Acid type saturated Weston cells that can be used as the standard of electromotive force unit were constructed and their characteristics in a specially designed enclosure were evaluated. Cadmium sulfate was purified by recrystallization under vaccum. Mercurous sulfate with dispersed mercury and the exact composition of cadmium amalgam were obtained by means of electrolysis. The enclosure was constructed using a commercial circulator. Temperature of the enclosure was only drifted in the range of ${pm}$5 mK in the case of extreme change of room temperature. The electromotive force of standard cells was measured over the temperature range of 5∼${sim} 30^{circ}C$. The standard deviation of the electromotive forces was about 1 ppm.