Amplificador e electrical telegraph: o que têm em comum

1. Passo 1. Amplificador — faz parte de — Hearing aid. An amplifier.
2. Passo 2. Hearing aid — evoluiu de — Telephone. The first electric hearing aids were built from telephone components.
3. Passo 3. Telephone — evoluiu de — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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