## What kind of unit is ohm-cm ?

Anything that doesn't fit elsewhere.
AGBX2
Posts: 18
Joined: Thu Jun 16, 2011 11:12 am

### What kind of unit is ohm-cm ?

Hi :)

When I use the units() method on the Ra (axial resistivity) it answers a unit combination I don't understand:

Code: Select all

``````oc>create soma
oc>units("Ra")
ohm-cm``````
Does it mean ohm/cm or ohm*cm ???

regger
Posts: 21
Joined: Sun Apr 22, 2012 9:49 pm

### Re: What kind of unit is ohm-cm ?

It's ohm*cm. You get the total resistance of a piece of wire of length L and cross-sectional area A by

R total = Ra * L / A

Since R total is of dimension [resistance], Ra must have dimension [resistance * length].

hope this helps,
Robert
AGBX2
Posts: 18
Joined: Thu Jun 16, 2011 11:12 am

### Re: What kind of unit is ohm-cm ?

Thanks a lot Robert. This is indeed very useful to me :)
I have not been able to find this information on the neuron website. Is it mentionned somewhere ?
ted
Posts: 5795
Joined: Wed May 18, 2005 4:50 pm
Location: Yale University School of Medicine
Contact:

### Re: What kind of unit is ohm-cm ?

AGBX2 wrote:I have not been able to find this information on the neuron website. Is it mentionned somewhere ?
Introductory physics and electrical engineering texts, plus (according to google) on the order of 10^6 places on the web.
AGBX2
Posts: 18
Joined: Thu Jun 16, 2011 11:12 am

### Re: What kind of unit is ohm-cm ?

ted wrote:
AGBX2 wrote:I have not been able to find this information on the neuron website. Is it mentionned somewhere ?
Introductory physics and electrical engineering texts, plus (according to google) on the order of 10^6 places on the web.
Sure and I use it. I was just a bit puzzled by the sign "-" bewteen ohm and cm which is a physical nonsens. A "." should have been clearer ;)
ted
Posts: 5795
Joined: Wed May 18, 2005 4:50 pm
Location: Yale University School of Medicine
Contact:

### Re: What kind of unit is ohm-cm ?

AGBX2 wrote:I was just a bit puzzled by the sign "-" bewteen ohm and cm which is a physical nonsens. A "." should have been clearer
Really? Haven't seen that anywhere--can you point me to an example in the literature? Doesn't seem like a good idea; there's enough notational confusion already, given the existence of countless examples of compound units that employ or don't employ one or more hyphens, depending on the whim of an author or editor. Not to mention the naming conventions used in UNIX's units.dat file. None of this should be surprising, really, especially when one considers that "uniform orthography" itself is a relatively recent convention, and, in practice, not always so uniform.
AGBX2
Posts: 18
Joined: Thu Jun 16, 2011 11:12 am

### Re: What kind of unit is ohm-cm ?

I don't really understand the meaning of your answers. I just wanted to disambiguate the use of the substraction symbol and to be sure that it was a product. Is it a problem ?
(http://fr.wikipedia.org/wiki/Resistivite).