Welding and telephone: what they have in common

1. Step 1. Welding — uses — Tungsten. TIG welding uses a tungsten electrode.
2. Step 2. Tungsten — is material for — Triode. The filament is tungsten.
3. Step 3. Triode — is part of — Amplifier. Tube amplifiers.
4. Step 4. Amplifier — is part of — Hearing aid. An amplifier.
5. Step 5. Hearing aid — evolved from — Telephone. The first electric hearing aids were built from telephone components.

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