General > General Technical Chat

why is the US not Metric

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TimFox:
Tooki:   I believe that the nominal size of floppies, e.g. 5.25 and 3.5 inch, comes from the “U” dimension (1.75 inch) for 19” rack panels, dating back to telephone relay racks.  This quantum persists, even for racks tapped with metric threads.

tooki:

--- Quote from: TimFox on February 20, 2020, 02:15:41 pm ---Tooki:   I believe that the nominal size of floppies, e.g. 5.25 and 3.5 inch, comes from the “U” dimension (1.75 inch) for 19” rack panels, dating back to telephone relay racks.  This quantum persists, even for racks tapped with metric threads.

--- End quote ---
Why would the size of the disk be multiples of the rack size? That doesn't make sense.

And again, the 3.5" floppy isn't actually 3.5", it's 9cm.

ebastler:

--- Quote from: tooki on February 21, 2020, 10:24:39 am ---
--- Quote from: TimFox on February 20, 2020, 02:15:41 pm ---Tooki:   I believe that the nominal size of floppies, e.g. 5.25 and 3.5 inch, comes from the “U” dimension (1.75 inch) for 19” rack panels, dating back to telephone relay racks.  This quantum persists, even for racks tapped with metric threads.

--- End quote ---
Why would the size of the disk be multiples of the rack size? That doesn't make sense.
And again, the 3.5" floppy isn't actually 3.5", it's 9cm.

--- End quote ---

Also, how would 8" floppies fit this size scheme?

TimFox:
3.5 inches is the “nominal size”.  Despite their actual dimension, I have never heard them called anything else.
The “U” is one of those practical international standards, like the M6.3 camera tripod screw and 2.54 mm pitch that originated in inches.

TimFox:
8 inch floppies:  I thought about that, maybe they were short for 8.75 (far-fetched) ? 

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