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.