JOULES LAW STATES:
H= I2 X R X T
In this equation the current, resistance and time are the variables. The controller or timer can alter both the time and the current. The resistance of the parts being welded is fixed but the resistance can be changed by altering the force/pressure used in the welding process. The question is which will have the largest affect upon the process. The answer is the current (I).
This variable is a squared factor in the equation (I2). The other variables are not. Any change in current up or down will have the change squared which can be very large. Current is generally the first variable that is adjusted to bring a weld process back to acceptable values when the process has drifting out of the acceptable range.
Of course time and resistance can also be altered to get corrections in the process but they won’t be as dramatic. In some applications they may be the preferred choice due to geometry, specification, power restrictions and quality specifications.