[2] In addition to the matters implied by the title of the lecture (radium, polonium, actinium and thorium), Hammer discusses: There are 21 pages devoted to "The properties and applications of selenium", including the use of its changing electrical resistance in different light levels.
The variation in the light falling on the sensor translated to a varying electrical signal which could be transmitted, then turned back into vibrations in the air.)
Ruhmer is also credited with using a selenium cell to turn off the lights of maritime navigation buoys during daylight hours, thus conserving the gas they burned.
It is claimed that by 1903, Finsen's work was approved of "In England by Queen Alexandra, and in Russia by her sister, the Czarina, they both investigating the system while visiting their father, the King of Denmark."
There is even a claim that Finsen was able to spare smallpox victims from scarring by keeping them in a room with red glass windows while the disease went through certain stages.