crystal detectors worked. Using an oscilloscope made with Braun's new cathode ray tube, he produced the first pictures of the waveforms in a working detector, proving that it did rectify the radio wave. During this era, before modern solid-state physics, most scientists believed that crystal detectors operated by some thermoelectric effect. Although Pierce didn't discover the mechanism by which it worked, he did prove that the existing theories were wrong; his oscilloscope waveforms showed there was no phase delay between the voltage and current in the detector, ruling out thermal mechanisms. Pierce originated the name crystal rectifier. Between about 1905 and 1915 new types of radio transmitters were developed which produced continuous sinusoidal waves: the arc converter (Poulsen arc) and the Alexanderson alternator. These slowly replaced the old damped wave spark transmitters. Besides having a longer transmission range, these transmitters could be modulated with an audio signal to transmit sound by amplitude modulation (AM). It was found that, unlike the coherer, the rectifying action of the crystal detector allowed it to demodulate an AM radio signal, producing audio (sound). Although other detectors used at the time, the electrolytic detector, Fleming valve and the triode could also rectify AM signals, crystals were the simplest, cheapest AM detector. As more and more radio stations began experimenting with transmitting sound after World War I, a growing community of radio listeners built or bought crystal radios to listen to them. Use continued to grow until the 1920s when vacuum tube radios replaced them. Crystodyne: negative resistance diodes Some semiconductor diodes have a property called negative resistance which means the current through them decreases as the voltage increases over a part of their I–V curve. This allows a diode, normally a passive device, to function as an amplifier or oscillator. For example, when connected to a resonant circuit and biased with a DC voltage, the negative resistance of the diode can cancel the positive resistance of the circuit, creating a circuit with zero AC resistance, in which spontaneous oscillating currents arise. This property was first observed in crystal detectors around 1909 by William Henry Eccles and Pickard. They noticed that when their detectors were biased with a DC voltage to improve their sensitivity, they would sometimes break into spontaneous oscillations. However these researchers just published brief accounts and didn't pursue the effect. The first person to exploit negative resistance practically was self-taught Russian physicist Oleg