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The J symbol can represent the unit of energy but it's also the symbol for the dimension of the candela (or luminous intensity).

For the energy unit, it clearly comes from the family name of the English physicist James Prescott Joule (1818–1889), but for the luminous intensity dimension, what's the origin? What does it stand for?

I'm also asking because the energy unit has the dimension $\mathsf{L}^2\cdot\mathsf{M}\cdot\mathsf{T}^{-2}$ which doesn't contain the dimension of the luminous intensity.

DevonDahon
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    This is a history-of-physics question. –  Aug 03 '19 at 17:59
  • @G.Smith Should I re-ask it on physics.meta ? Or can I move it there somehow ? –  Aug 03 '19 at 18:30
  • I don't think it's standard notation. Did you read the note there: «The symbols in this column denote dimensions; "L", "T" and "J" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.»? – Geremia Aug 03 '19 at 18:38
  • @Geremia It is standard notation as you can read here. –  Aug 03 '19 at 19:01
  • No, it doesn’t belong on meta. It is possible that a moderator will migrate it to the History of Science and Mathematics site. If they don’t, and if you don't get a good answer here, then you might want to ask it there. –  Aug 03 '19 at 20:52
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    The dimension symbol for current is $\textsf I$, presumably because $I$ is also the usual variable used for current in equations. (I have no idea why.) According to the Wikipedia article on luminous intensity, the variable $I_v$ is used to indicate luminous intensity in equations. (“I” for intensity, presumably.) So I speculate that the corresponding dimension symbol is $\textsf J$ because $\textsf I$ was already “taken” as the dimension symbol for current. They just picked the next letter in the alphabet. –  Aug 03 '19 at 21:06
  • @G.Smith this reminds me of $j$ being used for imaginary unit in electrical engineering... – Ruslan Aug 03 '19 at 21:19
  • See this related question/answer – Elements In Space Aug 04 '19 at 10:23
  • For what it's worth, the codification of the dimensions of units within the SI (and, with it, the setting of the $\operatorname{dim}(I_\nu) = \mathsf J$ standard), is extremely recent, and it was introduced to the SI Brochure in its eighth edition, in 2006. The next obvious target to look for are the CGPM resolutions from the period 1998-2006 that codify this as part of the SI, but they're not explicitly mentioned in the brochure. – Emilio Pisanty Aug 05 '19 at 10:58

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The symbol for candela is cd according to the NIST's website.

I gather you are mixing the concept of dimensions symbols (M, L, T) with dimensional analysis and energy units. The symbol for the mass dimension is M, it can be in any kg, mg, g etc, Length is L which can be millimeters, yards, kilometers etc. The dimension symbol for temperature is $\Theta$. So what should be the dimensional symbol for candela? It is J. Does it have to anything with Joule? No. It is by chance that the symbol matches with Joules. The problem is that English has only 26 letters. Have you heard of the pigeonhole principle?

In mathematics, the pigeonhole principle states that if $n$ items are put into $m$ containers, with $n>m$, then at least one container must contain more than one item. This theorem is exemplified in real life by truisms like "in any group of three gloves there must be at least two left gloves or at least two right gloves"

─ Wikipedia

The same applies on symbols and quantities. From the official website of SI Units, this is the list of symbols to be used for these dimensions: https://www.bipm.org/en/publications/si-brochure/

enter image description here

See the section on base units on Wikipedia.

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AChem
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    Thank you for this answer. I'm not mixing the concepts (or at least, I don't think so ;-)), I was actually more wondering why most symbols for dimension are pretty obvious while only this one is not clear and explained nowhere. I guess the correct answer is actually the one given by @G.Smith : " was already “taken” as the dimension symbol for current. They just picked the next letter in the alphabet." – DevonDahon Aug 04 '19 at 15:50
  • Dear maxagaz, In science and history, one cannot rely on casual coincidences, e.g. I is the symbol for current, J was chosen as the next symbol for candela. No need to rationalize such choices until and unless it is explicitly written by the SI committee. This makes an assumption someone was sitting and thinking about the symbols sequentially and he/she must have chosen I for current first and then thought what should we choose for candela? For instance, J or sometimes j is used for current density. How can we rationalize this choice? – AChem Aug 04 '19 at 16:11
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    I think G. Smith's suggestion should be taken with caution. I and J are closely linked (they were one letter for many centuries), so that may have been the reason rather than the proximity in dictionaries. "I was already taken" is solid, but the reason for choosing J is speculation until someone finds evidence. – Peter Taylor Aug 06 '19 at 07:48
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Candela contains the power emitted and the power is energy per sec or Joules/sec. So the J in candela is still Joules.