Exposure meter and davy lamp: what they have in common

1. Step 1. Exposure meter — is a kind of — Photometer. A photometer.
2. Step 2. Photometer — uses — Reflection of light. Brightness is judged from reflected light.
3. Step 3. Reflection of light — is used in — Headlight. A reflector gathers the light into a beam.
4. Step 4. Headlight — uses — Acetylene. The first headlamps burned acetylene.
5. Step 5. Acetylene — is used in — Carbide lamp. Burning acetylene.
6. Step 6. Carbide lamp — evolved from — Davy lamp. Miners’ lamps.

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