Mark Willis wrote:

> To be picky a little here - I'd think your weight doesn't change
> there.  Your body probably moves a little though, in response to the
> pressure change when your heart beats - and that's what they measure;

  This measurement is referred to as ballistocardiography.  I'm sure you
can follow the derivation.

  Electrical measurement is called electrocardiography.

  Measuring the expansion of tissues such as the finger or earlobe is
called pulse plethysmography.

  A rather alliterative form of plethysmography is used for another
application but the topic might be a little delicate on this list...
This was *supposed* to be demonstrated to us during our course, but for
some reason that particular demonstration didn't work out properly.
--
  Cheers,
        Paul B.