In article , Art Unwin
KB9MZ wrote:

I have never seen a reason to spend big money on
a Bird wattmeter but I do have a question.
What exactly is inside the various slugs
that one must use with this meter ?
Art

A coupling network and a diode are inside the slug. Slugs are
calibrated by twiddling (literally) components inside the slug, and
then putting the label on the front of the slug.

You can find the meters on eBay, but you need to be careful buying the
slugs sight unseen -- some of the lower-power slugs can be thrown out
of cal by being dropped (on a hard surface).

An easy way to check a slug is to put two high quality 50 ohm loads in
parallel (use a "T") on the output of the meter. Bird loads are also
easy to find on eBay and swap meets -- I have one that's probably 40
years old, and a newer one, only 20 years old.

Try and keep your readings midscale for best accuracy. Put the
shorting slug in the meter when the meter isn't in use, or turn the
slug so the arrow points up at the meter to protect the movement.

The Bird is a wonderful, stable design that's simple and well
understood. You can get fancier (from Bird, among others), you can get
more accurate (my old HP), but it's hard to get simpler.