Theory and experiment went hand in hand in the work of Lord Rayleigh, in which the quest for rigor was a ubiquitous theme. To Rayleigh’s mind, though, and in contrast to mathematicians, physicists could proceed in their investigations without seeking absolute rigor. In his experimental practice, pursuing rigor involved the application of control strategies, which pervaded his work at various levels. Moreover, experimental control had various aims, such as standardizing measurement units in determining the ohm and validating experimental results in the discovery of argon. In the former case, Rayleigh and his team varied the design of their apparatus to control the experimental conditions. Dealing with errors was the main aim of their control practices and lay at the heart of their methodology. In the latter case, control was present in every step of the discovery process: the detection of discrepancies between the densities of atmospheric and “chemical” nitrogen, the identification of argon as a constituent of the atmosphere, and the subsequent exploration of its properties. The aim of this paper is to investigate and contrast the strategies of control employed in those two cases and to clarify their various purposes.