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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.^[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.

By Karl Ferdinand Braun. Nobel lecture delivered 11 December 1909; cited printed p. 234, Strasbourg receiver experiments.

Karl Ferdinand Braun investigated how coupled electrical circuits transfer and build up oscillatory power for wireless telegraphy. His lecture describes coupling several circuits of the same frequency into the transmitter's excitation circuit. Making them start to discharge at nearly the same time was a substantive experimental difficulty; he discusses a timing requirement of order $10^{-7}\mathrm{s}$ and an arrangement called energy coupling. The example connects circuit geometry and timing with the useful oscillation delivered to an antenna.^[ref]

A physicist.^[ref]

Wireless telegraphy connects an oscillating electrical circuit to a distant receiver. Tuning and coupling determine how effectively the circuit transfers a signal.

Guglielmo Marconi and Karl Ferdinand Braun shared the award equally for developing wireless telegraphy.^[ref] Braun's receiver coupling experiment connects that history to resonance and the selectivity of a receiver.

Wireless telegraphy conveys discrete symbols without a connecting message wire. Here the mechanism is radio communication: keyed excitation produces radio waves, a receiving antenna drives a selective electrical circuit, and a detector produces a readable event. This makes a beautiful bridge between resonance and information: accumulating many appropriately timed small electrical disturbances can distinguish one transmission from another. Selectivity is finite, however; resonance alone cannot guarantee immunity to strong unwanted signals or noise.

The 1909 Nobel Prize in Physics recognized Guglielmo Marconi and Karl Ferdinand Braun for contributions to wireless telegraphy.^[ref] Their apparatus should be understood as a developing collection of systems. An early coherer receiver, a later tuned receiver and a modern envelope detector are different instruments.