# Why is the unit for resistivity sometimes typeset as $\rm m\Omega\text{-}cm$, with a hyphen instead of a dot or a space?

Quote from published paper:

and moderately low electrical resistivities ($\rho<5\:\rm m\Omega\text{-}cm$)

Why is this $\Omega\text{-} \rm cm$?

Resistivity is volume normalized, and markup is $\Omega$*(physical dimension) like $\Omega$*cm?

Is the - sign there just marking a compound 'word', to imitate rhythm, intonation and structure of spoken language?

Or why?

• Common usage. But it probably said "$\rho < 5$ mOhm-cm".
– user137289
Commented Feb 6, 2018 at 12:01
• It did say p not q. It oddly changed in copypaste. Anyway I edited to fix that. So the mOhm-cm is same as mOhm*cm. I get why sometimes negative powers are used instead of / but it does not make sense to me why would anyone want use - to replace * "here is added some text to add between the message and the signature because the signature has minus sign and can be confusing in this context" Commented Feb 6, 2018 at 12:28
• Can down voters comment what they do consider bad in this question. Better yet say how and where I should have asked this. Commented Feb 6, 2018 at 12:49
• So it is there just for readability, mOhmcm does look stupid I agree. Commented Feb 6, 2018 at 12:52
• @Doege Context is always relevant. You never know when there are parts of the context that inform the answer and which a potential answerer can then use; by taking out the reference, you are making that decision for them. For future reference, our tutorial for typesetting mathematics is here, and if you want to reply to comments you should use @user notation. Commented Feb 7, 2018 at 13:42

Presumably you're referring to this paper, which uses the notation

and moderately high electrical resistivity, $$\rho \sim 0.8 \: \mathrm{m\Omega\text{-}cm}$$.

Generally, this usage of a hyphen to separate unit symbols is highly discouraged. The core style guides for the scientific literature specify clearly the guidance for

• The NIST Guide for the Use of the International System of Units (SI), in point (10) of its initial checklist, specifies:

There is a space between the numerical value and unit symbol, even when the value is used as an adjective, except in the case of superscript units for plane angle. (See Sec. 7.2.)

a 25 kg sphere $$\qquad$$ but not: $$\quad$$ a 25-kg sphere

with further confirmation of the same usage in §6.1.5.

• The SI Brochure on the subject:

In forming products and quotients of unit symbols the normal rules of algebraic multiplication or division apply. Multiplication must be indicated by a space or a half-high (centred) dot (⋅), since otherwise some prefixes could be misinterpreted as a unit symbol.

• The AIP Style Manual (ostensibly the style guide that governs J. Appl. Phys., where the paper you're presumably looking at was published), has a similar opinion, in p. 10:

The product of two or more units may be indicated in either of the following ways:

$$\mathrm{N\cdot m}$$ $$\quad$$ or $$\quad$$ $$\mathrm{N\,m}$$

The formally correct way to typeset this is using a thin space between the two unit names,

$$\rho \sim 0.8 \: \mathrm{m\Omega\,cm}$$

though this can be traded for a centered dot if desired:

$$\rho \sim 0.8 \: \mathrm{m\Omega \cdot cm}$$

The indication of a multiplication using a hyphen is generally clear enough, particularly for units that see a lot of everyday usage in a given field, but it is still strongly discouraged by the usual style rules.

Journal typesetting tends to be good, but it need not be perfect; just because it's in a published paper doesn't mean it is right.

mOhmcm does look unreadable, ambiguous, so it has been split up with hyphen for more language-like readability. Hyphen similarity to minus sign is unfortunate coincidence. SI system does not define this kind of usage convention, and the use of hyphen is purely a contemporary habit, undocumented jargon in a sense that should be obvious to enlightened reader from the context.

Mostly in any subject the common practice that 'everyone' knows is the biggest hurdle for outsiders because that part is not mentioned or explained anywhere. So even if this is not pure physics question, it seems related enough to justify leaving this question intact with comments and this answer.

• Your second paragraph doesn't seem that relevant to me. I know quite a few entry level textbooks do discuss units and how they are best represented. It's definitely mentioned and explained places (especially if you look at standards like in Emilio's answer). You also don't have to try to justify keeping the question open in your answer. That's not a relevant place for that.
– JMac
Commented Feb 6, 2018 at 15:20