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Comment by markdown | Hacker News Reader

> and the units are in Metric

As they should be. Why is this worth mentioning?

Floppy disks are typically measured in inches, common sizes being 8", 5.25", 3.5" and 3". Like any distance, these may also be measured in metric units. However, for whatever reason, when it comes to disks, they pretty much never are.

Huh, is this true even outside the US? As in, did people in the UK call them 3.5 inch floppy disks, just like us Americans?

Yes. Only a few things proliferate in inches, but products that are defined in inches always do. Subway sandwiches are a mixture, in the netherlands they are 30cm (an extra 30mm), in other metric countries they remain as a footlong.

30cm leaves you 4.8mm short of a foot. This is how we lose spacecraft.

In germany it was called "3 1/2 Zoll Diskette/Floppy" so yeah we didn't use the word inch, but I can confirm that this was true at least for us.

"Zoll" is a direct translation from "inch". Inches were used when measuring wood.

The actual size was different from region to region, because there was no agreement on whether it is 1/10 or 1/12 of a feet, and 1 foot itself would correspond to different lengths: https://en.wikipedia.org/wiki/Obsolete_German_units_of_measu...

A good rule of thumb is that if it says somewhere "x inch" in a common kind of measurement, then the device never has any length of x inches.

Eaxmples:

- 3.5 in floppy disks have no 3.5 dimension (the disk is 86 mm, the cartridge is 94x90 mm, the drive is 100 mm). - 3.5 in hard drives have neither 3.5 in platters nor are any of their external dimensions 3.5 in - Ditto for 2.5 in drives, likely also true for 5.5 and 8 in drives. - Any kind of inch-sensor size (e.g. 1/2, 2/3) is not the size of the sensor measured in any way. - ...

5.25" disks are 5.25"/133mm along each side, making 5.25" a perfectly good description, in my view, for an object at this sort of scale. The disc inside must be 130mm in diameter, the case is presumably 3mm larger, and the whole thing is named after the size of the case. 5.25" = 133.35mm. I expect you could actually make a perfectly usable 5.25" disk that was exactly 5.25" wide...

Apparently 8" disks are 8"/203mm wide, no doubt for exactly the same reason, and with a rather similar result.

You are correct that a document from ECMA is expected to be in metric. Floppy disks are tech from the US though, and everything about them is hence in inches, from physical specs to tech specs, so the metric docs translate the units.

It's like when flat screen TVs came out. When they first appeared in Europe, they sold in inch sizes. You'd go to a store in France and buy a 50 inch TV. Years later they started labeling things as "87.3cm" - and only much later started making sizes in even metric units - which are still uncommon to this day.

If you were reading a spec doc for the US 50" TV, and it was in metric, it absolutely is not something that "should be" - and is a very notable change. Just a friendly question - are you one of the scientists that worked on that mars probe that crashed?

> Floppy disks are tech from the US though, and everything about them is hence in inches, from physical specs to tech specs, so the metric docs translate the units.

Do read the specs someone else linked in this thread. All major dimensions are even metric measurements and thus are not even imperial measurements.

yes, because the document is from Europe for a device created in California. A European document is going to be in metric. That is what the OP was pointing out - that the document is European. The US document, the original one with the specs, would be in inches. I would recommend looking up the acronym ECMA.

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