This is a paper from Drs. Nitsche and  Paulus at the University of Goettingen in 2000. I gather that this paper was responsible for the resurgence of interest in tDCS.

Abstract
In this paper we demonstrate in the intact human the possibility of a non-invasive modulation of motor cortex excitability by the application of weak direct current through the scalp.
Excitability changes of up to 40 %, revealed by transcranial magnetic stimulation, were accomplished and lasted for several minutes after the end of current stimulation.
Excitation could be achieved selectively by anodal stimulation, and inhibition by cathodal stimulation.
By varying the current intensity and duration, the strength and duration of the after-effects could be controlled.
The effects were probably induced by modification of membrane polarisation. Functional alterations related to post-tetanic potentiation, short-term potentiation and processes similar to postexcitatory central inhibition are the likely candidates for the excitability changes after the end of stimulation. Transcranial electrical stimulation using weak current may thus be a promising tool to modulate cerebral excitability in a non-invasive, painless, reversible, selective and focal way.

via Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation.
Full Paper Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation