Monday, June 06, 2011

Vacuum Tubes and Thallium

Vacuum tubes are all said to descend from one tube: the Fleming valve. they all have something in common with Fleming, it's a glass tube with no air in it, and a filament of some type. For all the variety that we have today it's clear there have been many generations of refinement. Before DeForest and before modern mass-produced tubes I immediately think of two other very significant stops along the way: Geissler tubes and the Crookes tubes.
The Geissler tube was invented by Heinrich Geissler in 1857.  Heinrich wasn't a physicist, or a chemist, he was a glass blower. Consequently the Geissler tube wasn't a empirical device, it was a toy. The tube was filled with rarefied gas, usually neon or argon. When current was applied, electrons would break off from gas atoms creating ions. When those ions recombine with other gas atoms they glow. Depending on the contents of the tube, they can glow in different colors. Geissler probably got invited to a lot of good parties. More here.

The Geissler tube was clever, but Geissler left it to others to further develop it. D. Mcfarland Moore (for example) attached it to a battery and observed how making and breaking the current increased the power of the electromagnetic field. But he wasn't alone. This curiosity was tinkered with in the labs of Philipp Lenard, Johann Hittorf, Juliusz Pl├╝cker, Eugen Goldstein, and Heinrich Hertz

Crookes  was a totally different man from Geissler . His parents were wealthy, and he attended the Royal College of Chemistry. While a student, he became the assistant of August Wilhelm von Hofmann. With Hofmann he attended meetings of the Royal Institution. That's how Crookes  met Michael Faraday. The chance meetings changed his focus from chemistry to physics. He discovered thallium in 1861 guaranteeing a place in the History books. But there he was with a new element and he was trying to determine it's properties. Flame spectroscopy was a new tool, with multiple uses but determining atomic weight wasn't one of them. But he thought he could use a modified Geissler tube. Thallium oxidized very easily, so weighing it inside a vacuum was ideal.

He put a tiny amount of thallium in the Geissler tube and sealed it with the aid of a pump to keep out the air. Then he tried to weigh it on a vacuum-balance, which is just what it sounds like, a balance inside a vacuum. But he had a problem, the balance is very delicate and  the scale would occasionally move inexplicably. Crookes figured out that the very scant bit of air that remained in the vacuum tube was beig effected by light, and thereby effecting the scale.This was such a revelation to him, he studied that next.

In trying to replicated the regulate that thermal effect on rarefied gas, Crookes invented the radiometer.  It was a set of four small vanes balanced upon a pin in a vacuum tube. Two sides of each vane were painted different colors: one side black, the other silver. When light shown on the vanes the black sides would heat up,causing them to turn as the excited air molecules struck them. In the same way that the Geissler tube was a toy so was the radiometer. Nonetheless James Clerk Maxwell used it to prove the kinetic theory of gases. Toys and tools are not so different... and these were all steps on the way to the invention of the cathode tube. More here.