I was working with my son on his science project “Impressed Current Cathodic Protection in Presence of AC Interference.” Impressed current cathodic protection is used to protect pipes (oil, gas, water lines) from rapid corrosion. Rectifiers are used more than any other source of impressed current power. Cathodic protection is a method of supplying electrons to a pipe such that when chemical corrosion processes occur at the surface of a pipe the pipe will not shed any electrons, but rather the cathodic system will supply electrons from an external power source, rectifier. It is often a difficult task to determine just the right amount of protection. One of the critical components of the experiment is the soil resistivity. Soil resistivity determines the amount of current flow from pipe into the soil. Higher soil resistivity results in lower currents flow. Basically pipe will corrode faster in wet soil than in dry soils or sands. I needed to measure soil resistivity and regular multimeter gives Open Line because it supplies DC current to the soil and soil gets charged like a capacitor. Multimeter + on the other hand measured soil resistivity very accurately. The reason is that instead of supplying DC current, Multimeter + sends a sort pulse during which it takes resistance measurement. After the reading is taken, pulse value drops to zero potential and soil has enough time to discharge. Then the process is repeated.