This is from memory from circa 1980. I was twelve, and my local library had a pre-first-world-war book of electrical experiments. Most of the devices I recall from the book I can fond documented on the web, but one remaining enigma is the photovoltaic cell.
This is not like modern solar cells made of silicone and semiconductors. The photovoltaic cell of the early 20th century was an acid battery made from metal plates in a glass jar of chemicals. I don't know what the liquid in the jar was (acid is my assumption based on what I know of old wet batteries) and I don't know what the plates were but I suspect that one of them was probably a strip of silver.
I don't think that it was a true solar battery; I think that the electrodes are probably consumed by the acid to produce energy just as happens in a regular chemical cell, but the unique feature of this battery is that it only works when light shines on it.
The experiment book described a use for the cell: the small current that it produces can be used to trip an electromagnetic relay to switch off electric lights when the sun comes up. That's a fairly impressive feat of automation for that era.
So I am asking of anyone can provide any more information? What kind of metal were the electrode plates made from? What was the chemical solution in the jar? Do the electrodes require any kind of treatment to make them photosensitive?
I would be grateful for any information that anyone can provide.
