A precise measurement of the electrical voltage and the resistance has become possible by utilising fundamental quantum effects, namely Josephson effect and quantum Hall effect. They allow the reproduction of the physical units Volt and Ohm with a very high precision and largely uninfluenced by environmental parameters at any place in the world.
Also the electrical current can be related to quantum units. Single electrons are driven at a precisely known frequency through a ‘wire’ of sub-micron dimensions to produce a current I=ef. Though conceptually simpler than the realisation of the Volt and the Ohm mentioned above the current produced this way is either too small for a technical use or it is not precise enough.
Though in an electrical circuit at known voltage and resistance Ohm's law determines the current, an independent quantum realisation of the Ampere would constitute an important experimental test of the consistency of the relations between the fundamental constants which are illustrated in the "Electrical Triangle" in the figure below. For this reason PTB as well as other metrology institutes intensely advance the work in this direction.