# Question #f4143

Oct 13, 2015

Differing coefficients of expansion.

#### Explanation:

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. If the wire and the glass expanded at different rates, the seal would be broken.

Brass expands at $18.7 \times {10}^{- 6} \text{m}$ for each metre of length if its temperature changes by $\text{1 K}$. Plate glass only expands $9.0 \times {10}^{- 6} \text{m}$, 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.