Electronics
Resistors in parallel
Two resistors against their combined resistance.
Readings
Drag the red line, or type into any two boxes — the third is solved. The marked box is the one being solved for.
Print at 100% scale, then check the 100 mm bar with a rule before trusting a reading.
Why this one is an instrument
The reciprocal scales bunch their large values into almost nothing, and that crowding is the point rather than a flaw in the drawing: it shows you, geometrically, that a large resistor in parallel with a small one barely matters. You can see the law of diminishing returns as a physical crush of tick marks at one end of the scale.
- Equation
- 1/R = 1/R₁ + 1/R₂
- Ranges
- R₁ 10–1000 Ω · R₂ 10–1000 Ω
- Reading
- Any two values fix the third. Outside the printed ranges the instrument stops being valid — that is what the end marks are for.
These are drawn from the stated equation and nothing else. Where an equation is itself an approximation — hyperfocal distance and tap drill sizes both are — the instrument inherits that approximation exactly, no better and no worse.