 Braun's receiver coupling experiment

In his 1909 lecture, Karl Ferdinand Braun recalled two experimental stations on Strasbourg forts in summer 1902. Known capacitances and calculated inductances set the transmitter tuning; the receiver searched for its two coupled oscillations. Its receiving coil fed a second coil in a tuned indicator circuit. Separating these coils increased both the sharpness of resonance and excitation intensity, until further separation reduced the response. The varied coil spacing was the control; transmitter coupling need not be loose for this receiver effect. For intensity measurements, Braun described an air-thermometer readout in the related resonant measuring apparatus.^[ref]

The comparison tests receiver coupling, not an unrestricted law that looser coupling always helps. The paragraph does not identify the particular indicator used in the coil-spacing test or give a response curve from which to reconstruct its optimum. The plotted curves below are an original linear model with stated losses, rather than invented historical measurements.