Roller mill and atomic clock: what they have in common

1. Step 1. Roller mill — evolved from — Gristmill. Rollers instead of millstones.
2. Step 2. Gristmill — has part — Gear. The drive from the power source to the millstone.
3. Step 3. Gear — is part of — Pendulum clock. A wheel train.
4. Step 4. Pendulum clock — evolved into — Quartz clock. Quartz replaced the pendulum as the frequency standard.
5. Step 5. Quartz clock — evolved into — Atomic clock. The quartz oscillator is tuned to an atomic standard.

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