The apparatus described provides a pulse of biasing voltage to a probe system so that electron temperatures up to 2 × 10⁵ ° K and ion densities up to 5 × 10¹⁴/cc can be measured. The system consists of a low impedance voltage sweep generator, a transformer-coupled measuring circuit and a logarithmic amplifier. The apparatus can be used to sample a portion or the whole of the electron energy distribution by using double probes of equal or greatly unequal areas. The probe plots are displayed directly on an oscilloscope.