Vacuum tube and railway semaphore: what they have in common

1. Step 1. Vacuum tube — evolved into — Diode. The first diode was a vacuum tube (Fleming); later diodes became semiconductor devices.
2. Step 2. Diode — includes — LED. A light-emitting diode.
3. Step 3. LED — is part of — Traffic light. LEDs.
4. Step 4. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

Open Evothings