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There have been many discussions on the reason for these balls and spires, also known as finials, however they seem to overcomplicate the matter by suggesting they were filled with mercury or some other kind of substance. I believe there may be a simpler function that is being overlooked.
Build a cavity with a set shape and funnel and it will create a specific frequency when vibrated.
So how was it vibrated? That's where St. Elmo's fire comes in. St. Elmo's fire is a corona discharge induced by a high difference in potential, or voltage. For anybody who isn't already aware and is reading this thread. The voltage in the atmosphere increases about 100v for every meter.
With a tall spire and a good ground, that's quite a lot of potential difference. The potential difference can be increased further with a good ground and many older churches and cathedrals were built over aquifers and wet ground is a much better conductor than dry ground. The potential difference in the atmosphere may have been even higher when the buildings were originally built. You don't need that much to induce a corona discharge depending on the geometry of the shape at the top of the spire such as having sharp points.
