As of 2011, the SET chip developed
at the PTB may well be the key to a
novel definition of the ampere which
links it to a fundamental physical
constant, the charge of an electron.
The chip transports and counts elec-
trons. This quantum effect – called
‘single electron tunneling’ – only
occurs at low temperatures, there-
fore the chip needs to be cooled
down to 20 mK with a cryostat – at
right – and liquid helium.

[A] ‘The AMPERE is that con-
stant 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 2·10–7 newton per meter
of length.’