Jandel HM21 Portable Four Point Probe Test Meter, designed for use with a four point probe to make sheet resistance and/or volume resistivity measurements. The unit is battery powered, however, it also includes a universal AC power adapter so that it does not have to run on batteries when portability is not important. The HM21 supplies a constant current and displays either the resultant voltage or the sheet resistance in ohms/square depending which function has been chosen. More info: https://www.jandel.co.uk/product/jandel-hm21-test-unit/
