Crookes tube
Philipp Lenard wanted to see if cathode rays could pass out of the Crookes tube into the air. See diagram. He built a tube with a "window" "(W)" in the glass envelope made of aluminum foil just thick enough to hold the atmospheric pressure out (later called a "Lenard window") facing the cathode "(C)" so the cathode rays would hit it. He found that something did come through. Holding a fluorescent screen up to the window caused it to fluoresce, even though no light reached it. A photographic plate held up to it would be darkened, even though it was not exposed to light. The effect had a very short range of about . He measured the ability of cathode rays to penetrate sheets of material, and found they could penetrate much farther than moving atoms could. Since atoms were the smallest particles known at the time, this was first taken as evidence that cathode rays were waves. Later it was realized that electrons were much smaller than atoms, accounting for their greater penetration ability. Lenard was awarded the Nobel Prize in Physics in 1905 for his work.