The piece has parts
A steel tumbler looks like one solid piece. What is it made of?
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The cathode is the negative metal plate in the tube. The anode is the positive plate.
- A glass tube holds a gas at very low pressure.
- High voltage sends rays from the cathode to the anode.
- Change the cathode metal. The rays stay the same.
- Change the gas. The rays stay the same.
The idea
It is made of atoms, and atoms have still smaller parts.
Over 2,000 years ago, thinkers in India and Greece asked one question. What is everything made of? Acharya Kanada reasoned that dividing matter must end somewhere. The last pieces cannot be divided. He called them parmanus. The Greek word atomos means indivisible. These were ideas from thought. They did not come from experiments.
In 1808, John Dalton gave an atomic theory built on experiments. His atoms could not be broken into smaller parts.
Later, some elements gave out invisible radiation. This is radioactivity. It suggested that atoms have smaller parts inside.
In 1897, J. J. Thomson sent electricity through a gas at very low pressure. Rays went from the cathode (−) to the anode (+). These cathode rays were streams of negative particles. Each was far lighter than an atom. We call them electrons.
The rays looked the same for every cathode metal and every gas. So electrons must be inside atoms of every kind.
A charged plate bends the rays toward the positive plate. Opposite charges attract. So the rays carry negative charge.