An amplifier which is based on the scheme of heterodyning the signal to be measured with a locking‐in voltage of the same frequency, as proposed originally by Cosens, is described and its characteristics are discussed. The particular design used, with the locking‐in voltage applied to the screen grids of two pentodes, possesses a distinct advantage over earlier instruments because the tubes operate on a linear portion of their transfer characteristics, reducing harmonic response, and also because the main power supply is direct current, so that there is little drain on the control supply. It is found that the response band can be given a width of the order of one cycle per second without undue inconvenience or expense.