The colourful episodes discussed in preceding pages enable us better to understand the changing and contested practices for measuring electrical performance in the latter part of the nineteenth century. In his 1883 ICE lecture, Sir William Thomson predicted that the practice of electric lighting would transform electrical-measurement techniques just as telegraphy had done since the early 1860s (Chapter 3). Chapters 4–6 of this book describe how the new technological enterprise did indeed stimulate practitioners to develop new kinds of instruments that embodied new techniques and new understandings of what constituted measurement – both in the general case and in the specific case of electrical practice. Yet in this process they brought lingering unresolved problems into the foreground about what measurement actually was, who counted as a measurer, and what and whom should be trusted or otherwise in the measurement process. Moreover, it raised awkward questions about whether accuracy was an easily identifiable and readily quantifiable attribute of a measurement.