Nice site. Good work and nice idea using mineral oil for cooling and a recycle paint can. Very "McGyver". XD
@Nick Gammon
I did the simulation using Circuit Wizard. http://www.new-wave-concepts.com/.
During the simulation, the pot was set around 65 %, the current going through the MOSFET and the resistor was 2.8 A. A saturated value. I increace the R value, the current max was reduced, but the pot at low setting ( under 10 % ) the current at R was under 100 mA. Decreasing the R, will increase the I across the R. The low setting of the pot will be increase ( start around 250 mA ). Also, it depend the MOSFET type ( resistance across the MOSFET ) , the voltage at the gate ( to optain saturation value ) and the power resistor ( controlling the max current ).
I will figure out a better way to design and make the variable dummy load. It a matter of tweeking the circuit so I can get a low current through the R ( at low pot setting ) and set a max value ( at max pot setting ), so I can test my PSU I built / test.
I may use the Arduino to monitor the current going through the R. Right now, a current meter wil do the job.