Why is nitrogen used in transmission lines?

Nitrogen is used in transmission lines primarily to prevent moisture from entering the line. When moisture infiltrates a transmission line, it can lead to a variety of problems, including corrosion of the conductors, degradation of insulation, and decreases in overall performance. By filling the transmission line with nitrogen, which is an inert gas, it creates a barrier that effectively keeps moisture out, thereby maintaining the integrity of the line and ensuring reliable signal transmission.

The other options provided do not apply to the use of nitrogen in this context. While enhancing signal strength and minimizing electromagnetic interference are important considerations in transmission lines, they are not the direct benefits of using nitrogen. Additionally, improving the velocity factor relates to the physical characteristics of the transmission medium and not specifically to the use of nitrogen as a moisture barrier.