...and why would Kirk need to jettison it in (the excellent episode) "Court Martial?"
As many times as I've seen this, while I was watching the blu ray today at lunch, it occurred to me that I don't really know what one does or why Finney would get caught in it.
The September 26, 1966 Final Draft script for "Court Martial" has a brief comment that explains this a bit--a comment which never made it into the final cut of the episode:
Weatherscan indicated an ion
storm, dead ahead. I sent
Finney into the pod.
That's outside the ship.
It's attached to the skin. One
of our missions is to get
electron readings in abnormal
conditions, such as ion storms.
This can only be done by direct
exposure on ion-sensitive plates.