Cathode Ray Deflection Tube, D series. is intended for investigating the deflection of electron beams in electrical and magnetic fields.It can be used to estimate the specific charge of an electron e/m and to determine the electron velocity v. The electron-beam deflection tube comprises an electron gun which emits a narrow focussed ribbon of cathode rays within an evacuated, clear glass bulb. A tungsten "hairpin" filament hot cathode is heated directly and the anode takes the form of a cylinder. The deflection of rays can be achieved electrostatically by means of a built in plate capacitor formed by the pair of deflection plates or magnetically with the help of the Helmholtz coils D magnetically. The cathode rays are intercepted by a flat mica sheet, one side of which is coated with a fluorescent screen and the other side of which is printed with a centimetre graticule so that the path of the electrons can be easily traced. The mica sheet is held at 15 Deg to the axis of the tube by the two deflecting plates.