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Note that R1 is nearest to the Positive + rail and R2 is nearest to the Negative  rail. With the two resistors in this configuration a voltage somewhere between the maximum and minimum would be measured at the point marked "out" depending upon the values of the resistors. This fixed combination is usually replaced by a single resistor in which the output can be changed by moving a slider along the track.
The output could be taken from anywhere along the resistor. Without necessarily knowing it you have all used such a potentiometer on the audio gain control on your transceiver / receiver !!! Potential divider formula However a potential divider can be two or more resistors but what ever number it is the resultant formula would be as we would have to do separate calculation to end up with two resistors. Looking at the diagram, you can see that the output is leaving the potential divider of R1, Ra and Rb at the link between R1 and Ra. With Ra and Rb are in series we can simply add together their values and come up with R2. This R2 we can use in the formula ................. simple eh ??
The V_{out} is thus dependent upon :
Example if Volts in are 12V R1 = 10 ohms and R2 = 30 ohms then Vout= 12 x 30/(10 + 30) Vout = 12 x 0.75 Vout = 9V




