It's from my dad's years as a generator mechanic. Permit me to explain the instrument briefly.
Gasoline or diesel engine powered electrical generators all run at a fixed, governed engine speed in order to produce a fixed frequency of electrical power output, just as the utilities do. In North America, that frequency is 60 Hertz (Hz -- cycles per second or cps).
One could set governed engine speed by using a tachometer, but a more direct, fool-proof method is to measure actual generator output frequency; that's what the pictured instrument does. Here's a view of it in operation.
Note the oscillating vanes. The instrument is reading just shy of 60 Hz.
I'm disinclined to believe that Toronto Hydro's power would be off at all from exactly 60 Hz, so I'll open up the instrument and see if there's a calibration adjustment.
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Nope. The meter appears to be a sealed unit, with no provision for adjustment.
Anyway, that's ok. The meter's accuracy is plenty good enough to be useable, and it wouldn't be hard to remember what Toronto Hydro's 60 Hz indication looks like.
This instrument can go in a bin along with some other electrical test gear.
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