Force measurements on magnetic reeds

With recent trends towards miniaturization, the magnetic reed relay or dry reed switch has become so small that force measurement by ordinary mechanical methods is extremely difficult. However, knowledge of such forces enables the prediction and control of opening and closing currents, contact forces, and general behavior of the device. An experimental method is given of measuring the quasi-static magnetic attractive force between two bodies, however small, providing that an appropriate flux measurement can be made. The magnetic reed relay is treated as a specific example to demonstrate the utility of the measurement. The reed relay is defined as a closed thermodynamic system. A free energy is derived for this system and it is shown that the magnetic attractive force may be obtained from this free energy even when magnetic hysteresis is present. The use of the measurement in studying reed relay behavior is explained, and typical experimental findings are presented.