On Wed, 10 Oct 2007 02:44:57 -0700, jim
wrote:

IMHE, these things are OK so long as you don't need something to be
accurately level.

If you want a consistent incline, such as for setting out drainage, then
they're quick and convenient at delivering an acceptable accuracy.

If you want "level", then you typically need better accuracy. They're
not up to this.

1. How do the levels work?

IMHE, there are lots of methods in detail, but they all use a swinging
pendulum to establish a local datum. There's then a sensor to read the
offset of this from the level's body. Older high-accuracy models used
an optical scale, cheaper ones use a non-contact inductive or capacitive
sensor. These have linearity problems on larger angles.