Glass Blowing for Vacuum Devices

Glow Tube

A straight length of 8mm diameter tube would suffice for this glow tube, but I decided to use one of the 'U' tubes from the Bending Tubes page.

One end of the tube has to be sealed off. I used the same method for this as the preparatory stage of Blowing Bulbs.

It would have been a little easier to seal the end of the tube before bending it.

For the glass to metal seals, I used pins recovered from an old TV valve. I have found that they can be sealed in more easily if they are pre-beaded with glass. This can be done with a glass rod which has been drawn out thin. I hold the pin with a pair of pliers and heat the glass and the pin at the same time. I wrap one end of the thin rod around the necked part of the pin and then draw the rod away so that it detaches. I turn the gas up high and rotate the pin in the flame until the glass melts into a round bead.

These are the pre-beaded valve pins. The part to the right of the glass bead is the pin which plugged into the valve socket. The part to the left of the bead was originally inside the valve. I have cut it short so that it will fit inside the 8mm diameter tube.

The necked part of these pins is Dumet , which produces a bubble free seal.

The seals have to be fitted one at a time. A small area near the end of the tube needs to be blown out in order to fit a glass to metal seal pin. This requires a very sharp flame. This is one of the few times where I blow into the tube while it is in the flame. A picture of this is shown below. The aim is to blow out an area just big enough for the wire to go through, but too small for the bead. I find that to get the glass to extend outwards, it is necessary to blow very gently.

This is the end of the tube after the side has been blown out. It is a bit wonky but adequate for a good seal.

To seal in the pin, the pin and the blown out side of the tube have to be heated at the same time. I used the bunsen type burner for this. I then simply pushed the softened bead into the hole in the tube. I then annealed it and allowed it to cool slowly.

This is one of the completed seals. The glass bead has fused into the surrounding glass of the tube.

The other pin now has to be sealed into the other arm of the 'U' tube. This is done in the same way as the first pin.

Here the tube is mounted on top of the vacuum rig. The tube contains air at 10mBar. A very high voltage is required to strike the discharge. Here I was using about 8kVac.

The tube could be sealed off at this point, but I decided to wait until I have some way of filling it with neon gas. This would give much better results.