[syhprum]

is a Barn-yard the imperial measure of nuclear cross sectional area i.e one Barn yard equals 0.8359 * 10^-28 square meter

[Soul Surfer]

Going back into ancient history the barn (note no yard)  is another good example of humour on science when the the first scientists doing nuclear science needed a measure for the sort of target that an atomic nucleus presented.  Now a big nucleus is about 10^-14 meters across and so its cross sectional area is about 10^-28  sq metres  when they first realised that that was the size of their target someone said its as big as a barn door (from the saying that its poor shooter that couldn't hit a barn door) and the name stuck.   There are a whole load of other target sizes for lower probability events see  http://en.wikipedia.org/wiki/Barn_(unit)

I have also just done a search on another interesting but old unit  the "hide"  which is the area of land needed to completely support an extended family unit  and is around 30-120 acres.  This could come into modern units on population and self sufficiency.

[syhprum]

I know the barn is not a S.I unit but it is tolerated but in a recent Phsyorg article I was reading a milli-barn was quoted as being 10^-27 cm^2 which I think is stretching our toleration a bit far

[Bored chemist]

The barn yard atmosphere is a unit of energy (a rather small one).
An area  multiplied by a distance gives a volume, and a volume multiplied by a pressure gives energy.
BTW if you don't like the "milli barn" as a unit for capture cross section you would really hate the alternative. Cross sections for improbable events are sometimes quoted in sheds.

[damocles]

There is actually a perfectly convenient and respectable SI alternative to millibarn.

1 barn = 1.E-28 m², therefore 1 millibarn = 1.E-31 m²

1 fm = 1.E-15 m, therefore 1 fm² = 1.E-30 m² = 10 millibarn

[Geezer]

There is actually a perfectly convenient and respectable SI alternative to millibarn.

1 barn = 1.E-28 m², therefore 1 millibarn = 1.E-31 m²

1 fm = 1.E-15 m, therefore 1 fm² = 1.E-30 m² = 10 millibarn

Yes, but how many sheepies would that hold?

[damocles]

There is actually a perfectly convenient and respectable SI alternative to millibarn.

1 barn = 1.E-28 m², therefore 1 millibarn = 1.E-31 m²

1 fm = 1.E-15 m, therefore 1 fm² = 1.E-30 m² = 10 millibarn

Yes, but how many sheepies would that hold?

approximately 0.314 attosheep

[acsinuk]

Have just looked up Oliver Boyd and found that a 1 kWh is equivalent to 2.247 * 10^25 electron-volts which means that as average light photons have 1.1 eV each then 2*10^25 is good aproximation for a one square metre PV panel which gets 1 kW peak at noon on equator after allowing for third haze reduction. The power is received per square metre but to change it into energy we must multipy by time.  So each photon is not a magic massless ray but a volume of 3 dimensional magnetic flux energy which is absorbed at the surface of the PV cell.
CliveS