Leakage is equivalent to a resistor in parallel with the capacitor. Thus the input voltage approximately equals the voltage across the resistor. A capacitor can also act as an AC resistor. Consider a divider consisting of a resistor and capacitor as shown in Figure 3. The joystick potentiometer acted as a variable resistor in the RC network. Another type of thermistor is a silistor, a thermally sensitive silicon resistor. The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2. In electrical terminology, the plasma membrane functions as a combined resistor and capacitor. The impedance may be a resistor, or an inductor (coil). Not all power supplies contain a small "bleeder" resistor to slowly discharge this capacitor. This consists of a capacitor in series with a resistor. Suppose that the output voltage of interest is the voltage across the resistor. Applying a voltage through a safety resistor repairs the oxide layers. The output current is that resulting from dropping the reference voltage across the resistor. It keeps up this constant voltage across the constant sense resistor.