The paper describes a combination of a projection oscillograph, a special amplifier, and a variable condenser coupled to the rocking mirror of the oscillograph, by means of which response curves can be thrown on to a screen for observation or may be recorded photographically.The amplifier may also be used with the oscillograph in the ordinary way for low-frequency observations, the amplifier then functioning as a special form of valve voltmeter giving an output suitable for operating the oscillograph.