Why is platinum wire used to seal over glass?

Incandescent light bulbs need a wire is needed to carry the electrical current inside the bulb to the tungsten filament and get very hot when operating. The bulb is filled with inert gas to prevent the filament from burning (no oxygen) , so a good seal is needed between the glass and the wire. read more

Different materials expand at different rates when heated depending on the physical property of the material. Solids like copper, iron, glass, etc. have different expansion rates. The change in length of a specimen one unit long when its temperature is changed by one degree is the ‘coefficient of linear expansion’. read more

The bulb is filled with inert gas to prevent the filament from burning (no oxygen) , so a good seal is needed between the glass and the wire. If the wire and the glass expanded at different rates, the seal would be broken. Brass expands at. 18.7×10−6m. for each metre of length if its temperature changes by. 1 K. Plate glass only expands. 9.0×10−6m, which is the same expansion rate as platinum. Since both the glass and the platinum move at the same rate, the seal between then is undamaged. read more

It was used as the earliest glass-to-metal seal and is still in use for liquid seals for e.g. rotary shafts. Mercury seal. The first technological use of a glass-to-metal seal was the encapsulation of the vacuum in the barometer by Torricelli. The liquid mercury wets the glass and thus provides for a vacuum tight seal. read more