Electroencephalography, procedure for obtaining a record of the electrical activity of the brain by means of electrodes attached to the surface of the skull. Normal patterns on the electroencephalogram, or EEG, include the alpha wave, indicative of a relaxed state, and the theta wave, usually found in children and thought by some to reflect creative activity in adults. Medically the most important use of electroencephalography is to aid in the diagnosis of epilepsy. The EEG tracing can identify a specific site at which damage to the brain has occurred. Study of EEGs has contributed to research into the nature of sleep, helping to identify four stages of sleep. Scientists have also analyzed brain waves stimulated by sensory input, such as a flash of light or a sound, as a means of determining which parts of the brain carry out various functions. The finding of a flat, or waveless, EEG in persons in coma has been interpreted as an absence of brain function and used as legal evidence of death

Xerography, electrostatic dry-printing process for the reproduction of images or documents, widely employed in commerce and industry in copying machines

The process was invented by the American physicist and patent attorney Chester F. Carlson in 1937 and first commercially developed in 1950. It makes use of the principle of photoconductivity, that is, that certain substances resist passage of an electric current except when struck by light. Silicon, germanium, and selenium are poor conductors of electricity, but when light energy is absorbed by some of their electrons, the electrons are able to pass from one atom to another, thus allowing a current to flow when a voltage is applied. When the light is removed, their conductivity again becomes low. Xerography employs a photoconductive insulating layer, such as selenium, on an aluminum or other conductive metal support

The layer is charged electrostatically, either with positive or negative ions (see Ion), the polarity