When resistors are placed in series the same current will flow through each of them. This means that we can use resistors to divide up the supply voltage into smaller amounts, since each resistor will have a voltage drop across it that is dependent on its resistance.
In figure 1 we have a power supply connected to two resistors, which are in series. To calculate the voltage across each resistor we can use this formula: Vout = (Vin x R2) / (R1 x R2)
Although voltage dividers are very useful, an important consideration is the power flowing through the resistors!