MOTIVATING THE DERIVATION In pursuit of an empirically determined value for the electron charge-to-mass e/m ratio we must guide electrons in a variable circular path using a homogeneous magnetic field. The Helmholtz configuration of two current-carrying coils with radius _R_ separated by a distance _d_ as shown in Fig. [fig1] provides the necessary field. To determine the magnetic field $}$ we will first find the magnetic field due to a single current-carrying loop using the Biot-Savart Law.