Ian, G3SEK wrote:
"The detected voltage (now DC) is squared by the meter scale to
indicate power."

That`s right. It only takes about 1.4X the deflection to represent 2X
the power.

Cecil was saying the same with his slide rule illustration. To square a
number, you multiply the logarithm of the number by 2 and take the
antilogarithm of the product. The slide rule is designed to do this for
you with its "engine divided scales".

Each sample taken in the Bird wattmeter, line volts and line amps, is
exactly half the total sample, so one of them is really redundant to the
power indication, were it not needed to cancel sensing of the unwanted
direction of power transmission. The 1:1 ratio between samples is
ensured by the lock Zo places on the ratio between volts and amps in the
transmission line. A dynamometer has no such lock so both both volts and
amps must be sampled in the absence of constant volts or constant amps.

Best regards, Richard Harrison, KB5WZI