Similar to a DMM, electrometers offer multiple measurement functions such as volts, current and ohms. However, the electrometer has special input characteristics which offer special advantages for measurement of low values of current or high values of resistance.
For example, the Volts function uses a much higher input impedance than a DMM which will minimize circuit loading.
The Resistance function offers extended ranges into the Giga or Tera ohm range. DMMs typically determine resistance by sourcing a current and measuring the resulting voltage. An important difference from the DMM is the ability to source a known voltage and measure the resulting current to determine resistance. This is especially important for materials whose resistance will vary as a function of the applied voltage such as is seen with insulating materials.
The Current function of an electrometer has much lower sensitivity than a DMM. Electrometers also offer an additional Charge function.
Picoammeters, such as the 6485 or 6487, measure current only. For these current measurements, Picoammeters are similar to the electrometer design. However, electrometers offer more range sensitivity, providing better resolution and improved accuracy for the extremely low currents (<1pA).