What is the equation for a potentiometer with dual power supply?
How about,  V = 20x-10
where x is the potentiometer wiper position as a fraction from 0 to 1.

Other related points (to consolidate those made in the follow-up comments and answers):


General form: \$ V_{out} = x(V^+-V^-)+V^-\$
Potentiometers (pots) are generally used in low power applications. Make sure to check your part's specs to ensure it will operate properly in your circuit.
Pot wiper terminals are typically designed to carry low current. Again, check the datasheet.
In response to your question in the comments section: if you intend to test your device with an ohmmeter, be sure to first disconnect the power source in order to obtain accurate measurements.