tanishqjain340 wrote this instructable detailing the build of his analog calculator project:

Do simple calculations with your math box.

The next time you need to crunch a couple of numbers, resist the urge to grab a digital calculator. Instead, round up some variable resistors, also known as potentiometers, and wire them into an analog mathematics rig. By twisting the potentiometers’ knobs and measuring the resulting voltage or resistance with a digital multimeter, you can perform simple multiplication and addition without a microprocessor in sight.

This is awesome :P Now add a capacitor and do integration!

Shame it’s flawed (and plagiarised as mentioned in the comments on that page, from here: http://www.popsci.com/article/diy/go-analog-resistance-based-calculator )

Originally I thought the circuit may have been a conventional analog calculator based on op-amps, but the author seems to believe he can achieve multiplication with resistors alone. The error is quite large, clearly trying to reduce the error by specifying a very low resistance for the first potentiometer (1k).