This article is from the Scientology and Dianetics FAQ, by firstname.lastname@example.org (Scientology Information Server) with numerous contributions by others.
The E-Meter is a device that measures fluctuations in electrical resis-
tance, when a tiny current is passed through the body. The fluctuations
show up in needle movements.
The definition that is found in the booklet "The Basic Dictionary of
Dianetics and Scientology" is:
E-Meter: [short for "electropsychometer"]...An electronic
instrument for measuring mental state and change
of state in individuals, as an aid to precision
and speed in auditing. The E-Meter is not intended
or effective for the diagnosis, treatment or preven-
tion of any disease.
The argument comes in, when the question is posed: Is the body the only
thing influencing the resistance?
Scientologists believe that the mind is an energy mechanism, indepen-
dent of the brain, and is capable of influencing the E-Meter. Those who
don't believe that the mind is anything more than the brain, feel that
the body is the only thing influencing the E-Meter.