The HiZ-223 Dual Channel Differential Electrometer is a high input impedance amplifier featuring two small active headstages that allow the first stage of amplification to take place at the electrode site. The amplifier, for measurements using high resistance ion-selective microelectrodes, reduces shunt capacity and minimizes electrical interferences such as electrostatic noise and motion artifacts. The probe housing includes a signal-driven guard that further reduces noise and stray capacity. A probe test port permits testing of the electrode as well as setting of the probe leakage current (IG). A standby mode is included and should be used when attaching microelectrodes or electrode holders to the probe input. Warner Instruments, 800-599-4203, www.warnerinstruments.com.