I made this as an aid for repairing things like flowmeters or drywells, but the current source part is also usable to test leds etc.
It is very simple. The PT100 part is made from two 10 turn precision pots. One of them is bridged to decrease the max resistance. The values are not very important but the resolution is. The bridget one has a geared knob so it makes more as ten turns and gives me 1 mOhm resolution. I also bridged the whole trough a switch so the right hand dial reads in degrees centigrade. Not exact but that is often not needed. That function is just to make it easy while testing flowmeter transmitters, pt-100 based thermometers, dry-wells, oil baths etc. It can also be used as a sort of precision resistor substitution box.
The build current source has more electronics as drawn. Things like protection diodes, a fuse, a switch to turn the output off etc.