Faraday's disk dynamo needs a magnetic field in order to produce an electric current. Is it possible for the current which it generated to also produce the magnetic field which the dynamo process required? That, in a nutshell, was what Larmor proposed was happening in sunspots.
At first sight this looks like a "chicken and egg" propostion: to produce a chicken, you need an egg, but to produce an egg you need a chicken--so which of these came first? Similarly here--to produce a current, you need a magnetic field, but to produce a magnetic field you need a current. Where does one begin? Actually, weak magnetic fields are always present and would be gradually amplified by the process, so this poses no obstacle.
One could, for instance, link two Faraday dynamos, each supplying the current needed to produce the other's magnetic field. They could (in principle) form a feasible self-excited dynamo, deriving its energy from whatever force was turning the disks. However, whether a fluid dynamo could mimic this behavior--e.g. swirls of fluid flow in a tank of liquid mercury--is a much harder question and took decades to resolve.
(More about dynamos in the next section and in part #14 of
"A Millennium of Geomagnetism".)
Next Stop: 13. The Self-Sustaining Dynamo in the Earth's Core
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