• French physicist André-Marie Ampère discovered that passing a current through two parallel wires generated a force between them. The current definition of the ampere is based on this concept.
  • However, the International Conference of Scinetists in 1908 decided on a different solution of the ampere: the amount of current that deposits 0.00118 grams of silver per second from a silver nitrate solution.
  • The device used to measure the „international ampere“ was called a silver voltameter. This had to be calibrated, and scientists quickly found it wasn’t accurate enough for their experiments.
  • In 1948 the General Committee on Weights and Measures went back to André-Marie Ampère’s ideas, and redefined the ampere as the current flowing in two parallel conductors that produces a force between them of 0.2 micronewtons.
  • In May 2019, the ampere will be redefined once again to take advantage of the fact that a current is made up of a flow of electrons. Now, we can count the number of electrons passing through a conductor in a given time.