Re: clever nonlinear meters

["John S. Denker" <jsd@MONMOUTH.COM>]

At 12:27 PM 3/20/01 -0500, Clarence Bennett wrote:
> I have an old meter called Thermocouple Milliammeter.
>
> It is not for temp reading, but the input current raises the temperature of
> the input resistor, whose temperature is sensed by a thermocouple driving
> the D'Arsenval meter movement.
>
> Would you call this a real True RMS meter?

Yup.  You can still buy things that work on this principle.  They're nice
because they have a rather flat frequency response, over the entire
frequency range from DC to daylight.

  http://www.google.com/search?q=bolometric+power+detector+OR+meter

============================

While we're on the subject of clever meters, I love the three-terminal
device the electric company uses to measure mean(V*I) with due regard for
phase (i.e. power factor).  It's basically a motor driving a
well-characterized mechanical brake.

The motor has one low-resistance winding in series with the customer's
circuit, and one high-resistance winding in parallel with the customer's
circuit.  The mechanical brake relies on eddy current damping.

For this application (and many others) it turns out that mean(V*I) is more
interesting than mean(V^2) or mean(I^2).