Seeing the Invisible – Electrons in a Cathode Ray Tube
Electrons… tiny, negatively charged particles that you can’t see, touch, or smell.
Yet in the lab, we can make them visible.
One of my favourite demonstrations (and a classic in physics) is using a Cathode Ray Tube (CRT) to “see” electrons in action. It’s a beautiful mix of theory and real-world evidence.
What is a Cathode Ray Tube?
A CRT is essentially a vacuum tube with three key parts:
- Electron gun – fires electrons from a heated cathode
- Deflection plates – control the path of the electrons
- Fluorescent screen – glows when struck by electrons
In a vacuum, electrons travel in straight lines. When they hit the screen, they produce a glowing spot — suddenly, the invisible becomes visible.
What Does This Show Us About Electrons?
This simple setup reveals some fundamental properties of electrons:
1. Electrons have mass
They travel in straight lines and can be deflected. Anything that changes direction must have mass.
2. Electrons carry negative charge
Apply an electric field across the plates and the beam bends toward the positive plate. That tells us the charge is negative.
3. Electrons can be accelerated
By increasing the voltage, the beam moves faster and hits the screen with more energy.
4. Electrons behave predictably
They follow well-defined paths under electric and magnetic fields — essential for understanding circuits and modern electronics.
The Key Experiment
This work traces back to J. J. Thomson in 1897, who used a CRT to measure the charge-to-mass ratio of the electron.
His conclusion?
Atoms were not indivisible after all.
That discovery completely changed physics.
Why This Still Matters Today
CRT technology may feel old-fashioned (unless you’ve still got an ancient TV in the loft), but the principles are everywhere:
- Oscilloscopes in school labs
- Electron microscopes
- Particle accelerators
- Even the foundations of modern electronics
And more importantly for students:
👉 It’s a perfect exam topic – linking electricity, fields, and particle physics in one neat experiment.
A Classroom Twist
When I run this demonstration, I often ask students:
“Are we actually seeing electrons… or just the effect of electrons?”
It’s a great way to push thinking beyond memorising facts and into understanding evidence.
Exploring Electron Properties with Teltron Tubes
There’s something rather magical about switching on a Teltron tube in a darkened lab…
A faint green glow appears… then a beam… and suddenly you are watching electrons move in real time.
For students, this is often the moment when abstract physics becomes real.
What Are Teltron Tubes?
Teltron tubes are modern versions of the classic cathode ray experiments. They allow us to investigate electrons under controlled conditions with much clearer visual results than older equipment.
Typically, they include:
- A low-pressure gas-filled tube (so the electron path glows)
- An electron gun to produce a beam
- Electric and/or magnetic field controls
- Often Helmholtz coils to create a uniform magnetic field
The result? A visible beam of electrons that we can bend, shape, and measure.
What Can We Learn?
Using Teltron tubes, students can explore several key properties of electrons:
1. Electrons Travel in Straight Lines
With no external fields applied, the beam travels directly from the cathode to the screen.
Evidence that electrons behave like particles with momentum.
2. Electrons Carry Charge
Apply an electric field and the beam deflects.
Just like in the experiments of J. J. Thomson, the direction of deflection shows the electron is negatively charged.
3. Magnetic Fields Curve Electron Paths
Switch on the Helmholtz coils and something wonderful happens…
The straight beam becomes a circle.
That circular motion is caused by the magnetic force acting perpendicular to the velocity of the electrons.
4. Measuring the Charge-to-Mass Ratio
This is where it gets really interesting.
By adjusting the magnetic field and measuring the radius of the circular path, students can calculate the specific charge (e/m) of the electron.
The relationship is:
(Combined with energy from accelerating voltage in full derivations.)
This is not just theory — this is a real experimental measurement of a fundamental constant.
Why Teltron Tubes Are Brilliant for Teaching
From years of teaching, these are a few reasons they work so well:
- Students can see the beam move instantly
- Adjusting controls gives immediate feedback
-
It links multiple topics:
- Electricity
- Magnetism
- Circular motion
- Particle physics
And perhaps most importantly…
It encourages curiosity.
Students start asking:
- “What happens if I increase the voltage?”
- “Why does the circle get bigger?”
- “Can we stop it completely?”
That’s when real learning begins.
A Classic Demonstration Trick
I often start by asking:
“If electrons are so small… how can we possibly see them?”
Then I switch off the lights and power up the tube.
The reaction is always the same.
A quiet:
“Whoa…”
Bringing It Back to Exams
Teltron tube experiments regularly underpin exam questions on:
- Magnetic fields and forces
- Circular motion
- Energy and accelerating voltage
- Experimental methods and uncertainties
So while it looks like a bit of fun…
It’s also serious exam preparation.
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