= Braun's receiver coupling experiment
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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.<Electrical Oscillations and Wireless Telegraphy by Karl Ferdinand Braun>[1909 lecture, printed p. 234: Strasbourg stations, coil-spacing test and air-thermometer readout]{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.
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