William Thomson, Lord Kelvin electromagnetism |

## Ampere balance

- To measure the ampere,
- slide a weight on a scale
- to balance the current to be measured
- coiled against the weight.

## Ampere calculation

- The strength of the current
- when the weight is balanced
- is proportional to the product
- of the square root of the weight
- and its displacement
- from its zero position.

## Balance

- Nothing is that delicately balanced
- that the result comes from a simple equation.
- The scale must be level; we must know precisely
- the dimension and winding of the coil;
- and how do we measure the mechanical rigidity
- of linkages and pivots?
- In balancing a current against a weight,
- either current or weight might be off.
- We should apply Bayes’ theorem
- and balance the significance of what we know
- against the weight of what we don’t know.

The Kibble balance is seen as a more accurate version of the Ampere balance even though Kibble’s purpose was to measure a weight and Kelvin’s purpose was to measure a current.

See also in

The book of science:Galvanometer—Luigi Galvani, Hans Christian Ørsted, Johann SchweiggerElectromagnetism—Hans Christian Ørsted, André-Marie Ampère, Michael FaradayReadings on wikipedia:

Other readings: