# How do we measure the units in physics?

1. Oct 24, 2014

### christian0710

Hi,

When i Look at SI derived base units like Joule, Newton, pascal and base units like the Ampere which is defined as
"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length."

Then 2 questions come to mind
1. How are theese units measured - With what equipement? How is the joule
2. How does it work - How does the measuring equipement measure it? What is i measuring?

How is the Joule = kg⋅m2⋅s−2 Measured?
How do you measure the force of 2 × 10−7newton of two currents one metre apart in vacuum? This seems so vague to me?

Are there any good textbooks or websites which describe this in detail? I'm interested in understanding how things are done in physics.

2. Oct 24, 2014

### christian0710

Hi,

When i Look at SI derived base units like Joule, Newton, pascal and base units like the Ampere which is defined as
"The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length."

Then 2 questions come to mind
1. How are theese units measured - With what equipement? How is the joule
2. How does it work - How does the measuring equipement measure it? What is i measuring?

How is the Joule = kg⋅m2⋅s−2 Measured?
How do you measure the force of 2 × 10−7newton of two currents one metre apart in vacuum? This seems so vague to me?

Are there any good textbooks or websites which desgribe this in detail? I'm interested in understanding how things are done (measured) in physics, and also if possible, who discovered the different physical quantities and how they measured them back in time.

Help is deeply appreciated.

3. Oct 24, 2014

### Staff: Mentor

You are looking for http://en.wikipedia.org/wiki/Metrology

In most cases you don't have to realize definitions exactly, as it is possible to scale the measurement (so for example you don't need two wires meter apart, they can be closer, the force will be different, but we know it scales with the distance, so we know what to expect).

4. Oct 24, 2014

### Staff: Mentor

The Ampere definition is indeed a tricky one. We do not have access to conductors of infinite length, and the tiny force between them would be hard to measure.
Luckily, we can use this definition to calculate forces between conductors with different geometries - like the force between two coils close together. This force can be measured like everything else - attach the coil to a spring with known spring constant (see below), measure the displacement (with a ruler or with more precise equipment).
This is not how it is actually done, but it can be done.

To calibrate the spring, you need some known force. To do this, you can use it as a scale: Put a mass m on it, it will lead to the force F=m*g.
The kilogram is defined by the mass of an object in Paris, so it easy to make something with a mass of 1kg (or fractions or multiples of it). The gravitational acceleration of earth can be measured simply by letting something fall down and using a ruler and a clock.

So you have to calibrate the ruler and the clock....
Today this is done with atomic clocks and the speed of light ("1 meter is the distance light travels in vacuum during 1/299792458 second" - that's something you can actually measure), historically it was done with different references.

I don't know which textbooks describe that best. Most should give some general introduction.

5. Oct 24, 2014

### NTW

From memory, I believe that the original (not the current) definition of the Ampere was based in the definition of the Coulomb, that had, in its turn, a 'hard' definition: off the head, and with the risks involved, the definition of the Coulomb was the charge that deposited 1,1180 mg from a silver nitrate solution...

6. Oct 24, 2014

### Staff: Mentor

It will likely change again (back to Coulombs, and this via the elementary charge), as the current definition is impratical.

7. Oct 25, 2014

### DennisN

You could have a look at these sites (there is at least some history and info on them, I'm not sure about the levels of detail, but there may be more links on those sites which may be helpful):

National Institute of Standards and Technology (NIST)
Detailed contents: http://physics.nist.gov/cuu/Reference/contents.html

Bureau International des Poids et Mesures (BIPM)
Measurement units: the SI: http://www.bipm.org/en/measurement-units/