The rapid development of the hot-filament type of cathode-ray tube is outlined, andvarious sources of error which are encountered in its use as a measuring instrument are discussed, including errors inherent to the tube, especially those due to the effect of gas conduction, then errors of manipulation and those due to external influences are reviewed.Recording is next dealt with, including photographic methods, and various methods of calibrating are considered. The chief part of the paper, however, deals with what is known as the “threshold effect”, a quantitative examination of which is given, together with graphs showing its relation to the various variables, such as frequency and the effect of resistance in series with the deflecting plates.Suggestions are put forward as to the cause of the threshold effect, and the paper concludes with recommendations for accurate working.In an Appendix particulars relating to the latest pattern of low-voltage tube are given, and its sensitivity is discussed.