A measurement of the radius r of the circle that the beam follows then determines the charge to mass ratio of the particles that make up the beam. That means that this experiment measures a basic property of the electron, one of the fundamental particles of nature. In the experiment we will do, the magnetic field is generated by a pair of Helmholtz coils. You have already found that in such a pair the field between the coils is nearly uniform if they are separated by their own radius and the current through them is such that the field they generate is the the same direction. overall By, over BX1 0.9 0.8 0.7 Coil 1 Coil 2 0.6 Magnetic Field Strenght B/Bo 0.5 0.4 0.3 0.2 0.1 -1.5 0.5 0 0.5 1.5 R R R B = (4/5)3- MO N IIR