switches to sense the rotor's position relative to the field poles. This allowed switching transistors to act as a commutator, but without the contact problems, wear and electrical noise of a traditional DC commutator. The motor design could also be "inverted", placing permanent magnets onto the rotor and switching the field through the external, fixed coils. This avoided the need for any rubbing contact to provide power to the rotor. Such motors were used in low-power long-service-life items, such as computer cooling fans and disk drives. As cheap Hall effect sensors became available, they replaced the reed switches and gave even longer service lifetimes. Reed switches are used in at least one brand of endoscopic capsule to switch on the power source only when the unit is removed from the sterile packaging. Reed switches may be selected for a particular sensor application when a solid-state device does not meet requirements such as power consumption or electrical interface compatibility. Life The mechanical motion of the reeds is below the fatigue limit of the materials, so the reeds do not break due to fatigue. Wear and life are almost entirely dependent on the electrical load's effect on the contacts along with the properties of the specific reed switch used. Contact surface wear occurs only when the switch contacts open or close. Because of this, manufacturers rate life in number of operations rather than hours or years. In general, higher voltages and higher currents cause faster wear and shorter life. Depending on the electrical load, life can be in the range of thousands of operations or billions of operations. See also List of sensors Notes References Further reading External links Glossary of Commonly Used Terms Relating to Reed Switch Products FAQ about Reed Switches Reed Switch Cross Reference Switches Electromagnetic components Soviet inventions