While the discovery of the ionosphere through wireless communications technology in the mid-1920s had built a close connection between atmospheric science and radio, the full-fledged use of radio waves as a geophysical probe did not start until an instrumental development occurred a few years later. This article examines this advance, which centered on the invention of radio sounders of the ionosphere. Like the earlier radio experiments on the ionosphere, the sounders bounced radio waves off the upper atmosphere and inferred its properties from the returns. But the new apparatuses contained valuable innovations. In the 1930s, researchers at American Telephone and Telegraph Company (AT&T), Britain's Radio Research Board (RRB), the Technical University of Munich, and the U.S. National Bureau of Standards (NBS) automated data recording with oscilloscopic displays and inscription gadgets. They also devised control mechanisms to synchronize the acquisition of data with the change of transmitting signals. On the one hand, such automation was consistent with Lorraine Daston and Peter Galison's "mechanical objectivity," which minimizes human interference, makes experiments repeatable, and reduces personal equations, most notably perhaps with the Bureau of Standards' automatic single-frequency sounder. On the other hand, however, the automation also embodied mechanically theory-laden experimental procedures—it instrumentalized a theory that underlay the experimental scheme. The British and Americans' sweep-frequency recorder reckoned precisely the mandate of the magneto-ionic theory, the dominant model of radio-wave propagation. The sweep-frequency recorder incorporated the magneto-ionic theory by producing an easy-to-measure critical condition that the theory had predicted.

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