1. National Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK
2. Federal Institute of Metrology METAS, Lindenweg 50, CH-3003 Berne-Wabern, Switzerland
3. National Standards, FPS Economy, Koning Albert II laan 16, 1000 Brussels, Belgium
4. Physikalisch-Technische Bundesanstalt, Abbestraße 2, 10587 Berlin, Germany