File:Bell reed resonator.jpg

English: An electromagnetically-driven metal reed sound resonator used by US scientist Alexander Graham Bell around 1874-1875 to reproduce sounds during his pioneering audio research that resulted in the invention of the telephone. It consists of a springy steel reed suspended over an electromagnet. Bell used these devices initially as part of his effort to develop a "harmonic telegraph" that could send multiple telegraph signals over a common telegraph wire, using different pitch tones for each signal. He connected two resonators in separate rooms in a wire circuit with a battery to provide electric current. When he plucked one reed, making it vibrate, the varying reluctance caused a varying magnetic field in the coil, resulting in a varying current in the wire to the second device. This caused a varying magnetic field in the second coil, causing the second reed to vibrate, reproducing the tone. In a breakthrough on June 2, 1875, Bell heard overtones produced by a reed in another room plucked by his assistant Watson, and realized that a diaphragm and electromagnet could transmit not just tones but the more complicated waveforms of speech through a wire.