Artificial geysers (Water Heaters) work on the Heating Effect of Electric Current.
Such water heater have copper coils. When electric current is passed through it, it heats up a metal plate, which heats up the water.
The heat energy produced can be calculated from the equation:
H = I² R t
where H = Heat Energy produced
I = Electric Current passing through coil
R= Resistance of coil
t = Time for which current flows.
This equation is called Joule's Law.
Generally, the coil used is of high resistance so as to obtain more heat energy.