Hi,
Sounds to me like you want to make an artificial resistor,
presumably to provide loading conditions for a battery or some
power supply unit to test that the power supply works ok.
Presumably you want to be able to vary the resistance of it
to provide a heavy or a light load.
My own 'dummy loads' from years ago were made with simply a heavy
resistor of low ohms value in series with a power transistor, the
load being set by adjusting a small pot across base to collector.
This was very primitive and only made up as and when needed,
such a unit would only work properly as a load for a small power
supply or a battery, as its clearly only for DC.
As Ron says, you don't mention if you are concerned with AC or DC,
and you dont mention the sort of wattage you are thinking of.
Also, the lowest voltage that your 'dummy load' is likely to meet
because making something that will mimic a resistor down at low
voltages gets increasingly difficult as the voltage drops below
less than a couple of volts.
John