Detection of small a.c. voltages over a wide range of frequencies with sensitivity comparable with d.c. necessitates methods, equipment, and technique far in advance of that required for the equivalent d.c. measurements. The portable d.c. meter offers the ultimate in simplicity, compactness, and ruggedness for use as a d.c. null indicator. A simple method of detecting a.c. voltages is by use of a d.c. meter in conjunction with a rectifier. Perhaps the metallic thin film rectifier is the best to date for above use from the standpoint of simplicity, ruggedness and compactness. The d.c. meter and rectifier as conventionally used are unreliable for null indications because of inherent characteristics of the rectifier.