Selenium photocell

Willoughby Smith was not looking for a way to convert light into electricity. He was testing materials for submarine telegraph cables. The transatlantic cable had been successfully laid in 1866, but signal transmission remained finicky—the enormous lengths of cable required high-resistance conductors to prevent signal degradation. Smith, chief electrician of the Gutta Percha Company, was evaluating selenium rods as potential high-resistance components when he noticed something strange: the rods' resistance changed with lighting conditions. In darkness, selenium resisted current strongly; in sunlight, resistance dropped dramatically. Light was directly affecting electrical conductivity.

Smith reported his findings to the Journal of the Society of Telegraph Engineers in 1873, and a summary appeared in Nature. The discovery was accidental, embedded in work that had nothing to do with light—but its implications were immediately recognized. Here was a material that could transform light into electrical signals. If light intensity could vary the current in a circuit, then visual information could be electrically encoded. The dream of 'seeing by electricity'—transmitting images through wires—suddenly seemed physically possible.

The physics was not understood at the time. Selenium is a semiconductor, and its photoconductivity arises from light freeing electrons in the crystal structure—concepts that would not be clarified until quantum mechanics emerged decades later. But engineers could exploit the effect without understanding it. Within years, selenium cells were being incorporated into experimental devices.

Alexander Graham Bell used a selenium cell in his 1880 photophone, transmitting sound via a modulated beam of light. A speaker's voice vibrated a mirror, varying the intensity of sunlight reflected to a distant selenium receiver. The varying light produced varying current, which drove a speaker. Bell considered it his greatest invention, though it found no commercial application at the time. The photophone demonstrated that selenium could convert light modulation to electrical modulation—the essential operation for image transmission.

The Nipkow disk, proposed in 1884, used a spinning perforated disk to scan an image sequentially, focusing light from each point onto a selenium cell. The cell converted the varying light into varying current that could be transmitted to a receiver, where a synchronized disk and modulated light source reconstructed the image. This mechanical television—crude, low-resolution, requiring intense illumination—was the first practical scheme for transmitting images electrically, and selenium cells remained central to television experiments through the 1920s.

Wirephoto services, which transmitted newspaper photographs by wire, also relied on selenium cells to scan photographic images and convert the varying tones to electrical signals. The Associated Press wirephoto network, launched in 1935, enabled newspapers across America to print the same photograph on the same day—a revolution in visual journalism made possible by the accidental discovery in Smith's submarine cable laboratory.

The selenium photocell was eventually superseded by more efficient photoelectric devices: vacuum phototubes, then solid-state photodiodes. But it established the principle that would define modern imaging: light could be converted to electricity, and electricity could represent visual information. Every digital camera, every television, every scanner traces its conceptual ancestry to Smith's observation that selenium resistance changed in sunlight. The age of electronic imaging began when an engineer noticed an anomaly while testing telegraph cable materials.