The Hewlett-Packard Models 400D 400H and 400L Vacuum Tube Voltmeters are general purpose, portable electronic a-c voltmeters of high sensitivity and stability. They are suited to both laboratory and field use. Models 400D H measure a-c voltages from 0. 001 to 300 volts and Model 400L from .003 to 300 volts rms full scale, with a frequency bandwidth cover- ing 10cps to4 megacycles. The voltmeters are com- pact, accurate, and rugged and have fast meter re- sponse, high input impedance, stable calibration ac- curacy, and freedom from the effects of normal line voltage variations. The voltmeters are designed for long instrument life with a minimum of servicing.