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The techniques used to produce magnetic sensors encompass many aspects of physics and electronics. Eleven of the most common technologies used for magnetic field sensing are described and compared. These are search coil, flux-gate, optically pumped, nuclear precession, SQUlD, Hall-effect, magnetoresis tive, magnetodiode, magnetotransistor, fiber optic, and magneto-optic. The usage of these sensors in relation to working with or around €arth's magnetic field is also presented. I. MAGNETIC SENSOR TECHNOLOGIESMagnetic sensors have assisted mankind in analyzing and controlling thousands of functionsfor many decades. Computers have unlimited memorythrough theuseof magnetic sensors in magnetic storage disks and tape drives. Airplanes fly with higher safety standards because of the high reliability of noncontact switching with magnetic sensors.Factories have higher productivity because of the precise stability and low cost of magnetic sensors.There are many ways to sense magnetic fields, most of them based on the intimate connection between magnetic and electric phenomena. In the first half of this paper, the more popular sensor technologies will be described with examples of products. I n the second half, the major applications of magnetic sensors are discussed in relation to three categories: measuring fields stronger than Earth's field, measuring perturbations in Earth's field, and measuring gradients in generated or induced magnetic fields. A common theme among all applications is that magnetic sensors provide a more rugged, reliable, and maintenancefree technology compared t o other sensor technologies.Magnetic sensing techniques exploit a broad range of physics and chemistry disciplines. Eleven of the more common magnetic sensor technologies are listed in Fig. 1, which compares approximate sensitivity ranges. In some cases, projected ranges based on further improvements are indicated by dashes. It is important to note that the sensitivity range for each concept is influenced by the readout electronics. There are many other factors, particularly frequency response, size, and power, that affect what sensor is best suited for an application. A description of each of these concepts follows.

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