At the outset it should be emphasized that this article is not concerned with the measurement of distortion; it deals with a method of diagnosis aimed at recognizing, locating and removing the source of any distortion which is found.

The diagnosis is made by examining an oscilloscope trace which represents all the defects of the apparatus which is being tested. By comparing the outline of this picture with certain standard shapes, examples of which are given here, the various sources of distortion can be recognized