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The calculable capacitor is a classical and fundamental experimental apparatus in precision electromagnetic measurements. It is the alternating current (AC) impedance primary standard, and an important tool in measuring the fine structure constant. The calculable capacitor provides a way to directly link the capacitance unit to the mechanical unit of length. In the calculable capacitor, the displacement measurement of the guard electrode is an essential part, because the average value of the cross capacitances is directly proportional to the linear displacement of the moving guard electrode. In order to measure the displacement with a high accuracy of 10-9 or lower, a Fabry-Perot interferometer, whose cavity length is traceable to a stabilized laser by the phase sensitive detection technique, is employed. Considering that the Fabry-Perot interferometer is irradiated by the Gaussian laser beam, the effect of the phase shift of the Gaussian field, relative to the plane wave, should be carefully considered in the displacement measurement. The amplitude of the Gaussian laser beam disperses out of the region where it can be assumed to be plane-wave propagation, so its wavefronts bend and their spacing is different from that of the plane wave. As a result, the corresponding distance of an interference fringe from the coherent Gaussian laser beams is not strictly equal to /2, and it means that the displacement correction based on the phase shift of the Gaussian laser beam in the Fabry-Perot interferometer is inevitable. Therefore, the measured result should add or subtract the correction value to obtain the actual displacement of the interferometer. In order to determine the Gouy phase correction, an interferometer model based on the calculable capacitor is studied analytically and numerically. Using the free space propagation and lens transformation of the Gaussian beam field, the complex amplitude of the partial beam transmitted through the interferometer is obtained, and its phase versus the longitude propagation distance is analyzed. The amplitude and phase of the total transmitted beam, which is the coherent superposition of all the partial beams, are presented. Since the Fabry-Perot interferometer in the calculable capacitor is actively locked to a stabilized laser at two different cavity lengths, the phase of the transmitted beam at each cavity length is calculated individually. The phase difference between the two transmitted beams versus the longitude propagation distance is also analyzed numerically. The simulation result demonstrates that the minimum value of the displacement correction can be obtained by actively detecting the laser light at a distance of 560 mm from output mirror, when the Fabry-Perot interferometer moves from the cavity length of 111.3 mm to 316.3 mm, and it means that a displacement correction value of 0.7 nm, with a relative value of |L|/|L| = 3.410-9, should be added to the measured displacement of the guard electrode.

An energy comparison in industrial frequency of 50Hz has started in China. The metrology institutes of the big regions and of some important provinces participate in the comparison. The comparison purpose and main technical problems are introduced in the paper. The comparison will be finished in June 2004 and the main results would be published in the conference. Introduction A huge sum of electrical power are produced and consumed in China. At the same time a marketable management for power utility is in progress. It promotes greater development of energy metrology in China. Some provinces set up the energy standards with level of 0.01%. In 2000 the national standard of three-phase energy was completed at NIM [1] and then the result of the Key Comparison of CCEM-K5 including the results of NIM was published by CCEM [2]. Under the conditions the General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China (AQSIQ) organized an energy comparison in the whole country range. The metrology institutes of the big regions such as south, northeast, east and so on, participate in the comparison and some important provinces such as Zhejiang, Jiangsu and Tianjing are including too. NIM National Institute of Metrology is as pilot laboratory. The measurements of comparison started from September 2003 and will finish in June 2004. The organization experience of CCEM-KC is taken as reference and some of them are direct adopted. The purpose, the difference from CCEM-KC, test points, travelling standard, pre-test results and preparatory statistics method are introduced in the paper. The comparison is now in progress and up to the end of 2003 the half participants have finished their test. The detailed results and analysis would be shown in the conference in end of June 2004.

A so called "cross conductivity standard" based on a new principle developed from Thompson-Lampard Theorem (1956) has been established at N I M by the end of 1992.The conductivity range of the standard is approximately from 3% IACS to 100% IACS and with an uncertainty about 0 . 0 7 % .By using an AC measuring system and a new method, the unknowns can be calibrated with an uncertainty of (0.1% rx + 0.12% IACS).