Amplifier and electrical telegraph: what they have in common

1. Step 1. Amplifier — is part of — Hearing aid. An amplifier.
2. Step 2. Hearing aid — evolved from — Telephone. The first electric hearing aids were built from telephone components.
3. Step 3. Telephone — evolved from — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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