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From: anthony@convex.csd.uwm.edu (Anthony J Stieber)
Newsgroups: comp.sys.ibm.pc
Subject: Re: DS HD floppies
Message-ID: <13624@uwm.edu>
Date: 30 Jun 91 02:41:19 GMT
References: <1991Jun18.234702.9028@bronze.ucs.indiana.edu> <13264@uwm.edu> <1991Jun28.220658.23540@eagle.lerc.nasa.gov>
Sender: news@uwm.edu
Organization: University of Wisconsin - Milwaukee
Lines: 63

In article <1991Jun28.220658.23540@eagle.lerc.nasa.gov> afdenis@lims05.lerc.nasa.gov writes:
>In article <13264@uwm.edu>, anthony@convex.csd.uwm.edu (Anthony J Stieber) writes...
>>In article <1991Jun18.234702.9028@bronze.ucs.indiana.edu> ntaib@silver.ucs.indiana.edu (Nur Iskandar Taib) writes:
>>>Incidentally, why don't HD diskettes have hub 
>>>rings? You'll also notice that HD floppies 
>> 
>>I don't really know, I remember reading about how HD drives don't
>>need them.  It seemed to me that hub rings are there to protect
>>the hub from dammage, perhaps someone out there knows the real reason?
>> 
>One possible explanation I heard is that HD disketts have their magnetic 
>media impregnated to thicker mylar disks for stability. The reasoning was 
>that the hubrings "thicken" the DS DD disks for positive clamping, and the 
>HD disks, being thicker initially, don't require them.
>
>Could be BS but it made sense to me.

Well, I got out a micrometer and measured the thickness of some
floppies.  The 5.24" DSDD, 5.25" DSHD and 3.5" DSDD disks I tested all
measured 0.003 inches in thickness.  Oddly enough, the MEI/Micro Center
catalog has listed the HD disks as being 0.005 inches in thickness
while DD as only 0.0003 (within spec).  I'll check around on other HD
disks and see what I find.  Thickness is not given in the specs for
either of the 3.5" disks.  The hub ring on the 5.25" DD disk was 0.008
inches thick.

I don't know what this means, maybe the cheap 5.25" disks are cheap
because they are thin, maybe not.  There are other HD disks
I can check out later.

By the way, here are the coercivity specs that started this whole thing.

size  density	oersteds
------------------------
5.25" DD	320
5.25" HD	640
3.5"  DD	600
3.5"  HD	720

Higher coercivity corresponds to higher write currents in the
read/write heads on the disk drive.  HD disks in a DD drive don't work
well because the high coercivity medium actually resists being written
to by the low write current head.

And some definitions:
co-er-civ-i-ty \,ko^--,er-'siv-et-e^-\ n
(1898)
:the property of a material determined by the value of the coercive
     force when the material has been magnetized to saturation 

oer-sted \'er-sted\ n
[Hans Christian Oersted]
(ca. 1889)
:the centimeter-gram-second electromagnetic unit of magnetic intensity
     equal to the intensity of a magnetic field in a vacuum in which
     a unit magnetic pole experiences a mechanical force of one dyne
     in the direction of the field 
--
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