1888: Ampere balance - The book of science

Ampere balance
Ampere calculation
Balance
Commentary

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:

Readings on wikipedia:

Other readings:

“Amperemeter,” 1911 Encyclopædia Britannica, in Wikisource