Editing Nomogram (section)

===Parallel-resistance/thin-lens===
[[Image:Nomogramparallelresistance.svg|right|200px|thumb|Parallel [[electrical resistance]] nomogram]]
The nomogram below performs the computation:

<math display=block>f(A,B)=\frac{1}{1/A+1/B}=\frac{AB}{A+B}</math>

This nomogram is interesting because it performs a useful nonlinear calculation using only straight-line, equally graduated scales. While the diagonal line has a scale <math>\sqrt{2}</math> times larger than the axes scales, the numbers on it exactly match those directly below or to its left, and thus it can be easily created by drawing a straight line diagonally on a sheet of [[graph paper]].

''A'' and ''B'' are entered on the horizontal and vertical scales, and the result is read from the diagonal scale.  Being proportional to the [[harmonic mean]] of ''A'' and ''B'', this formula has several applications.  For example, it is the [[Series and parallel circuits#Parallel circuits|parallel-resistance formula]] in [[electronics]], and the [[Thin lens|thin-lens equation]] in [[optics]].

In the example, the red line demonstrates that parallel resistors of 56 and 42&nbsp;[[ohm]]s have a combined resistance of 24&nbsp;ohms. It also demonstrates that an object at a distance of 56&nbsp;cm from a [[lens (optics)|lens]] whose [[focal length]] is 24&nbsp;cm forms a [[real image]] at a distance of 42&nbsp;cm.
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