Improvements of AC/DC calculable standard resistor with double helix arrangement and Kelvin resistance bridge at NIM

In order to calibrate the frequency dependence of ac standard resistor more accurately, improvements of double helix standard resistor and Kelvin resistance bridge at NIM are made and described in this paper. The experimental results show that the dc resistance stability of the AC/DC calculable resistor is enhanced by one order and the measurement uncertainty of the bridge is decreased.