Galvanometer and davy lamp: what they have in common

1. Step 1. Galvanometer — evolved into — Voltmeter. A voltmeter is a galvanometer with a large series resistor: a tiny current, proportional to the voltage, flows through it.
2. Step 2. Voltmeter — has part — Resistor. The series resistor gives the voltmeter a high resistance so it barely draws current from the circuit.
3. Step 3. Resistor — is made of — Carbon. Later — carbon.
4. Step 4. Carbon — includes — Coal. Mostly carbon.
5. Step 5. Coal — uses — Coal mine. Extraction.
6. Step 6. Coal mine — uses — Davy lamp. Protection against methane explosions.

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