I type devices use a converter circuit with a capacitor as the feedback element. The output is the voltage across the capacitor. Using a capacitor to integrate suppresses noise and allows small input currents.
I type devices are good for measuring fast pules and very small currents.
128 Channel Charge Integrator Digital Electrometer
32-Channel Charge Integrator Digital Electrometer
Precision Electrometer Digital Current Integrator with High Voltage
4 Channel Charge Integrating Digital Electrometer
64 + 1 Channel Charge Integrator
4 Channel Biased Input Charge Integrating Electrometer