own, roughly consistent with either picture. It was the combination of measurable momentum (favouring particles) and consistent deflection in both electric and magnetic fields (which a simple wave in a neutral ether shouldn't show at all) that tipped the argument firmly toward the particle picture — but a wave advocate could still, in principle, dispute the interpretation of any single result taken in isolation.

Resolving this argument decisively, and going further to measure the actual properties of whatever cathode ray particles were, was the problem J.J. Thomson set out to solve with the experiment covered in the next two lessons.

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