Modern Field Recording

To simplify the field detection system, Gauss attached a mirror to the magnetic pointer needle axis. A ray of light directed toward the mirror cast a spot onto a far wall to allow an easier deflection measurement. It wasn't until the development of photography in the early nineteenth century that the Gauss magnetometer became a modern self-recording device. With the improved instrument in a darkened room, the light spot deflected by the mirror was directed to photographic paper attached to a drum, which rotated once a day, capturing a continuous record of the daily change in field (Figure 5.2). By applying special torsional biases and separate axis suspensions, the instruments could measure field changes separately in the magnetic northward, eastward, and vertical directions. These magnetometers were given the more specific

Light Source

Light Source

FIGURE 5.2 ► The simple variometer, developed by the mid-nineteenth century, consists of a light beam focused on a mirror attached to the suspension of a bar magnet. The light beam, moving with the magnet, shines on a slowly turning drum covered by photographic paper. A second light beam is reflected from a stationary mirror to inscribe a simultaneous baseline on the photopaper.

FIGURE 5.2 ► The simple variometer, developed by the mid-nineteenth century, consists of a light beam focused on a mirror attached to the suspension of a bar magnet. The light beam, moving with the magnet, shines on a slowly turning drum covered by photographic paper. A second light beam is reflected from a stationary mirror to inscribe a simultaneous baseline on the photopaper.

name variometers to indicate that only the variations in field were recorded, not the full main field strength.

The exact strength of the Earth's main field, whose force tugs the compass magnetic needle northward, could be obtained from a knowledge of the oscillation period resulting from the restoring force on the needle after it is manually pushed away from its rest position. Notice how your home compass needle, after being moved, oscillates before coming to rest northward. The strength of the main field causing this oscillation varies inversely with the square of the oscillation period. Scientists can determine the Earth's field strength by comparing the period of oscillation of a compass needle in the Earth's field with the oscillation measured at a fixed distance from a calibrated magnet. Records of compass needle oscillations were used by Baron Alexander von Humbolt to determine the Earth's field strength in his 17991803 surveys of the American continents. He made the significant discovery that the main field magnetic intensity decreased toward the equator.

Today, many other systems are used to record the field. Proton magnetometers use the aligned precession of the spinning hydrogen atom nucleus to find the total main field strength. Fluxgate magnetometers use the distortion properties of saturated fields in special magnetic material. Rubidium optically pumped magnetometers use special unique atomic energy-level light stimulation and emission properties. Cryogenic magnetometers (Figure 5.3) use the unusual quantum-wave conditions occurring in materials near absolute zero temperatures (-273° C or -460° F).

FIGURE 5.3 ► Modern, but complex SQUID (super-conducting quantum interference device) magnetometer, which uses some unique properties of quantum physics to detect minute magnetic fields as small as 0.00001 gamma. The niobium metal sensor, which is only approximately 1.5 cm (0.6 inches) in diameter, is immersed in liquid helium for cryogenic cooling. Figure from J. Zimmerman of NIST.

FIGURE 5.3 ► Modern, but complex SQUID (super-conducting quantum interference device) magnetometer, which uses some unique properties of quantum physics to detect minute magnetic fields as small as 0.00001 gamma. The niobium metal sensor, which is only approximately 1.5 cm (0.6 inches) in diameter, is immersed in liquid helium for cryogenic cooling. Figure from J. Zimmerman of NIST.

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