The e-meter is, fundamentally, a wheatstone bridge, with the person holding cans being one of the resistances and the tone arm being another. The needle is the voltage meter in the centre of a wheatstone bridge diagram.
The voltage of electricity going through the person holding the cans is very small. It is approximately the same as a small battery, approximately 1.5 volts Direct Current.
The e-meter, in the hands of a competent operator, can detect changes in the mind as a result of questions or statements made to the person holding the cans, even when the person holding the cans are not aware of such changes. The changes observed by the operator are the nature of needle movements and the rising or falling of the "tone arm". Therefore, an e-meter can be used to assist auditing or word clearing, which is its principal purpose.
There have been several versions of e-meters within the Church of Scientology. Some of them are
- Mark V
- Mark VI
- Mark VII
People outside the Church of Scientology have developed e-meter alternatives.