Electroencephalography (EEG) is the recording of electrical potentials generated by nerve cells n the cerebral cortex.
Routine electroencephalography is the recording of these electrical activities on the scalp. However it provides a highly limited view of cerebral activity due to inherently poor spatial resolution and low amplitude as well insensitivity to electrical fields that are either not perpendicular to the scalp or distant from it.
Recording the low amplitude cerebral activity, especially in the context of the larger amplitudes of muscle potentials, EKG and ambient noise requires powerful amplifiers and special techniques of canceling the extracerbral electrical noise. 
Recording electrodes consist of small metal cups or discs which are attached to the scalp so that they make good mechanical and electrical contact.They cover the surface of the head at regular intervals. The location of electrodes on the scalp are standardized by international 10-20 system of electrode placement.This system uses standard measurements of the head with reference to visible landmarks. This minimizes the variation in electrode placement among the laboratories and technologists and provides a uniform reference to corresponding brain structures among patients.
The EEG machine receives electrical inputs from the scalp electrodes and after several levels of amplifications as well as passing through different filters it projects it on writing devices or the computer monitor.
The main purpose of doing EEG on a patient is recording certain abnormalities which are well defined as certain disease features. These abnormal electrical discharges will show epileptic activities, certain metabolic or toxic conditions affecting the brain. Particular abnormal patterns are present in certain infections of the brain.
EEG is one of the diagnostic tool utilized by the neurologists to assist in the diagnosis of number of illnesses( such as epilepsy)  which may adversely affect the brain. The findings must be interpreted in the context of the whole clinical presentation.
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