The unit bar for pressure is clearly a metric unit, but its order of magnitude is a bit strange. In the centimeter–gram–second system of units we have:
1 bar = 1 000 000 baryes = 1 000 000 dyn/cm²
so the bar is not "coherent" with this system (the factor is not one). Also in the meter–kilogram–second (and SI) system we get:
1 bar = 100 000 pascals = 100 000 N/m²
while in the meter–tonne–second system:
1 bar = 100 pièzes = 100 sn/m²
So my question is simply, where does the conversion factor for bar come from, since it seems to not fit into usual systems? According to the Wikipedia article bar this unit was created already in 1909 by British meteorologist Shaw, but not much detail is provided.
Maybe the factor was simply chosen as the power of ten making the unit closest to the atmospherical pressure at sea level (which is 1.01325 bar by convention, and close to 1.01 bar on average)?
Best Answer
The $\text{pascal}$ seems of a much later date than the $\text{bar}$. In fact, it seems that, at some time, the $\text{bar}$ was adjusted a bit away from the average air pressure on earth (its originally intended definition), to get it "in line" with the SI units, and therefore also with the new or later $1~\text{Pa}=1~{\text{kg}\over\text{m}\cdot\text{s}^2}$.
Pascal (unit)
As noted in the comments below, this answer (and, perhaps, the Wikipedia quote) might or might not stand up to scrutiny. Further digging in history seems necessary, but I dug a bit and can't find anything really substantiating this reading. However, I also didn't find anything conclusively and explicitly pointing towards a different reading (i.e., that the magnitude of the $\text{bar}$ is the same now as it was when the $\text{bar}$ was adopted initially).