HOME  |  NEWS  |  BLOGS  |  MESSAGES  |  FEATURES  |  VIDEOS  |  WEBINARS  |  INDUSTRIES  |  FOCUS ON FUNDAMENTALS
  |  REGISTER  |  LOGIN  |  HELP
Blogs
Made by Monkeys

Time Ticks Backwards on This Clock

NO RATINGS
View Comments: Newest First|Oldest First|Threaded View
<<  <  Page 4/5  >  >>
Rob Spiegel
User Rank
Blogger
Re: Mind teaser
Rob Spiegel   1/10/2012 7:01:20 AM
NO RATINGS
I would guess the answer is once -- in the spirit of "Even a broken clock is correct twice a day." You propose half a day, so the answer would be once. Right, Kenish?

kenish
User Rank
Platinum
Mind teaser
kenish   1/10/2012 1:57:36 AM
NO RATINGS
A thought problem, not a trick question:  Assume two identical 12 hour clocks set accurately to the current time.  One runs normally and the other backwards.  In a 24 hour period, how many times will the hand positions of the two clocks exactly match?  (Answer is the same whether a second hand is included or not).

Milton Hathaway
User Rank
Iron
Which is more accurate?
Milton Hathaway   1/9/2012 9:50:32 PM
NO RATINGS
So, which is more accurate, the older style synchronous motor movements or the new quartz movements?  Think carefully!

 

bdcst
User Rank
Platinum
Dyslexic Synchronous Motors
bdcst   1/9/2012 2:59:38 PM
NO RATINGS
Well, that's an old issue with 100% symetrical synchronous motors as they can rotate and lock to 60Hz in either direction.  I believe GE Telechron clock motors had sufficent armature asymetry to put the armature at rest slightly in the clockwise magnetic aligment with the stator poles to make them always start in the correct direction.  As for some 1960's vintage turntables, I believe at least one model from AR (Acoustic Reseach) used two motors, a small induction motor to accelerate the platter in the correct direction and a synchronous motor to lock it to 60Hz.

Ampex used fairly powerful hysteresis synchronous motors in their commercial audio tape recorders designed in the 1950's.  They'd always start up in the correct direction.  And they had two sets of windings to provide two speeds to pull tape at 7.5 or 15 inches per second.  The one idiosynchracy of these motors was a cogging problem.  If you switched motor speed windings while it was energized and turning, you would get temporary magnetization of the rotor such that it would cog sufficiently to be heard as tape flutter.  Merely powering down the motor would void the magnetic memory and the flutter would go away.  Easy to do since a spring loaded tape guide lever (tape break switch) controlled the capstan motor to stop transport motion if the tape snapped or slipped off of the take-up reel.

Rob Spiegel
User Rank
Blogger
Re: clock runs backwards
Rob Spiegel   1/9/2012 2:49:16 PM
NO RATINGS
That's pretty good Ichien52. I would imagine with the reversed face, you could tell time fairly well even with the hands moving backwards. What it not possible to reverse the alarm trigger?

Rob Spiegel
User Rank
Blogger
Re: New product
Rob Spiegel   1/9/2012 2:32:10 PM
NO RATINGS
You're right, Chuck. I would imagine this family was quite disapplointed when the clock started working correctly. You're probably right that a company could profit from making backwards clocks.

Battar
User Rank
Platinum
Quartz alarm - same trick
Battar   1/9/2012 11:07:51 AM
NO RATINGS
I've got a cheap quartz alarm clock thats doing the same trick. I can actually get it to change direction at will by removing and inserting the battery at the time the alarm function is tripped. I don't dare take it apart to see how it got to this state, because knowing a thing or two about how these things are put together, I know it's designed to be a one-way process.

Ratsky
User Rank
Platinum
Re: clock runs backwards
Ratsky   1/9/2012 10:36:57 AM
NO RATINGS
Thanks !  I had forgotten the "shaded pole" name for these motors.  It was only 48 years ago...

lchien52
User Rank
Bronze
clock runs backwards
lchien52   1/9/2012 10:14:54 AM
NO RATINGS
Back in 1972 at Cornell, we still had motors and electrical machinery courses. The text said cheap clock movements (no one had the ubiquitous battery operated quartz analog movements until the 80's?) used shaded pole synchronous motors - the shading was a loop of copper around one of the armatures.

I took my clock and moved the loop around the opposite armature and sure enough it ran backwards. I could not use the alarm function because of the way it engaged it would be going the wrong way.  I pried the plastic face off and made a new face with the numbers labeled counterclockwise and I had that around for many years. At the time i just had drafting tools and made the face of paper, it looked a bit rough. Maybe I should make a new face now that I have CAD stuff... Have to dig the clock out, I'm sure its still somewhere around. Too bad the case is a hideous 1970's green color.

 

 

 

 

Quacker
User Rank
Bronze
Offensive Avatar
Quacker   1/9/2012 10:08:41 AM
NO RATINGS
Please request that "Staber Dearth" remove his offensive avatar.

Thank you.

<<  <  Page 4/5  >  >>
Partner Zone
More Blogs from Made by Monkeys
Made By Monkeys highlights products that somehow slipped by the QC cops.
Made by Monkeys highlights products that somehow slipped by the QC cops.
Made By Monkeys highlights products that somehow slipped by the QC cops.
Made by Monkeys highlights products that somehow slipped by the QC cops.
Made By Monkeys highlights products that somehow slipped by the QC cops.
Design News Webinar Series
7/23/2014 11:00 a.m. California / 2:00 p.m. New York
7/17/2014 11:00 a.m. California / 2:00 p.m. New York
6/25/2014 11:00 a.m. California / 2:00 p.m. New York
5/13/2014 10:00 a.m. California / 1:00 p.m. New York / 6:00 p.m. London
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
Aug 4 - 8, Introduction to Linux Device Drivers
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6


Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Next Class: August 12 - 14
Sponsored by igus
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Sponsored Content

Technology Marketplace

Copyright © 2014 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service