Hubbard L. Ron. E meter essentials

1961
The meter tells you what the preclear's mind is doing when
the preclear is made to think of something.
The meter registers before the preclear becomes conscious
of the datum. It is therefore a pre-conscious meter. The meter
passes a tiny current through the preclear's body.
This current is influenced by the mental masses, pictures,
circuits and machinery. When the unclear pc thinks of
something, these mental items shift and this registers on the
meter.