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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).