When observing voltages in high impedance circuits by means of an ordinary cathode‐ray oscilloscope, one often encounters disturbance of circuit conditions by the relatively low impedance to ground of one of the oscilloscope amplifier input terminals. The authors present a coupling circuit for eliminating this difficulty in which the voltage to be observed modulates an oscillator isolated from ground. The resulting output is loosely coupled to a detector built into the oscilloscope. The paper shows that the system not only fulfills its purpose and indicates wave forms correctly, but is also economical and easily adjusted.