This CleanTechnica miniseries about photovoltaics, celebrating the UN’s 2015 Year of Light, is the fourth article of seven. We continue our dialogue with physicist John Perlin, author of Let It Shine: The 6000-Year Story of Solar Energy. Perlin’s expertise on the subject provides an excellent archive for any person wishing to keep abreast of today’s solar revolution.
In the third post of our miniseries, we finished reporting on Willoughby Smith and his startling experiments proving the photosensitivity of selenium. The year: 1872. It was this discovery that stirred keen interest in two British scientists, William Grylls Adams and Richard Evans Day. (Other previous posts in this miniseries include: Author John Perlin Celebrates the Coming Year of Light, Author John Perlin & the Solar Cell, The Pathway to Today’s Solar Revolution: Discovering the Photosensitivity of Selenium.) Adams and Day had received from Smith the bars of selenium he had experimented with. Their goal? To delve deeper into the peculiar behavior Smith had discovered.
This interview with Perlin chronicles the sequence of events following the original discovery.
CleanTechnica: Please describe the nature of the experiments that took place using these selenium bars from Willoughby Smith.
Perlin: In one experiment, Adams and Day passed a battery-generated current through the bars. After detaching the battery from the selenium, they discovered to their astonishment that the induced current had reversed itself. To see if light would have the same effect as the battery had, the the two scientists placed candlelight close to the selenium. The flame forced the current to flow in the opposite direction as if it were a battery.
CleanTechnica: What was their reaction?
Perlin: They stated: “Here there seemed to be a case of light actually producing an electromotive force within the selenium, which in this case was opposed to and could overbalance the electromotive force of the battery.” Adams and Day immediately changed the course of their research to examine, in their words, “whether it would be possible to start a current in the selenium merely by the action of light.”
CleanTechnica: What then took place concerning this experiment with light?
Perlin: Adams and Day let the selenium bars rest overnight. The next morning, they checked the selenium for any residual current and found none. Then a candle an inch away from one of the bars was lit. The needle of their galvanometer – an instrument for detecting electric current – jumped instantaneously. Shading the candle from the selenium caused the needle to immediately drop to zero.
CleanTechnica: Thus proving the cause for fluctuation was not from the heat of the candle?
Perlin: Exactly. The rapid change in the movement of the needle ruled out the possibility that heat from the candle flame was responsible for generating the current. The phenomenon of thermal electricity was already known and when heat was applied and withdrawn in thermal electric experiments, the needle always gradually rose and fell. They therefore knew they had discovered something previously unknown to science – that a current could be started by the action of “light alone” in a solid-state material.The two scientists called the flow of electricity caused by light, “photoelectric.” Today, we say, “photovoltaic.” So began in 1875 the first stirrings of today’s solar revolution.
CleanTechnica: What reaction took place from the scientific community regarding this discovery? Open arms?
Perlin: This will be covered in the next two episodes. Suffice it to say the reaction supported the observation by Abraham Pais in his acclaimed scientific biography of Einstein, that good physicists “are conservative revolutionaries, resisting innovation as long as possible, but embracing it when the evidence is inconvertible.”
Photo via Let It Shine
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