Many years ago I had the delight in learning how RF ammeters worked.
It was a panel meter 0-5 amps. Normal DC shunts could do it for DC,
but for AC ? and RF ?

There was a four terminal 'tube'. There was a heater and a
thermocouple. The meter was measuring the voltage generated by the
thermocouple!

Using this method, any waveform or complex AC riding on or modulated
within an RF signal can be measured.

Martin

On 3/12/2011 8:59 PM, Josepi wrote:
In the case of a filament it should be totally resistive and cause no
distortion or phase shift on it's own but RMS amperes x RMS voltage is
not always accurate for power measurements with distorted waveforms.

Yes, most good DVMs, these days, measure a reasonably accurate form of
RMS with various methods.
As Jim's posts says the current is usually measured via a shunt
resistance IR drop.

---------
wrote in message
...
Which brings up a couple of points. The temperature of the filament
in vacuum tubes does vary if you have AC applied to it. So you will
get some distortion from that. But most of the better DVM's are true
RMS meters, at least on Voltage. I would suspect they would also
measure true RMS current. But does anyone know for sure?

Dan


On Mar 12, 2:02 pm, "Josepi" wrote:
Careful. Variacs are known waveform distorters. The government would not
let
us use them for precision power measurements as the waveform adopts many
harmonics in some units.