Abstract: Laboratory EMF measurements were conducted in Fe+C_(sat)+Si system using ZrO₂ (MgO) solid electrolyte equipped with a new auxiliary electrode SiO₂+CaF₂. The results showed that the measured EMFs reaches stable value within 5 seconds after immersion of the developed silicon probe into the above melt; the relation between measured EMFs and silicon activity corresponds with the theoretical calculation; the measured EMFs were nearly independent of measured temperature from 1 694 to 1 749K. Due to the rapid response and accuracy of the silicon probe developed in this study, it is expected to be used in situ.