The motor and cognitive functions of the brain produce neuronal electrical activity of characteristic patterns. These can be recorded and measured through different techniques.
The neurons in brain are polarised by ion exchange with extracellular milieu for maintaining resting potential (-70mV) or propagation of action potential. K+ ions are intracellular and Na+ are extracellular to the neurons. There are always waves of ions travelling through neurons (volume conduction) and these reach the scalp too. When electrodes are placed on the scalp, the ions can push or pull electrons on the electrode which can be measured by a voltmeter.
Electroencephalography (EEG) is a technique in which the overall neuronal electrical activity patterns of the brain are amplified and recorded by means of electrodes glued to the scalp. The sum total of oscillations of various assemblies of millions of neurons form the oscillations in an EEG.
Another technique called Magnetoencephalography (MEG) measures the weak magnetic fields generated by the electrical activity in brain and thus record the brain actions by devices called SQUIDs. "Any electrical current generates a magnetic field around it". MEG can detect the total magnetic field generated by millions of neurons. This is a very sensitive technique but requires the surroundings to be free from magnetic field as very low magnetic fields from the brain are measured.
Electrical Impedence Tomography (EIT) is a technique in which an image of the conductivity of brain can be obtained by surface electrical measurements. Conducting electrodes are attached to the scalp and small alternating currents are applied. The resulting electrical potentials are then measured. A tomographic imaging of the neural activity in brain is obtained using EIT.