The design and construction of a low frequency vacuum tube amplifier capable of detecting 5·10−10volt is described. The amplifier is intended to measure the alternating output of a rapid‐response thermocouple in an infra‐red spectrograph, the incident radiation being interrupted at frequencies between 1 and 5 hertz. (1 hertz=1 cycle per second.) The inherent noise level of the amplifier is less than the thermal noise of the thermocouple. This low amplifier noise level is attained by use of an excellent high gain input transformer, by careful selection of the first vacuum tube and operating its heater at low current, by use of inverse feedback, and by use of two power supplies, one for the heaters in series and a second for the plate and screen potentials, both of which have very good electronic voltage regulators. The noise level is further reduced by the use of inverse feedback and twin‐T resistance‐capacity networks to give a narrow and variable band width. The amplifier is operated from the a.c. power line. It feeds a short period galvanometer whose entire swing is recorded photographically so that the record has the appearance of a modulated carrier wave. A portable microammeter fed from a vacuum tube voltmeter is used as a visual monitor of the output. A simple photoelectric test signal generator is described, and typical records show the amplifier noise as well as its output for input signals as low as 6·10−10volt. The effect of too narrow a band width on the recording time of an absorption spectrum is discussed briefly and is illustrated by water vapor spectrum records.