Willoughby Smith (6 April 1828, in Great Yarmouth, Norfolk – 17 July 1891, in Eastbourne, Sussex) was an English electrical engineer who discovered the photoconductivity of the element selenium. This discovery led to the invention of photoelectric cells, including those used in the earliest television systems.

In 1848, he began working for the Gutta Percha Company, London where he developed iron and copper wires insulated with gutta-percha to be used for telegraph wires. In 1850, Smith superintended the manufacture and laying of 30 miles of underwater telegraph wire from Dover to Calais. Though the first cable failed almost immediately, another laid the following year was a success and over the following decades, Smith and the company he worked for were involved with many other underwater telegraph cable projects.

In 1866, Smith developed a method for continually testing an underwater cable as it was being laid. For his test circuit, he needed a semi-conducting material with a high resistance and selected selenium rods for this purpose. The selenium seemed to do the job properly, except in actual use, the device gave inconsistent results. Upon investigation, it was discovered that the conductivity of the selenium rods increased significantly when exposed to strong light. Smith described the "Effect of Light on Selenium during the passage of an Electric Current" in an article that was published in the 20 February 1873 issue of Nature.

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