Lord Kelvin famously stated that “when you can measure what you are speaking about and express it in numbers you know something about it; but when you cannot measure it … your knowledge is of a meagre and unsatisfactory kind” (Thomson 1891: 80). Today, in an age when thermometers and ammeters produce stable measurement outcomes on familiar scales, Kelvin’s remark may seem superfluous. How else could one gain reliable knowledge of temperature and electric current if not through measurement? But the quantities called “temperature” and “current” as well as the instruments that measure them have long histories during which it was far from clear what was being measured and how-histories in which Kelvin himself played important roles (see Chang 2004: 173-86; Gooday 2004: 2-9).