Nanoamps on the DMM: Measure Tiny Currents Using an Ordinary Digital Multimeter

Even the most bargain-basement LCD digital multimeters (DMMs) these days have more than adequate accuracy and range coverage for any normal use. However, sometimes less ordinary situations arise and you’ll find yourself out of luck: for example, a cheap 3.5-digit meter will normally have a lowest current range of 200 µA full scale. What can be done?