When an acceleration occurs, the mass moves with respect to the anchored ends of the tethers. Roughly speaking, the amount of acceleration is proportional to the amount of displacement of the mass. This is not quite true in this case since the spring system is not an ideal spring as presented earlier. The next problem which needs to be solved is that of measuring the displacement of the bar. The principle upon which this is based is that of the electrical property of capacitance.
Capacitors are electrical components which store charge.
A simple capacitors is formed by placing two metal plates in parallel with each other as shown in Figure 10 . The amount of capacitance that a device such as this would exhibit is exhibit is given by , where k is a property of the material between the two plates. The ADXL50 takes this technique one step further and uses two capacitors configured as in Figure 11 . If the device is at rest, and the spacing between each of the plates is , then each of the capacitors exhibits a capacitance of . The ADXL50 uses a negative feedback control loop to make sure that the movement of the mass is kept small so that the above expression remains correct.