The kilogram before the reform:
The kilogram has been defined on the basis of the mass of a prototype kept in Paris since 1889. Several copies have been made of the prototype, including the one at VTT MIKES, No 23, which Finland received in 1890. The mass of the prototypes changes based on comparisons between the copies, but no-one knows by how much exactly. It is nevertheless clear that the mass of a kilogram is no longer what it was originally.
Copyright: BIPM – International Bureau of Weights and Measures
The kilogram after the reform:
The reform of the SI system is enabled by the Kibble balance shown in the picture, which can be used to relate mass to the strength of an electric power, which can then be fixed to the Planck constant by means of quantum phenomena. The Kibble balance is a highly sophisticated precision instrument, but research teams have nevertheless been able to build various versions of it around the world. Therefore, the size of the kilogram will no longer depend on a single prototype or device after the reform of the SI system. Fixing the kilogram to the Planck constant ensures that it will not change systematically over time either.
The ampere before the reform:
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length. The current definition of the ampere is almost impossible to realise with high precision in practice. On the other hand, the newton ties the ampere to the mass of the prototype of a kilogram, which is known to change.
The ampere after the reform:
The new definition of the ampere is fixed to the elementary charge of a single electron. The most direct way to realise the new definition is to transport electrons one by one, which can be done using microscopic single-electron pumps like the one shown in the picture. This particular device, developed by VTT, is based on CMOS technology, which is also used in computer processors. The dotted lines illustrate the parts of the pump hidden by the outer layers. The turquoise structures are used to guide electrical current in the wire marked in blue. Dealing with individual electrons requires quantum phenomena and cooling the device down to within approximately one degree of absolute zero. The black-and-white electron microscope image has been coloured in by hand.