Narrow-beam tube

1. PURPOSE AND DESCRIPTION The narrow beam tube 06950.00 is used in conjunction with the pair of Helmholtz coils 06960.00 to study the behaviour of an electron beam in a magnetic field normal to the electron trajectory, which is made visible by the fluorescence of the insert gas with which the tube is filled. The tube has an axial scale shaped like a ladder, whose metal 'rungs' are given a coat of luminous paint to enable the diameter of the electron trajectory to be measured accurately in order to determine the specific charge e/m of an electron. After evacuation, the tube is filled with argon at a pressure of about 10-I Pa mbar). This residual pressure of inert gas plays an important role in the focusing of the electron beam. Along their trajectory the electrons collide with gas molecules and ionise them, so that the beam is made visible by the luminescence produced by the recombination of the argon. Since, on the other hand, only the secondary ions arising escape from the beam during ionisation, the sluggish positive argon ions form a thread along the path of the electron beam, which as a result of electrostatic forces of attraction makes an important contribution to the sharp focusing of the beam. even over large distances. Each of the similar Helmholtz coils is cantilevered off a plastic foot. The coils are positioned by three spacer rails so that their axial spacing is equal to the average coil radius. (The rails can be removed after undoing knurled screws; allowing the coils to be used individually). Two fo these rails are provided with a clip holding the narrow-beam tube in the centre of the pair of coils. The sockets of the coil winding are cast into the plastic foot of the coil; and the connecting leads can be used to connect the coils in parallel or series as required. The sockets are numbered (1 ; 2) to make it easier to wire the coils. Fig. 2: Winding arrangement of the Helmholtz coils