Measuring the latter allows a constant power load mode (as power = current * voltage). In his article, [Kerry] shows that he can simulate a load of up to 200W.

for which the load voltage is essentially constant and the current varies with the load. A transformer of this type is designed to operate at or just above the knee of the magnetization curve in order ...

The ‘317 can be configured in a constant current mode by connecting the chip’s adjustment pin to the output through a series resistance. A multiturn pot provides current adjustment ...