point of a sword, Radium (element)
銧 has two unrelated meanings: the point of a sword and the element radium. These do not share a clear conceptual center; the sword-point sense is a rare, possibly obsolete usage, while the radium sense is a modern chemical coinage.
銧 is a modern Japanese coinage combining 金 (metal) with 光 (light), created to represent the element radium, which is radioactive and emits light. The sword-point meaning is obscure and its historical origin is uncertain; it may be a separate, rare usage.
The left side 金 means metal, and the right side 光 means light. Picture a radioactive metal that glows with light: that's radium. For the sword-point meaning, imagine the tip of a metal blade catching the light.
For コウ, imagine a glowing metal sword-point that makes a sharp 'koh' sound when it strikes: koh -> コウ.