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Pair of spiral conductors with metal balls at their ends
Riess spirals, or Knochenhauer spirals, are a pair of spirally wound conductors with metal balls at their ends. Placing one above the other forms an induction coil. Heinrich Hertz used them in his discovery of radio waves. They are named for German physicists Peter Theophil Riess and K. W. Knochenhauer.
References
Jed Buchwald, The Creation of Scientific Effects: Heinrich Hertz and Electric Waves, 1994: Ch. 14: "A Novel Device", p. 219.