A bridge, composed of resistances in a pair of opposite arms and a capacitance and an inductance in another, each of the latter two having a resistance in series-namely the Hay's bridge-is here described as a new device to measure the frequency of alternating currents. The balance is obtained by adjusting the two resistances in series with capacitance and inductance. Then, the frequency in directly proportional to the resistance in the inductance arm. Thus the bridge has been called as a linear frequency bridge. By properly selecting the circuit constants of the bridge, the frequency may be given directly by the number of ohms of the resistance or any multiple of it. For instance, a bridege with two equal resistances of 594.39 ohms in a pair of opposite arms, a capacitance of 1 micro-farad and an inductance of 100 millihenries, each having an adjustable resistance in series, will give the frequency directly by the number of ohms of resistance in the inductance arm. In appendices, two modifications of bridges, respectively given by M Wien and by Mathes and Cone on described. One may be called, in the case used in the text, as an approximately linear frequency bridge and the other as an approximately linear period bridge.