1. Analytical Science Team, National Physical Laboratory, Hampton Road, Teddington, U.K. TW11 0LW, EffecTech Ltd., Dove Fields, Uttoxeter, U.K. ST14 8HU, Michell Instruments Ltd., Lancaster Way Business Park, Ely, U.K. CB6 3NW, National Grid, National Grid House, Warwick, U.K. CV34 6DA, Orbital Ltd.,Cold Meece, Swynnerton, U.K. ST15 0QN, and Advantica Ltd., Ashby Road, Loughborough, U.K. LE11 3GR