Electrostatic Origins of the Dirichlet Principle

The Dirichlet Principle is an approach to solving the Dirichlet problem by means of a Dirichlet energy integral. It is part of the folklore of mathematics that the genesis of this argument was motivated by physical analogy involving electrostatic fields. The story goes something like this: If an electrostatic potential is prescribed on the boundary of a region, it will extend to a potential in the interior of the region which is harmonic when the electric field is in stable equilibrium, and that electrostatic field has minimum Dirichlet energy. The details of this argument are seldom given and where they are, they are typically scant, redacted, and speculative while often omitting either physics details or mathematics details. The purpose of this article is to give a detailed reconstruction of the electrostatic argument by combining accounts in several contemporary and historical disparate sources. Particular attention is given to explaining the frequently omitted physics and mathematical details and how they fit together to give the physical motivation.