h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: The Metric System - A2220599
Author: Hussassan the Silicon Samurai - Proud User of Alabaster - U200779

OK, here's what I've written, and I'm putting it in here because there's probably some stuff I got wrong or left out. Any and all help is appreciated.

Well, a good jibe against the old British units, which are still used by the Americans. (They have had some very high profile mistakes when using metric units. Everyone knows when NASA makes such a mistake.) The subject of units is vast, and these days a matter of international agreement.

But there are some very important things that you do not say.

There are fundamental units, such as length, mass and time, and derived units, such as pressure. The derived units inevitably involve more than one fundamental conversion factor.

The metric system has evolved too. The scientific community for a long time used the CGS system, which is metric, and quite different from the present metric system, which evolved through MKS, MKSA to the present SI system.

The British system existed in two forms, roughly corresponding to the CGS and MKS system. In the first of these, the unit of mass was the pound, and the unit of force was the poundal. The more modern version has the unit of force as the pound, and the unit of mass as the slug. The difference is which one depends on g (the gravity thing). The CGS (centimetre, gramme, second) system had length in centimetres and mass in grammes, whereas the MKS (metre, kilogramme, second) system uses metres and kilogrammes, but both CGS and MKS use g in the same way. MKS cannot measure everything, and so the electrical unit of the ampere was added to give the MKSA system. The SI system developed from that.

The British system is different from the apparently similar US system. The units of capacity in particular do not agree: UK gallon=4.546litres, US gallon=3.7854litres

You do not mention that some of the scientific units have undergone change for various reasons. Some units would natural involve Pi, and whether Pi appears in the formula for a unit, or is absorped by changing to unit, has been a changeable fashion. Hence the so-called rationalised MKS system.

All of the units in the SI system are defined by international agreement. I think you should mention that, and give some relevant links, to various bureaux of standards.

There are some units that have been entirely omitted. For example, the units for light.

There are some units for special purposes, rather like the Astronomical Unit that you do mention, some historic and some which may persist. For example, millimetres of mercury for atmospheric pressure, inches of water for domestic gas pressure.


The are mistakes in the entry.

Your definition of the second is quite wrong. For an explanation see my (unedited) entry Time and Date at A2080630, in the section headed Time. In it, the link to the National Physical Laboratory will get you to a wealth of information about units, and is bang up to date. (The NPL is the agency responsible for UK national standards.)

Your definitions of length and time are circular, as you acknowledge, but your solution is not the correct one, as time is defined differently.

I am not certain, but fairly sure that the basic electrical unit is the ampere, which is defined in terms of the force between two parallel conductors.

Ok, it took a while, but I've done some research, and updated this entry significantly. for all you help, and your entry on time & date was quite useful. Incidentally, do you know how to type a capital omega in GuideML? All I can get is a small one.

Try Ω instead of ω - I think that should do it.

I did that and it worked perfectly. Thanks a lot! So I take it you like the new, improved version?

One brief note...

"In the SI system, there are six fundamental measurements: length, mass, time, electric current, and temperature."

Something's wrong there!

It would be more accurate to say I did not have time to read it. I just gave a quick answer to a small question.

I will comment again more fully when others have had some input.

*"In the SI system, there are six fundamental measurements: length, mass, time, electric current, and temperature."

Something's wrong there!*

Oopsie, I've fixed that typo and now await more input from other people who read the WW.

I have unfortunately only just noticed that there is a guide entry A471476 which covers much of the subject matter of this entry already. It is difficult to see what, if anything, this entry would add to the guide.

Damn. I was pretty proud of this entry too, so I guess I'll just leave it as an 'unedited' one.