This is a handy reference chart combining Ohm's Law and Joule's Law, manufactured as a 10 cm diameter printed circuit board. A hole is provided at the top for hanging the wheel.

The front side contains expressions for making calculations involving voltage, current, resistance, and power. To calculate one of the inner quantities, use two known quantities and the appropriate outer expression. Example: a 220 ohm resistor with 750 milliamperes of current flowing through it has a power dissipation of 123.75 watts and produces a voltage drop of 165 volts.

The opposite side features public domain artwork of Georg Simon Ohm (1789 - 1854) and James Prescott Joule (1818 - 1889).

We also have a few of these available in red/gold, black/gold, and laser etched wood (eBay item ID: 145407414242), as well as some with minor cosmetic defects that are offered for a bit cheaper.