It might be "The Magic Goes Away" by Larry Niven
The Warlock, whose actual name is both unknown and unpronounceable, is a powerful sorcerer in excess of 200 years of age. He observes that when he stays in one place too long, his powers dwindle and will return only when he leaves that place. Experimentation leads him to create an apparatus (now known as the Warlock's Wheel) consisting of a metal disc enchanted to spin perpetually. The enchantment eventually consumes all the mana in the vicinity, causing a localized failure in all magic. The Warlock realizes that magic is fueled by a non-renewable resource, which would cause great concern among the magicians, as it was through their magic that nations enforced their wills both internally and abroad. The widespread diminishing of magical power in The Magic Goes Away triggered a quest on the part of the most powerful of the magicians of the time to harness a new source of magic (the Moon), resulting in the events described in the book.
Description of the protagonist from the book:
He was an old man, tall and lean, his head bald as an egg, his voice thin and reedy.