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Fine beam tube or Double beam tube – measurement of e/m for the electron This tube is different from the deflection tube in that it is filled with low pressure helium gas. The advantage of this is that you can see the path of the beam of electrons since they ionise the gas which then glows as the electrons collide with helium atoms. The greater the potential difference between the anode and the cathode the further the electrons will travel through the gas before they lose their energy and are brought to rest. Put more exactly the range of the beam can be shown to be proportional to the accelerating voltage and therefore to the electron's energy. Tangential electron gun Low pressure helium gas Radial electron gun Figure 1 Magnetic field perpendicular to the figure Helmholtz coils There are also two electron guns, one firing the beam tangentially and the other radially through the tube (See Figure 1). For the tangential gun ½ mv2 = eV and Bev = mv2/R where v is the electron velocity. So combining these two equations: Double beam tube: e/m = 2V/B2R2