Ian, G3SEK wrote:
"1. The Bird "wattmeter" does not in fact sense power. At the detailed
level, it senses voltages and currents in its internal transmission
line---."

Since the ratio of voltage to current in either direction is always Zo,
all you need are voltage and current samples of one direction at a time.

The Bird is not alone in determining power from such voltage and current
samples. The dynamometer does too.

A d-c meter is useless on a-c because the needle tries to move first one
way then the other as the a-c current through it alternates. Inertia
keeps the indication on a-c close to zero.

The dynamometer eliminates the flicker and gives an appropriate
deflection on a-c. It does this by replacing the d-c meter`s permanent
magnet with an a-c coil. This is arranged to torque the needle upscale
no matter whch direction the current takes through the coils. The
moveable coil is always deflected the same way by interaction with the
fixed coil.

It is simple to calibrate a dynamometer for power and feed current
through one coil and place volts across the other. It senses volts and
amps but it reads out power, as does the Bird wattmeter.

Best regards, Richard Harrison, KB5WZI