Caesium and fitness tracker: what they have in common

1. Step 1. Caesium — is material for — Atomic clock. Essen’s clock (1955) used a caesium atomic beam.
2. Step 2. Atomic clock — evolved from — Quartz clock. The quartz oscillator is tuned to an atomic standard.
3. Step 3. Quartz clock — evolved from — Mechanical clock. Quartz clocks replaced mechanical ones: a vibrating quartz crystal took the place of the pendulum and balance.
4. Step 4. Mechanical clock — includes — Wristwatch. A wristwatch is a mechanical clock made small enough to wear.
5. Step 5. Wristwatch — evolved into — Smartwatch. The form factor of a wristwatch.
6. Step 6. Smartwatch — evolved from — Fitness tracker. Tracker functions moved into watches.

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