Caesium and fuel cell: 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 — is used in — GPS (satellite navigation). The satellites carry atomic clocks; the position is calculated from signal arrival times.
3. Step 3. GPS (satellite navigation) — is part of — Reusable rocket. Navigation during landing.
4. Step 4. Reusable rocket — has part — Liquid-propellant rocket engine. Liquid-propellant engines.
5. Step 5. Liquid-propellant rocket engine — uses — Hydrogen. Hydrogen is an efficient fuel.
6. Step 6. Hydrogen — is used in — Fuel cell. The fuel is hydrogen.

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