What Is A Crookes Radiometer
You have seen this large glass bulb with the four weather vanes inside, spinning with no apparent power source. There are no wires into the bulb, and no batteries or motors inside. You conclude that the vanes are spinning due to heat from one side of the vane being painted black and the other is silver; Or maybe something to due with photons striking one side of the vane, causing it to move. For such a seemingly simple device, there has been great discussion on how it works since it was invented by Sir William Crookes in 1873.
Sir Crookes wasn’t actually trying to create a “light mill” or “solar engine”, but was looking for a method to accurately weigh very small samples of chemicals. His initial design was to weigh materials in an evacuated chamber. In this partial vacuum environment, he thought the measurements would be less affected by any air currents. What he discovered was that sunlight was now causing variations in the weighing process, with more sunlight increasing the measurements.
Modifying the balance scale (e.g., two horizontal plates connected together by a wire which is exactly centered on a pivot point) configuration to the four weather vanes allowed him to test out his theories of how sunlight effects measurements. The “Crookes Radiometer” could now measure the radiation from sunlight. Within the glass bulb, the near vacuum condition eliminates the effect of atmospheric friction on the rotating vanes. One side of each vane is silver, the other is black.
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An early hypothesis was that light hitting the black side of the vane is absorbed and light hitting the silver side is reflected. This would mean there is a net effect of twice as much radiation pressure on the silver side compared to the black. However, the reality is that the vanes spin in the opposite direction of what this would dictate.
Sir Crookes experimented with different levels of vacuum within the bulb. At both extremes, the vanes didn’t move. Interesting! He concluded that with no vacuum, the air in the bulb was too heavy for the vanes to displace with small amount of energy provided by the sunlight. The total vacuum scenario was the more puzzling, and which led to his eventual realization of the true cause for the movement.
In a total vacuum, there were no longer any air molecules in the glass. So the air itself must be contributing to the movement of the vanes. His investigation moved from the faces and surfaces of the vanes to their edges. His theory was that the solar energy was causing the air in the bulb to heat on one surface of the vane, while remaining cooler on the other side. The air molecules on the warmer side are now moving slightly faster than those on the cooler side. The warmer, faster air impact the edges of the vanes providing a tangential force from the warmer side to the cooler side. This was later validated by using a stream of cooler air applied to the outside of the bulb, which caused the vanes to spin in the opposite direction.
The temperature gradient, also called thermal creep, due to the un-even solar heating of the air molecules inside the partially evacuated bulb causes the weather vanes to spin. Amaze your friends the next time you come across a Crookes Radiometer by explaining how this fascinating curiosity works.
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