Cathode Ray Experiment - 3D Virtual Lab (Thomson)

Tube at atmospheric pressure Start the pump to evacuate the tube

Observations Idle

ApparatusSet it up, then run

1 · Vacuum

1013mbar Atmospheric Air remaining: 100%

2 · Accelerating voltage

EHT across the tube2500 V

500 V5000 V

3 · Deflecting fields

Electric field strength0 V

OffLowMediumHigh

Magnetic field strength0.00 mT

OffLowMediumHigh

4 · Look closer

5 · Compare

Theory Read it after you have run the experiment

Quick check 0 / 8

8 / 8

What this experiment shows

Seal two electrodes into a glass tube, pump most of the air out and put a few thousand volts across them, and something crosses the tube. That something was called a cathode ray, because it came from the negative electrode, and for forty years nobody knew what it was made of. J. J. Thomson settled it in 1897 by pushing the ray about. He bent it with charged plates and found it moved towards the positive one, so whatever it was carried a negative charge. He bent it with a magnetic field and found it curved. Then he set the two fields against each other until the beam ran straight again, which gave him the speed of the particles, and from the curvature he got their charge to mass ratio. The number came out about eighteen hundred times larger than for a hydrogen ion, and it was the same for every gas and every cathode metal he tried. One particle, present in every kind of atom. The atom was not indivisible after all.

v = √(2 (e/m) V) · balance: eE = Bev → v = E/B · e/m ≈ 1.76 × 10¹¹ C/kg

What you need

A sealed glass discharge tube with a heated cathode and a slitted anode
A rotary vacuum pump and a pressure gauge on the tube
An extra-high-tension supply, a few hundred to a few thousand volts
A pair of parallel deflection plates inside the tube
A pair of Helmholtz coils outside it, one each side
A fluorescent mica screen printed with a centimetre grid, so the path shows
A light mica paddle wheel on two rails, which can be put in or taken out

How it is done

Switch the EHT on with the tube still full of air, and note that nothing crosses it.
Start the pump. Watch the gauge fall and the gas begin to glow as the pressure drops.
Keep pumping past the glow. A sharp beam appears on the screen once the gas is thin enough.
Switch the plates on. Note which way the beam bends and what that says about its charge.
Switch the plates off and the coils on. Note that the beam now curves rather than kinks.
Switch both on and adjust one until the beam is straight again, then read off E and B.
Take both fields off, put the paddle wheel in, and watch the beam spin it along the rails.

What you should see

With the tube full of air nothing happens: the beam cannot get a millimetre before it collides.
Between roughly 10 mbar and 0.01 mbar the gas itself glows, in bright bands with dark spaces between them.
Below that the gas stops glowing and a sharp beam is seen instead, travelling in a straight line from the anode slit to the far end.
A higher accelerating voltage makes the beam faster, and harder to deflect.
With the plates live the beam bends towards the positive plate: a parabola between the plates and a straight line after them.
With the coils live the beam follows a circular arc while it is in the field.
With both live and correctly set the beam is straight again, because the two forces cancel.
With the paddle wheel in the beam it turns and creeps along the rails away from the cathode; deflect the beam off the vanes and it slows to a stop.

Where marks are lost

Saying a cathode ray is a ray of light. It is a stream of particles - electrons - and that is exactly what the deflection experiments prove.
Forgetting the vacuum. Nearly every mark about why this experiment needs a low pressure is about the mean free path: the electrons must be able to cross the tube without colliding with gas molecules.
Confusing the glow discharge with the beam. The glow is the leftover gas being ionised; the beam only appears once the gas has almost gone.
Saying the beam bends towards the negative plate. It bends towards the positive one, because the particles are negative.
Treating the magnetic path as a parabola. A magnetic force is always at right angles to the velocity, so its size never changes and the path is a circular arc.
Claiming Thomson measured the charge of the electron. He measured the ratio e/m; Millikan measured e itself, in 1909.
Treating the paddle wheel as proof on its own. It is good evidence for momentum, but in a tube with gas left in it, heating at the vane surface pushes too - which is why the deflection experiments were the decisive ones.

Questions students ask

Why must the tube be evacuated?

Because at ordinary pressure an electron hits an air molecule after about a millimetre and never reaches the far end. Pumping the gas out lengthens the mean free path until the electrons can cross the whole tube without a collision, and only then is there a beam.

Why does the gas glow part-way down?

Between roughly 10 mbar and 0.01 mbar there is still enough gas to hit but a long enough gap between collisions for an electron to pick up the energy needed to ionise what it hits. The gas gives out light as it recombines. That is a glow discharge, and it is the same physics as a neon sign.

Which way does the beam bend in an electric field, and what does it prove?

Towards the positive plate. A charge is attracted to the plate of opposite sign, so the particles in the beam must be negatively charged. That single observation is why cathode rays could not be light.

Why is the magnetic path an arc and the electric path a parabola?

The electric force has a fixed size and direction, like gravity on a thrown ball, so the path is a parabola. The magnetic force stays at right angles to the velocity, so it changes the direction but never the speed - which is circular motion, radius r = mv/(eB).

How does balancing the two fields give the speed?

When the beam is straight the two forces are equal: eE = Bev, so v = E/B. Both E and B are known from the settings, so the speed follows without knowing the charge or the mass.

And how do you get e/m from that?

All the electrical energy an electron gains becomes kinetic energy, so eV = ½mv² and therefore e/m = v²/2V. Put in the speed from the balance and the accelerating voltage, and you have the charge to mass ratio: about 1.76 × 10¹¹ C/kg.

Why does a paddle wheel in the tube turn?

Because the rays arrive with momentum. Something with mass is hitting the vanes and pushing them, which is what you expect of particles and not of light. Deflect the beam off the vanes and the wheel stops, which shows it really is the beam doing the work. Be honest about one thing, though: in a tube with a little gas still in it, gas heated at the vane surface adds to the push, so the wheel alone was never conclusive.

Why did the same value for every gas matter so much?

Because it meant the particle was not a piece of the gas. Whatever the tube was filled with, and whatever the cathode was made of, the same particle came out - so it had to be a component of every atom. That is the discovery of the electron, and the end of the indivisible atom.

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Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.