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[SOLVED] Odd electronics test question

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xaos

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I could really use some help. I am taking an electronics test, and this is one of the questions:

Direct current would NOT ordinarily be used in which of the following?
A. Ships
B. Airplanes
C. Battery Charging
D. Arc Welding
E. Electric clocks



Now I may be totally crazy, but don't all on the list use or are capeable of using both AC & DC? I've pretty much narrowed this down to either battery charging or arc welding, but both of these have AC & DC versions... Battery chargers are typically run on AC, but output DC, though there are the 'quick charge' packs that use entirely dc...

Any help appreciated.
 

xaos,

AC clocks get their extraordinary long term accuracy from their synchronous motor that follows the line frequency. This frequency is constantly monitored, and is very accurate over a long time period.

Ratch
 
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    WimRFP

    WimRFP

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Ok since my future career hinges on a test full of assinine questions like this, and I have absolutely no clue what they are looking for to answer this question, I will take your advice and choose electric clocks. I appreciate your insight.

BTW, I believe this test is from the 60s. It has schematics that use 2D21 thyratrons that they ask questions about... So considering that battery tech was not as it is today back then, Electric clocks actually makes a lot more sense aas an answer.

Thanks for your help.
 

I cant remember clearly now but long long ago somebody told me that Airplanes use 400Hz AC!!!
AC can be produced without load carrying brushes in the generator, or alternator, they are more reliable, smaller and lighter.
I cant remember every details he said. He even asked me a tricky question - "Since ships and Airplanes use AC current, how they connect the earth line".

But since A synchronous electric clock doesn't contain a timekeeping oscillator like a pendulum, but instead relies on the oscillations of the AC utility current from its wall plug to keep time. Its clear that electric clock will be the correct answer
 
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    xaos

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Air planes commonly use 28 VDC power systems as well.
MIL-STD-704 defines power quality (transient) conditions for these systems.
Just a side note!
 

Thanks everyone for help. The correct answer was indeed electric clocks.
 

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