The Model 122A is a dc to 200 kc dual-trace cathode-ray Oscilloscope It has accurately calibrated input sensitivity and sweep-speed ranges. This Oscilloscope features dc-coupled differential Amplifier and dual-trace operation. Differential Amplifier are useful since they reject the common-mode (in-phase) part of the input while amplifying the differential (out-of-phase) part. For instance, they will reject hum pick-up while amplifying the desired signal. In addition, using differential Amplifier has other advantages. Normally dc Amplifier are difficult to keep stable. Changes such as aging and changes of potentials cause this drift. However, these changes are of the common-mode type and are rejected by differential Amplifier Thus differential Amplifier are also used to stabilize dc Amplifier Dual-trace operation is obtained with an electronic switch. This permits observation of two signals at the same time. The two signals may be viewed either alternately during consecutive sweeps or chopped on each sweep. Chopped operation involves switching rapidly between the two signals so that both appear to be traced simultaneously. The most useful type of presentation in each case will depend upon the frequencies involved. Generally, chopped operation is used with low frequency signals. These signals may also be viewed separately or with their difference (B-A) displayed on the screen.