Potentiometer and bellows: what they have in common

1. Step 1. Potentiometer — is a kind of — Resistor. A variable resistor.
2. Step 2. Resistor — is made of — Carbon. Later — carbon.
3. Step 3. Carbon — is material for — Coke. Coke is almost pure carbon: everything else is baked out of the coal.
4. Step 4. Coke — is used in — Blast furnace. The fuel and reducing agent is coke (formerly charcoal).
5. Step 5. Blast furnace — has part — Bellows. Air was blown in by bellows, later by blowers.

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