The photomill is an instrument Lyra recollects seeing whilst accompanying the Librarian of Jordan to a religious service at Gabriel College. The instrument and its unusual, perhaps inexplicable operation, is used to demonstrate the power of the Authority. Lyra described it and its mechanism thusly:
|"At Gabriel College there was a very holy object kept on the high altar of the oratory, covered (now Lyra thought about it) with a black velvet cloth, like the one around the alethiometer. She had seen it when she accompanied the Librarian of Jordan to a service there. At the height of the invocation the Intercessor lifted the cloth to reveal in the dimness a glass dome inside which there was something too distant to see, until he pulled a string attached to a shutter above, letting a ray of sunlight through to strike the dome exactly. Then it became clear: a little thing like a weathervane, with four sails black on one side and white on the other, that began to whirl around as the light struck it.... It was all done by photons, said the Librarian as they walked home to Jordan."|
|The Golden Compass, 148|
The photomill appears to resemble an object from the real world - called a radiometer. It was invented in 1873 by the chemist Sir William Crookes as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the balance. Investigating this effect, he created the device named after him: Crookes Radiometer.
These are a curio which do not actually measure anything. They operate in the same way as a photomill. The radiometer too is made from a glass bulb - from which much of the air has been removed to form a partial vacuum. Inside the bulb, on a low friction spindle, is a rotor with several (usually four) vertical lightweight metal vanes spaced equally around the axis. The vanes are polished or white on one side, black on the other. When exposed to sunlight, artificial light, or infrared radiation (even the heat of a hand nearby can be enough), the vanes turn with no apparent motive power, the dark sides retreating from the radiation source and the light sides advancing. Cooling the radiometer causes rotation in the opposite direction.
The causes are of course scientific and not divinely inspired. You can read more on Wikipedia about exactly how the radiometer works.