Ive made this little rheostat box, simple circuit. 12 volts goes to 3 rheostats in parallel to control 3 fans, but heres where Im confused: when I turn one up, all of the voltage drops slightly ...

However, does this mean that only parasitic capacitance is important? No, as your resistor is normally connected to other parts in the circuit which are bandwidth limited, that also plays a role. The ...