CoachA resistor is a component manufactured to have a chosen, steady resistance — a calibrated obstacle, sold from fractions of an ohm to millions. Circuits use resistors to set currents on purpose: taming the march that would burn a delicate part, dimming, dividing, adjusting. Their striped color bands spell out their value; their symbol is a plain rectangle.
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Example 58.3 (Resistances around you)
A metre of thick copper wire: hundredths of an ohm — the level road. A glowing flashlight bulb: some ohms. A small electric motor: a few ohms. The kettle’s heating coil: about . Dry human skin, hand to hand: tens of thousands of ohms — wet, catastrophically less: the old damp-hands rule is a resistance statement. An open switch: resistance beyond all measuring — the infinite obstacle.
Example 58.5 (A resistor’s confession)
A session’s table, for one resistor:
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Double the push, double the march; triple, triple: and are proportional, and every ratio returns the same number: . That constant ratio is no accident — it is the resistor’s resistance, , showing itself in every row.
Example 58.7 (The law computes everything)
Once is known, the law answers in every direction. The kettle coil, on the mains: — the ten-ampere appliance explained. A resistor must limit a delicate lamp’s current to from : — pick the next standard value. Through flows : the voltage across it is . One law, three recipes, the whole workshop.