We looked at glass bases earlier in my blog but today, I would like to add a little detail about the lead through wires. To recap, the lead through wires are of composite construction with one end forming the valve pin, the other end, the support for the electrode cage with the in between section being a short length of wire to pass through the glass base.
To make a perfect mechanical and impermeable seal between the metal and glass, the metal and glass require near identical co-efficients of expansion otherwise seal leaks and the destruction of valve vacuum would ensue.
The metal chosen for the middle section of the lead through wire is borated copper clad iron wire of 0.015 mm diameter and 2.5 mm length.
The valve pins are nickel of 1 mm diameter and 9 mm length.
The support wire is nickel of 0.5mm diameter and 12 mm length.
In the Mullard picture below you can see the individual components and the completed lead through joined pins: -