English: One of the first pre-vacuum tube AM radio transmitters, developed December 1906 by the German firm Telefunken (Gesellschaft für drahtlose Telegrafie). In experiments on December 14, 1906 it transmitted voice signals 36 km (24 mi) from Berlin to Nauen, Germany. Based on the Poulsen arc transmitter invented by William Duddell and Valdemar Poulsen in 1903, it used 6 short electric arcs between carbon an copper electrodes (inside vertical tubes) with 220 VDC applied, connected to a tuned circuit consisting of a coil and capacitor in series (located under the table) to make a continuous wave electronic oscillator. The negative resistance of the arcs excited radio frequency oscillations in the tuned circuit. The radio signal was modulated by the carbon microphone (cone, above) in the transmitter's antenna wire. The pressure of the sound waves varied the resistance of the carbon granules, which varied the current applied to the antenna. The small breadboard mounted on standoffs on the desk is the receiving apparatus. Due to lack of a magnetic blowout the efficiency was low. Arc transmitters like this were one of the first technologies that could transmit sound, but were only used for a short period until the early 1920s, when they were replaced by the vacuum tube invented by Lee De Forest in 1906. More information in Alfred N. Goldsmith, "Radio Telephony, Article 4" in Wireless Age magazine, April 1917, p. 472-476