Every character has a story #20: U+210e (PLANCK CONSTANT)

by Michael S. Kaplan, published on 2006/04/21 03:00 -04:00, original URI: http://blogs.msdn.com/b/michkap/archive/2006/04/21/580328.aspx

Unicode is a standard that fits all kinds of different needs.

Obviously the need to represent text that may contain important symbols used in science has always been important.

At some fundamental level, the identity of that symbol goes beyond an italicized small letter 'h', and therefore the need to represent Planck's constant even in plain text was a real one.

Thus, it was encoded at U+210e. And has been since at least Unicode 1.1 (and I believe 1.0).

There is a nice description of the constant itself up on Wikipedia. It looks something like this:

For fun you can even italicize it? :-)

Recently, Andreas Prilop asked on the Unicode List:

The symbol for Planck's constant (6.626E-34 J·s) is an italic "h". Why is there a special Unicode character U+210E for it?

The symbol for the elementary charge is an italic "e". But of course there is no special Unicode character for it.

The symbol for the speed of light is an italic "c". But of course there is no special Unicode character for it.

The symbol for the fine structure constant is an italic "alpha". But of course there is no special Unicode character for it.

etc. ad inf.

So what's this U+210E for? IMHO, this character should be listed as deprecated.

Deborah Goldsmith of Apple was first to point out the answers to these questions:

To differentiate it for purposes of representing mathematics in plain text....

....Note that there is not a MATHEMATICAL ITALIC SMALL H precisely because U+210E exists.

And she is right -- if you look at the Unicode charts at the Mathematical Alphanumeric Symbols block, there is a reserved space at U+1d455. The reason for these spaces is described in UTR#25: Unicode in Mathematics right after Table 2.1:

The plain letters have been unified with the existing characters in the Basic Latin and Greek blocks. There are 24 double-struck, italic, Fraktur and script characters that already exist in the Letterlike Symbols block (U+2100—U+214F). These are explicitly unified with the characters in this block and corresponding holes have been left in the mathematical alphabets.

Kind of says it all, doesn't it? :-)


This post brought to you by "ℎ" (U+210e, a.k.a. PLANCK CONSTANT)

# David on 21 Apr 2006 5:17 PM:

Maybe I'm missing something, but the response didn't seem to answer the question.  Why did the Unicode standard elect to create a PLANCK CONSTANT character instead of a MATHEMATICAL ITALIC SMALL H?  If PLANCK CONSTANT was created before the mathematical symbols were added to Unicode, why did they create PLANCK CONSTANT without all of the other constants?

I think it's a great idea to differentiate characters representing well-known mathematical constants when they might be used with regular text, but the response doesn't seem to say why they went about it this way.

# Maurits [MSFT] on 21 Apr 2006 5:40 PM:

Maybe U+210e got in on U+210f's coattails? ;)

# Michael S. Kaplan on 21 Apr 2006 6:58 PM:

That is pretty much a historical thing -- David. Long before the math support came in  fully, some standard had this one in it or some person requested it.

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