The development of the BA's ‘absolute’ units and standards of electrical resistance from the 1860s is well documented. William Thomson and his allies eventually persuaded many others to express their electrical measurements in interrelated units of length, mass, and time through the principle of energy conservation. Nevertheless, as Schaffer rightly observes, there was nothing inevitable about the way that the BA absolute unit of resistance (later the ‘ohm’) displaced its chief rival, the Siemens’ mercury unit.

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.

Of all the measurement devices used in early electric-lighting projects, the domestic meter was the most commercially significant. More of them were manufactured for the constituency of domestic consumers than the ammeters produced for practising electrical engineers. Yet, as the only electrical instrument expected to operate reliably for long periods in inclement conditions far away from company surveillance, it was also the most problematic to construct and operate within the commercially useful degrees of accuracy cited by James Swinburne in the second epigraph in the opening of this chapter.

As James Clerk Maxwell knew perhaps better than anyone else, dealing with electricity was no dull or easy matter. Like many contemporaries in industrial and academic spheres who sought to harness electricity to technological ends, he laboured extensively to comprehend its complex and occasionally shocking behaviour. Yet as Maxwell hinted early on in his famous Treatise, there was much uncertainty about what electricity actually was.

Like William Thomson, George Carey Foster and his colleagues were sure that measurement furnished a reliable grasp of how the world worked. They did not need to ask searching questions about what measurement actually was nor about how measurement produced knowledge. In regard to electrical measurement, such matters were so self-evident for them that they devoted little effort to theorizing their certainties.

Why did late nineteenth-century electricians decide to trust particular measurement instruments in preference to others? Was it a particular faith in the theory that informed the design of their electromagnetic mechanism? Was it their trust in the instrument-maker's effectiveness in enacting reliable designs for the devices? Or was it the fidelity with which users could take quantitative readings from performances yielded by the instrument? We need to consider these sorts of questions to recover how evaluations were made of the trustworthiness of the new species of ‘direct-reading’ current-measuring instruments that came onto the market in the 1880s. In contrast to the idealist tradition common in the history of science that treats instruments as essentially theory-laden devices, I consider measurement instruments from the perspective of users as technologies that had to be read. As in the previous chapter, my aim here is to show that pressing problems of trust were generated by the recalcitrance and fallibility of instrumental readings.

Preceding chapters discussed how the industries of submarine telegraphy and electric lighting prompted concern for measuring electrical properties and were the main audiences for new technologies that measured electrical resistance and direct current, respectively. This chapter extends this theme to consider the emergence of self-induction as an important but troublesome parameter in the technology of ac generation developed in the late 1880s. My main focus is the fate of the ‘secohmmeter’, an instrument designed by William Ayrton and John Perry to measure self-induction. Hitherto practitioners had often treated self-induction as a source of major errors in measurements of electrical resistance. For example, in 1881 Lord Rayleigh diagnosed that neglect of self-inductive effects in the operation of ‘whirling-coil’ apparatus had lead to an error of more than 1% in the BAAS Electrical Standards Committee's determination of absolute resistance fifteen years earlier (Chapter 3).

Whom and what should people trust or distrust? This question has long been a prominent concern not only in everyday human transactions but also in the most abstruse domains of science, commerce, and technology. Both Steve Shapin and Ted Porter have shown the significance of this question in the complex relationship between trust and quantification. They demonstrate that, to a certain extent, Restoration natural philosophers and nineteenth century engineers were able to win greater trust for their claims by giving them quantitative expression. At the same time, though, Shapin and Porter map some of the important historical contingencies of the subject. Quantification has not always been achieved to the satisfaction of all, nor has it necessarily made claims uniformly more highly trusted by all parties. Therefore, to avoid facile transhistorical generalizations about the relations between trust and numerical work, the historian has to ask questions rather more socio–historically specific in nature.