Glassblowing #2: Making a tungsten lamp and (bad) vacuum diode (Maurycy's blog)

Glassblowing #2: Making a tungsten lamp and (bad) vacuum diode

2026-06-18

Now that I have a way to run electrodes through glass, it's time to do something with that.<br>An old school lightbulb is rather simple: just a thin wire that gets hot enough to glow.

However, to produce anything approaching white light, the wire must get to around 2500 C.<br>While conductors like tungsten or graphite can survive those temperatures, they all burn on contact with air.

To prevent this, I'll be sealing my lamp in glass under vacuum: no air, no problem.

I lied.

Some ceramics start conducting once heated, and being oxides, are completely unaffected by oxygen.

Open air "Nernst lamps" did enjoy brief popularity in the 1890-1900s, but because of the added complexity of preheating the filament,<br>they were replaced by filament lamps once vacuum pumps became good enough for commercial production.

https://en.wikipedia.org/wiki/Nernst_lamp: those lamps.

https://www.youtube.com/watch?v=-spTvp5-sf0: Using one.

To start, I bent some some 0.3mm diameter tungsten wire into a "U" shape, and twisted a length of very fine 0.012 mm tungsten wire onto the free ends.<br>I cut one side of the lead frame shorter so that the filament would sit diagonally:<br>allowing it to be reasonably long without looping it around which could result in a short.

To make the bulb, I partially inserted the lead frame into a glass tube, heated the end with an oxy-propane torch and pinched the glass onto the wires:

Once one end was sealed, I connected the other to a rotary vane vacuum pump, and pumped it down while lightly heating the tube to remove moisture.<br>After a few minutes, I headed the middle of the tube to sale the bulb, and pulled off the excess tubing.

After the glass cooled, I cut the middle of the wire "U" to separate the leads:

Glowing at 4 volts

The finished lamp glows nicely between 200 and 400 mA (3-6 V and 0.5-2.5 W):

I didn't stretch the filament tight enough and it ended up touching the glass, which creates a dim spot.<br>The glass is borosilicate, so I'm not too worried about cracking, but it's still not ideal.

I've tested the bulb up to 10 volts (~7 W), which is bright to light up a whole room,<br>and the creates white light instead of the orange-ish color seen at low power —<br>but it also gets very hot and won't last very long.

In addition to color, the filament's temperature also affects the bulb's efficiency:<br>a low temperature filament emits the vast majority of it's light in the infrared.<br>A hot filament emits more visible light per watt, but tungsten evaporates faster leading to early failure.

This tradeoff between lifespan and color/efficiency is why most light bulbs have rather short lifespans...<br>or at least they did until we stopped using filaments.

As an experiment, I ran a length of wire through a hole in the glass tube before evacuating it:

When making seals like this, first melt a capillary tube (~1 mm) onto the tungsten wire before sealing it.<br>This ensures good contact to the metal and protects the wire from the torch flame.

The idea was to observe thermionic emission:<br>when the filament is white hot, the atom's have enough kinetic energy to knock electrons into the vacuum.

If the cold electrode is at a positive voltage, these electrons allow a small current to flow.<br>If it's negative, the free electrons are repelled and nothing happens.<br>While a diode isn't terribly exciting, it's the basis of more interesting devices like triodes, X-ray tubes and CRTs.

... and it's terrible: only conducting 1 uA with 700 V of bias between the filament and anode.<br>Reverse biased, it conducts around 50 nA, mostly from the photoelectric effect.

I suspect this is the result of two problems.

Tungsten wire contains trapped gasses, which are released when it's heated.<br>To avoid ruining the vacuum, the filament should be run while pumping down the tube, which I forgot to do.

Second, the anode area is really small: Its ~6 mm of 0.3 mm wire, several centimeters away from the filament.<br>Most emitted electrons will miss the anode and create a negative charge on the glass, which impedes current flow.

Real vacuum diodes surrounding the filament in a metal tube, but I didn't do that because I wanted it to work as a light bulb.

Just for fun, here's a photograph of my spool of filament wire, lit by the bulb made using it:

Related:

/projects/glass/1/: Considerations for sealing metal through glass

http://www.rhunt.f9.co.uk/Glass_Blowing/Filament_Lamp_A/Filament_Lamp_A_Page1.htm:<br>another homemade incandescent lamp