From the Latin for "measure", it's a small amount of something
[DOC] [TLE] Word List: Definitions of Scientific Instruments - The Phrontistery [PAR] Killers and Killing  Latin Adverbs and Prepositions ... chlorometer, instrument for 
measuring amount of chlorine in a solution ... clinometer, instrument used to 
measure slopes and elevations ... eriometer, instrument for measuring very small 
diameters ... iconometer, instrument for finding size of object by measuring its 
image.