Light pen and boiler: what they have in common

1. Step 1. Light pen — has part — Photodiode. A photocell catches the CRT beam.
2. Step 2. Photodiode — is part of — Exposure meter. A photocell.
3. Step 3. Exposure meter — is part of — Camera. A built-in light meter.
4. Step 4. Camera — is part of — Bubble chamber. Photographing the tracks.
5. Step 5. Bubble chamber — uses — Latent heat. A particle leaves a trail of boiling bubbles.
6. Step 6. Latent heat — is used in — Boiler. Evaporation needs latent heat.

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