The AC-powered clock. The power grid requires full synchronization of the entire grid, and there is no master. Thus once the grid is accurately sync'ed to exactly 60Hz, if any one generator starts to lag or lead, the power flows from the many other grid generators will pull the "offender" back into sync; if that fails, it will be cut off from the grid because the reverse power flows attempting to force it back will trip all its breakers! The conventional quartz-based timepieces at best can hold a few PPM considering temperature, aging, etc. Interesting sidepoint, though: those tiny cyindrical 32.768 KHz "watch" crystals have an interesting property. They have an inverted-U shaped temperature vs. frequency curve. The peak is generally very close to the average human body temp (37C or 98.6F). This happy accident has 2 consequences: as long as you wear a wristwatch (and don't leave it sitting on you dresser most of the time!) it will be most accurate. The other: at higher OR lower temperatures, the frequency DROPS, so your watch will lose time, rarely gain it!
HOWEVER... there is now a very common and even more accurate time source: your cell phone! The cell and hand-off structures of the networks require nanosecond-level sync (generally provided at each site by several GPS receivers in a "voting" redundancy configuration). Unfortunately, not ALL cell networks 'cooperate' properly, leading to (for example) my AT&T Blackberry to lag "real" time by as much as 2 minutes! My Verizon basic cell phone is dead on.
Back in the service, one of the guys in our shop took apart one of the typical government clocks to get the motor running again. He put it together backwards. One seeing the result, he relettered the dial.
We enlisted took delight watching visiting brass check the time and then re-look with a strange lost expression.
Answer is 4x per 24 hour period. 6am, noon, 6pm and midnight. The answer would be the same if only the hour hand is present.
Reminds me of the ubiquitous AA battery-powered clocks. They were invented in Japan. During postwar reconstruction, eastern Japan including Tokyo ended up 50Hz generator from Europe and the western part including Osaka with 60Hz from American hardware. The battery powered clock eliminated the pain of having to buy an AC powered clock with the appropriate motor or gearing. The dual line frequency exists in Japan today; rather backwards for a modern industrial nation. At least modern power supplies will accept either frequency.
My father had one of these syncronous clocks on our fireplace mantel back in the 40s. It had a small knob on the back you would turn after plugging it in to get the motor spinning. Once it got going, it would stay in sync with the AC. If you turned the knob in the opposite direction of the arrow printed on it, the clock would run backward. I used to drive my dad nuts by changing its direction whenever possible.
This sounds like either a Bizarro cartoon, the beginning of a Twilight Zone episode, or a ghost story. I also laughed out loud, for the same reasons Beth did. Like Rob, I'm also trying to figure out how it could be used in real life: how the heck does that work?
Back in the late 80's I built some custom equipment for a small clock mfg that had just received a shipment of clock faces with the numbers and text printed on the back of the clear laminate by a contract printing company. This was done to reduce costs as the laminate could be applied to several different clock face base colors. The printing company didn't notice that the laminate had been printed as viewed from the adhesive side thus creating reversed mirror image of the numbers/text when the laminate was applied its intended for use. While the clock company owner was greatly displeased at some of his employees assembling a few backwards-running mirror-image clocks, the clocks were a big hit at the local watering hole and it wasn't long before the new line of "novelty" clocks were selling more volume than the "normal" clocks.
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