Fort Wayne KM-1 standard (1900s) Up
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Rotating standards are used as a "standard" meter against which to check the accuracy of a given watthour meter. The two meters are connected in series, and a thumbswitch is connected to the voltage winding on the standard. The standard is reset to 0 by a lever, then the meter under test is allowed to make a given number of revolutions, using the mark on the edge of the disk and a given point on the case or nameplate. After the meter under test has completed the number of revolutions, the thumbswitch is opened and a reading taken from the standard's register. The difference between the two in revolutions is then used to compute the accuracy of the meter being tested so it can be adjusted accordingly.
Not much is known of the KM-1, but the internal construction was very likely similar to that of the other K series meters. Most standards use a set of terminals or a drum switch to set the current rating of the standard, but the KM-1 used a set of pins (similar to the Westinghouse B rotating standard).

 
Fort Wayne rotating standard
Phil Shelley