The Cathode Ray Tube Experiment Explained

Before flat screens, every television and computer monitor contained a large glass tube with a beam of electrons inside it, painting pictures on the screen. That device, the cathode ray tube (CRT), began as a nineteenth-century laboratory curiosity, and it’s the apparatus that led to one of the greatest discoveries in science: the electron. This article explains how the tube works, what scientists observed, and what those observations proved.

What is a cathode ray tube?

A cathode ray tube is a sealed glass tube with most of the air pumped out, containing two metal electrodes:

• the cathode: the negative electrode
• the anode: the positive electrode

The electrodes are connected to a high-voltage power supply, often thousands of volts.

In later versions, the anode has a small hole in it, so that a narrow beam passes through and continues along the tube to strike the far end, which is coated with a material that glows when hit (a fluorescent or phosphorescent screen).

Why remove the air?

At normal air pressure, particles leaving the cathode would collide with air molecules almost immediately and never get far. With most of the air removed (a partial vacuum), they can travel the length of the tube. Improvements in vacuum pumps in the mid-1800s made these experiments possible.

What happens when the voltage is switched on

1. The high voltage pulls electrons out of the cathode (in many tubes the cathode is also heated, which releases electrons more easily).
2. The electrons are accelerated towards the positive anode.
3. Those passing through the hole in the anode continue in a straight beam.
4. When they hit the coated end of the tube, the screen glows at that spot.

Nineteenth-century scientists couldn’t see what was travelling along the tube. They could only see the effects, so they called it a cathode ray, meaning “a ray coming from the cathode”.

The key observations

Over several decades, scientists including Julius Plücker, Johann Hittorf, William Crookes and J.J. Thomson made a series of observations:

Observation	What it suggests
An object placed in the tube casts a sharp shadow on the screen	The rays travel in straight lines from the cathode
A small paddle wheel placed in the path turns	The rays can push objects, so they may carry momentum (though this was later shown to be partly a heating effect)
The beam is bent by a magnet	The rays are affected by magnetism, as moving charges are
The beam is bent towards a positive plate (in a good vacuum)	The rays carry negative charge
A metal collector in the beam gains negative charge	Confirms the rays are negatively charged
The same results occur with any cathode metal and any gas	The particles are part of all matter

How deflection works

Electric fields

If two metal plates are placed on either side of the beam and connected to a voltage, the beam bends towards the positive plate and away from the negative plate. Opposite charges attract, so the particles must be negative.

Magnetic fields

A magnet held near the tube bends the beam sideways. The direction depends on which pole is used, and it’s the direction expected for a moving negative charge (this can be predicted using the left-hand rule from physics).

Balancing both

If an electric field and a magnetic field are applied at the same time, pushing the beam in opposite directions, they can be adjusted until the beam goes straight again. From the field strengths needed, the speed of the particles can be calculated, and from how much one field alone bends the beam, the charge-to-mass ratio (e/m) can be worked out.

Thomson’s conclusion (1897)

J.J. Thomson used exactly this method at the Cavendish Laboratory in Cambridge. He found:

• The particles are negatively charged.
• Their charge-to-mass ratio is enormous, about a thousand times larger (by his measurement; about 1,836 times by modern values) than that of a hydrogen ion, meaning they’re far lighter than any atom.
• The ratio is the same whatever the cathode is made of.

His conclusion: cathode rays are streams of tiny negatively charged particles that are found in all atoms. They became known as electrons. This proved that atoms aren’t indivisible, overturning an idea held since Dalton. See Thomson’s plum pudding model and the electron.

Related discoveries from the same tubes

The humble discharge tube led to several other major discoveries:

• X-rays (1895): Wilhelm Röntgen noticed that a screen near a covered cathode ray tube glowed. Electrons hitting the glass or a metal target were producing a new, penetrating radiation. He took the first X-ray image, of his wife’s hand.
• Canal rays (1886): Eugen Goldstein used a cathode with holes in it and found positively charged rays travelling the opposite way. These were positive ions, and their study led towards the proton and the mass spectrometer. See mass spectrometry.
• Radioactivity (1896): Henri Becquerel’s investigation of whether fluorescent materials emit X-rays led him to discover radioactivity. See radioactive elements.

Cathode ray tubes in technology

For most of the twentieth century, CRTs were everywhere:

• Televisions and computer monitors: an electron gun fired a beam that was steered by magnetic coils across the screen, line by line, many times a second. The screen was coated with phosphors that glowed red, green or blue. Colour TVs had three electron beams.
• Oscilloscopes: used electric plates to steer the beam and display electrical signals as graphs.
• Radar displays and early computer memory.

Flat-panel LCD, LED and OLED screens replaced CRTs from the early 2000s, but the principle of an electron beam steered by fields lives on in electron microscopes, which use focused electron beams to image objects at near-atomic resolution.

Why it’s still worth studying

The cathode ray tube is a model example of how physical evidence builds up into a conclusion. No single observation proved that cathode rays were particles. Shadows showed straight-line travel, deflection showed charge, the charge-to-mass ratio showed tiny mass, and the same result from every metal showed universality. Only together did they make the case for the electron. Exam questions often ask students to link an observation to its conclusion, so it’s worth learning the table above as pairs.

Safety note

CRTs operate at high voltages and contain a vacuum; old sets can implode if the glass is broken, and some internal parts store dangerous charges long after being unplugged. Old CRT televisions shouldn’t be opened, and they must be recycled properly because the glass contains lead.

Key takeaways

• A cathode ray tube is an evacuated glass tube in which a high voltage drives a beam from the negative cathode towards the positive anode.
• Cathode rays travel in straight lines, carry negative charge and are deflected by electric and magnetic fields.
• Balancing electric and magnetic deflection lets the charge-to-mass ratio be measured.
• In 1897, Thomson used this to show that cathode rays are electrons, particles found in all atoms.
• CRTs also led to X-rays and powered televisions and oscilloscopes for most of the twentieth century.