Diode and pneumatic tube: what they have in common

1. Step 1. Diode — evolved into — Triode. De Forest added a grid.
2. Step 2. Triode — uses — Vacuum. It works in a vacuum.
3. Step 3. Vacuum — is used in — Hyperloop (concept). A vacuum in the tube.
4. Step 4. Hyperloop (concept) — was inspired by — Pneumatic tube. Capsules in a tube — pneumatic post for people.

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