When commercial vacuum tubes are used to amplify small low‐frequency voltages it is found that random disturbances of the order of 100 to 1000 microvolts are present in the anode circuit. These disturbances exist almost entirely in the range below 100 cycles per second, and therefore fix the minimum voltage which can be measured over this low frequency band from 10 to 100 microvolts. These disturbances are shown to be caused by any or all of the following: (1) insulating material in or near electron path; (2) irregularity of filament emission; (3) gas; (4) positive ions emitted by filament; (5) insulating or foreign deposits on grid wires. A tube has been developed in which the above effects are removed or reduced to a point where the disturbances are nearly that of the shot effect of the electrons, as limited by space charge. This allows the amplification of low‐frequency voltages of less than 1 microvolt over the entire frequency band below 100 cycles.