Why does my picoammeter (or electrometer) give a non-zero reading when I short the input?

When measuring current, the input circuit is an op amp in a current-to-voltage converter configuration, also known as a transimpedance amplifier. As such, the feedback is 100% when the input is connected to a true current source.

But if the input is shorted the feedback is ZERO, and all the input stage noise and voltage offset shows up on the display.

A voltmeter should be shorted to check for offset, drift, etc., but an ammeter should be open circuited.

To determine the zero offset of your picoammeter or electrometer in current mode, use the Zero Check feature.