At 10:21 PM 11/20/99 -0000, Wilf Melling wrote:
>This has been most interesting watching this OT. Could our American friends
>confirm that the electricity supply meters (domestic)as stated by Bob
>McClure measure true power (KW) in the UK our meters only measure KVA.
>
Quoting from the 12th Edition of the Standard Handbook for Electrical
Engineers (Fink and Beatty, McGraw-Hill) p 3-31 et seq

81. General  The subject of metering electric power and energy is
extensively covered in the American National Standard C12, "Code for
Electricity Metering," American National Standards Institute.  It covers
definitions, circuit theory, performance standards for new meters, test
methods, and installation standards for watthour meters, demand meters,
pulse recorders, instrument transformers, and auxiliary devices.  Further
detailed information may be found in the Handbook for Electric Metermen,
Edison Electric Institute.

...
83. Energy is measured in watthours (or kilowatt hours) by means of a
watthour meter.  A watthour meter is a motor mechanism in which a rotor
element revolves at a speed proportional to power flow and drives a
registering device on which energy consumption is intergated.  Meters for
continuous current are usually of the mercury-motor type, whereas those
for alternating current utilize the principle of the induction motor.

...

This extensive section includes information on adjusting meters to insure
that *watts* and *not kva* are measured.  This includes testing the meters
at various power factors.  This concludes with acceptance accuracy limits
from the Code for Electricity Metering, ANSI C12-1975, which prescribes
that accuracy must be within 2% at a power-factor lag of .5.

KVA meters are occasionally used, but primarily for some specific industrial
applications that operate at extreme power factors.  I have never seen or
heard of one in use in residences.

And yes, the previous remark to the effect that the aluminum disk is indeed
the motor is correct, and has been since early in this century.

If I might be permitted an anecdote.  One of my professors of electrical
engineering (circa 1954) started his career after receiving his EE degree
in the midst of the depression in the 1930's as an ordinary lineman.  (It
was the only job he could get.)  He was assigned to find out why this old
farmer had electricity usage much lower than could be inspected.  The had
his meter tested and it proved accurate.  Then one day as he was starting
at the meter on the wall, he noticed an ant crawling inside.  Upon close
inspection a tiny hole was discoved drilled through the wall behind the
meter and into the meter.  It seems that the farmer would insert a wire
through the hole and stop the rotation of the rotor when he didn't want
his electricity usage recorded.

Bob McClure