Brownian Motion as a Limit to Physical Measuring Processes: A Chapter in the History of Noise from the Physicists’ Point of View Martin Niss Roskilde University In this paper, we examine the history of the idea that noise presents a fundamental limit to physical measuring processes. This idea had its origins in research aimed at improving the accuracy of instruments for electrical measurements. Out of these endeavors, the Swedish physicist Gustaf A. Ising formulated a general conclusion concerning the nature of physical measurements, namely that there is a definite limit to the ultimate sensitivity of measuring instruments beyond which we cannot advance, and that this limit is determined by Brownian motion. Ising’s conclu- sion agreed with experiments and received widespread recognition, but his way of modeling the system was contested by his contemporaries. With the more embracing notion of noise that developed during and after World War II, Ising’s conclusion was reinterpreted as showing that noise puts a limit on physical measurement processes. Hence, physicists in particular saw the work as an indication that noise is of practical relevance for their enterprise. 1. Introduction In this paper, we examine the history of the idea among physicists that there is a fundamental limit to physical measuring processes and that this limit is set by noise. In contrast to previous studies (see Beller 1988; Hon 1989), that have focused on the realization of the existence of such a limit, we focus on the noise aspect of this history. In his monograph entitled Noise from 1954, the Dutch-American physicist and pioneer of noise Alder van der Ziel (1910–1991) described how noise (or spontaneous fluctuations as he saw as the more scientific term) came to be seen as of practical impor- tance for measurements: The study of spontaneous fluctuations was primarily theoretical as recently as 1925. At that time it was known that the fluctuating Perspectives on Science 2016, vol. 24, no. 1 ©2016 by The Massachusetts Institute of Technology doi:10.1162/POSC_a_00190 29