The cable used to connect the RTG (Radioisotope Thermoelectric Generator) to the ALSEP Central Station had what was called a "shorting plug" on the free end that plugged in the base of the Central Station.  The shorting plug contained a shorting circuit that, from the time the RTG was fueled until the time when all the experiments had been connected to the Central Station, would mimic the electrical resistance of the full ALSEP array to the current produced by the RTG.  Once all the experiments were ready to be powered and the Central Station antenna had been aligned, the crew used a UHT (Universal Handling Tool) to either push a button (Apollos 12 to 16) or turn a switch (Apollo 17) that would send the current to the experiments.  While current was flowing through the shorting circuit, an ammeter (ampere meter) would read about 8  or 9 amperes.  Once current was no longer flowing through the shorting circuit, the ammeter would read zero. The meter could be difficult to read if dust coated.