Martindale mm39 auto ranging digital multimeter martindale mm39 p the martindale mm39 is an auto ranging multimeter with a large rotary switch for easy selection of the required test function even when wearing gloves there are two test buttons one to lock the range and the other to hold the results which are displayed on a 3 frac12 digit lcd screen p p the main test functions are ac voltage dc voltage dc current and resistance the mm39 measures both ac and dc voltage between 0 1mv