Activated carbon and optical telegraph: what they have in common

1. Step 1. Activated carbon — is a kind of — Carbon. Porous carbon.
2. Step 2. Carbon — is material for — Hearing aid. The first hearing aids used a carbon microphone.
3. Step 3. Hearing aid — evolved from — Telephone. The first electric hearing aids were built from telephone components.
4. Step 4. Telephone — evolved from — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).
5. Step 5. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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