Chasing the Negative Ray

Why it matters

In a glass tube with almost no air, a mysterious ray shoots from the negative electrode. It casts a sharp shadow of a cross placed in its path.

Predict first

You set up charged plates beside the tube and the ray bends toward the positive plate. What does this tell you about the ray?

Only charged particles are deflected by an electric field. Since the ray bends toward the positive plate, it must carry a negative charge.

• It is made of neutral light waves
• It consists of negatively charged particles — correct
• It is a stream of positive protons
• It is just heat moving through the vacuum

What you do

Play detective in Thomson's lab: match each cathode-ray observation with the conclusion it forced about the electron.

Check yourself

Why did J.J. Thomson conclude the electron has negligible mass?

Greater deflection in the same field implies a much lower mass-to-charge ratio.

Cathode rays are identical regardless of the gas or metal used. What does this prove?

Since the ray is the same from every source, the electron is a fundamental building block of all matter.

After discovering electrons, Thomson pictured the atom as a 'plum pudding'. What was the pudding?

Atoms are neutral overall, so Thomson balanced his negative electrons inside a smeared-out sphere of positive charge — the model Rutherford would soon overturn.

• It was invisible to the naked eye
• It deflected much more than hydrogen ions in electric fields — correct
• It came from the cathode, not the anode
• All atoms contain the same negative particles — correct
• Gases are made of electrons
• Electricity is a fluid
• Vacuum tubes are universal
• A sphere of positive charge with tiny electrons dotted inside — correct
• A miniature solar system with electrons in orbit
• A hollow shell with nothing inside