Several Bayard–Alpert-type ion gauges were constructed with a modulator electrode. Curves of ion collector current versus modulator potential were obtained for each gauge and as a function of other electrode potentials and electron current. The most striking difference is found to occur between gauges with and without grid endcaps. For gauges with open end grids, the ion collector current is found to exhibit a maximum at a modulator potential 20–30 V below grid potential. Such a modulation characteristic provides the opportunity to utilize the modulation technique for obtaining the true ion current and spurious current without producing gross potential changes within the gauge. This results in a minimum perturbation of electron orbits and reduces pressure changes due to electron desorption and subsequent adsorption on the modulator since the modulator is continuously bombarded by electrons.