The Potential Divider

A potential divider allows an output voltage to be adjusted, to a set level.
It consists of two resistors  
\[R_1, \: R_2\]
  in parallel, with an out taken from contacts across  
\[V_2\]
.

The output voltage is  
\[V_{OUT}=\frac{R_2}{R_1+R_2} \times V_{IN}\]
.
The voltage is split across  
\[R_1, \: R_2\]
  in the ratio  
\[R_1 : R_2\]
. One of the resistors can be a variable resistor, then  
\[V_{OUT}\]
  can be adjusted by turning a knob. Such a device is called a potentometer.