Glassblowing #3: A better thermionic diode
A glassblower built a better homemade thermionic vacuum diode by surrounding the filament with an anode. The tube, sealed in borosilicate glass, achieved several milliamps forward current and 22 nA reverse leakage at 200 V. After 24 hours, performance remained unchanged, though unusual high-temperature behavior may indicate gas discharge.
Background
- This is from Maurycy Z's personal project log (maurycyz.com), a hobbyist documenting his attempt to hand-build vacuum tubes (thermionic diodes) by glassblowing. He's not a company or institution—just an individual maker sharing detailed technical experiments.
- A thermionic diode is a simple vacuum tube: a metal filament (cathode) is heated until it emits electrons, which are attracted to a metal plate (anode) when a positive voltage is applied. This allows current to flow in only one direction.
- The key challenge: getting the geometry right so electrons reach the anode instead of building up on the glass envelope (which blocks current). His earlier tube failed because too much glass sat between cathode and anode.
- "Townsend discharge" refers to a phenomenon in low-vacuum tubes where accelerated electrons ionize residual gas molecules, creating extra current carriers—a crossover between pure vacuum behavior and gas-discharge behavior.
- "Lead-in seals" are the metal wires passing through the glass to carry current; making them airtight without cracking the glass is one of the hardest parts of DIY tube fabrication.