# Units of measurement

#### Besoeker

##### Senior Member
My '96 Honda Nighthawk 750 engine is rated at 56kW.
Excellent.
The Jag in my avatar was 224 kW.

#### kwired

##### Electron manager
My '96 Honda Nighthawk 750 engine is rated at 56kW.
I'm sure the 750 is most likely 750 cubic centimeter of displacement as well.

#### RichB

##### Senior Member
Most US engineers are bi-dimensional, I even think in metric in other than mechanical construction.

Perhaps we should all go back to Megalithic yards.

Wonder what electrical units would look like in Megalithic terms ? Other than any precursors of Bagdad battery, no archeological artifacts though
Is that like being "bi-sackual" at the store?? Paper or plastic ?:roll::?:angel:

#### drcampbell

##### Senior Member
Don't forget French Horsepower!
Phil Corso
I'm pretty sure the French horsepower is the metric horsepower, 75 kilogram-meters per second.
(which is actually a measure of impulse, not power, because the kilogram isn't a measure of force)

I'm also pretty sure the French don't use it any more.

#### kwired

##### Electron manager
I'm pretty sure the French horsepower is the metric horsepower, 75 kilogram-meters per second.
(which is actually a measure of impulse, not power, because the kilogram isn't a measure of force)

I'm also pretty sure the French don't use it any more.
My question is why 75 and not 100?

Newton is a unit of force that could be used instead of a unit of mass.

#### drcampbell

##### Senior Member
My question is why 75 and not 100? ...
Probably so it would be approximately the same as the British horsepower. (736 vs. 746 watts)

... Newton is a unit of force that could be used instead of a unit of mass.
Could'a been, but wasn't. The kilogram-force per square meter [sic] was also popular in a bygone era.

#### winnie

##### Senior Member
My question is why 75 and not 100?

Newton is a unit of force that could be used instead of a unit of mass.
75 because it was a round number which gave a result that was close to the 'imperial' unit.

In a 1 g field, lifting 75 kg 1 m per s is about 736W.

Sort of like 500g being used as a 'pound' in areas where metric is used but there is a tradition of using pounds.

-Jon

#### Besoeker

##### Senior Member
My question is why 75 and not 100?
Because that's what it works out to be.

##### Senior Member
75 because it was a round number which gave a result that was close to the 'imperial' unit.

In a 1 g field, lifting 75 kg 1 m per s is about 736W.

Sort of like 500g being used as a 'pound' in areas where metric is used but there is a tradition of using pounds.

-Jon
I thought a pound of beef at Asda was 400 grams..they are shorting me? Hmmmm

#### Phil Corso

##### Senior Member
For those of you who don't know...

We, the USA, signed the world treaty to accept and use the International System of Units (SI) in 1866! Now, why do you think it's use here, is spotty at best!

Phil Corso

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Now, why do you think it's use here, is spotty at best!

Phil Corso
Enlighten us, Dr. Phil!

#### Besoeker

##### Senior Member
Enlighten us, Dr. Phil!
I think he means it is not used consistently for all measurements,

##### Senior Member
For those of you who don't know...

We, the USA, signed the world treaty to accept and use the International System of Units (SI) in 1866! Now, why do you think it's use here, is spotty at best!

Phil Corso
But we do use many items from international treaties without realizing it. So many of the symbols we use in electronics are international symbols. Most of our road signs are using international symbols, so much so that I keep wondering will I return to the USA and see the round speed limit signs on the posts...lol... even the stop lines I got used to in many states are used in the UK and Europe.

##### Senior Member
One advantage of legal marijuana is having to know the metric system...
I agree metric units are much easier to work with, where I work all our lab work is metric, as most places are.
But with metric its all or nothing, which is why our previous attempts in the US have failed.
That is funny I don't care who you are! I must admit that for dry weight that is why I have a full grasp of grams.

##### Senior Member
English Oxford Dictionary.

Technically though, aren't all the units from that system interrelated? One gram of water is equal to 1 cubic centimeter of water? Really it doesn't matter what we call it, I am with you. I would gladly convert to SI across the board. It would be difficult at first, but the payoff is huge.

#### drcampbell

##### Senior Member
They're all interrelated, but not the way you seem to think.

The old metric system (c.1700s) was based on the boiling & freezing points and density of water, the distance from the equator to the North Pole, and so forth. But that's long obsolete.

In the centuries since then, we've discovered that the water's characteristics aren't constant - density varies with temperature and boiling & freezing points vary with pressure. So it became necessary to redefine the fundamental units in order to be able to make precise measurements that are the same everywhere. The meter was redefined in terms of wavelengths of cesium light, the kilogram was defined in terms of a pure silicon sphere and so forth. Those harmonizations & redefinitions mark the transition from the old metric system to the current SI system.

#### Carultch

##### Senior Member
They're all interrelated, but not the way you seem to think.

The old metric system (c.1700s) was based on the boiling & freezing points and density of water, the distance from the equator to the North Pole, and so forth. But that's long obsolete.

In the centuries since then, we've discovered that the water's characteristics aren't constant - density varies with temperature and boiling & freezing points vary with pressure. So it became necessary to redefine the fundamental units in order to be able to make precise measurements that are the same everywhere. The meter was redefined in terms of wavelengths of cesium light, the kilogram was defined in terms of a pure silicon sphere and so forth. Those harmonizations & redefinitions mark the transition from the old metric system to the current SI system.
The kilogram is still defined by the international prototype platinum-iridium cylinder known as "Le Grande K". The silicon sphere from the Avogadro project is one of the two proposed alternatives that could redefine it. The other alternative is fixing Planck's constant as it relates to an experiment known as the Kibble balance, and this is the solution that is most likely going to define the new kilogram. It has to do with whether the uncertainty of the new method is an improvement over the existing uncertainty in Le Grande K, as we know from comparing it to the sister kilograms stored in similar conditions.

The fundamental advantage of the new methods, is that its definition would be independent of any individual object in particular. Just as the international prototype meter stick was replaced with the speed of light in free space and the definition of the second in the Cesium atomic clock.

#### Besoeker

##### Senior Member
Technically though, aren't all the units from that system interrelated? One gram of water is equal to 1 cubic centimeter of water?
Yes, in the CGS system.

Really it doesn't matter what we call it, I am with you. I would gladly convert to SI across the board. It would be difficult at first, but the payoff is huge.
Some units you already use are SI. There is, for example, there is no Imperial equivalent of the Amp or Volt.
For others like the pound and the inch, there are readily available conversion tools if you don't remember the factors. It isn't really very difficult.

One of the obstacles to changing the systems used in the US is cost I suppose. You have say, a motor with a 2" drive shaft. To accommodate that, you need bearings that fit which won't be in SI units. That's true for old equipment here (UK) and I've often come across motors that were built in the 1950s and still in service.

#### winnie

##### Senior Member
The US currently uses SI as the actual basis of its system of measure, so that any changes in the definition of the meter are reflected as a change in the definition of the inch.

The inch is _defined_ as 2.54cm, _exactly_.

-Jon

#### JoeStillman

##### Senior Member
Système international.
C'est en français! Cela règle le problème pour moi. Utilisez des unités impériales s'il vous plaît.