Fields Making a Difference Travel and Exploration

Airplane travelers all have a first-hand experience with magnetic fields and field measuring devices, called magnetometers. At concourse entrances, airport security personnel require all passengers to pass through a large rectangular security door (Figure 2.1). We can credit this invention to Michael Faraday's (see Figure 1.7) research on the induction of currents in conducting materials. Within the security passageway, an oscillating field is generated, which induces secondary currents in metal objects carried by the passenger. A field sensor within the door compares the phases (see Figure 1.12) of the returned and generated signal oscillations. If (because of induced fields in metal objects) a phase difference is detected, an alarm is sounded. The sensor responds to metal eyeglass frames, keys, coins, watches, knives, guns, etc. The triggering phase-difference measurement is adjusted to eliminate small effects, for example, from rings.

FIGURE 2.1 ► The airport security magnetometer senses the effect of magnetic fields from small electric currents induced to flow in metal objects carried by the passenger.

Hikers, back-country travelers, forest rangers, and small-boat captains all know the value of a map and compass. By universal convention, most maps and charts indicate the geographic north as the direction toward the top of the display. (Although some Australians have merrily rejected their "down under" status by producing a revised world map with south toward the top.) Usually marked at the lower edge of a detailed map is a magnetic declination arrow showing the direction that a compass needle will point toward the magnetic north in that map area (Figure 2.2). Because the declination changes slowly over the years (Figure 2.3), the map date, printed nearby, is also important for the traveler. Ships at sea, from fishing vessels to luxury cruise ships, have a compass as part of their navigation system. In addition to the magnetic northward direction, the special navigation charts also indicate the declination adjustment expected for each year following the chart publication date so that a ship's captain can estimate the correction to be applied when using a compass direction for a voyage. On the modern vessels,satellite-dependent






FIGURE 2.2 ► Magnetic declination, along with the date, is always displayed on the best maps.

1900 1910








1980 1990

FIGURE 2.3 ► The change of declination at Tucson, Arizona, from 1910 to 1986. Figure from the Geomagnetism Section of USGS.

Angle North Star From Stockholm
FIGURE 2.4 ► The North Star's elevation angle above the horizon equals the latitude of the ship at sea.

global positioning systems (see Section 2.1.9, p. 49) have replaced the older navigational aids. Nevertheless, modern electronic systems have been known to occasionally fail, so the stately ship compasses are kept for reliable backup service.

In the years of global exploration the geographic northward direction and ship's latitude were always rather easy to determine using the North Star horizontal direction for northward and that star's elevation angle for latitude (Figure 2.4). Navigators in the Southern Hemisphere, where the North Star cannot be seen, used the Southern Cross constellation for guidance. For years, longitude at sea was poorly determined from estimates of a ship's direction and speed. It wasn't until the production of sea-worthy chronometers after 1757 that a ship's longitude could be found using the midday time of the maximum elevation of the Sun determined with a ship clock that was originally set so its noon hour corresponded to the port clock at the departure longitude.

West Longitude (degrees)

West Longitude (degrees)

Fossils Greenwich

Local hour when it 1« noon st Greenwich

FIGURE 2.5 ► After the invention of an accurate nautical clock in 1757, a ship's location longitude was determined using the timing of the Sun's highest position. In this example, 12 noon was determined while the ship was anchored in port at Greenwich, England.

Local hour when it 1« noon st Greenwich

FIGURE 2.5 ► After the invention of an accurate nautical clock in 1757, a ship's location longitude was determined using the timing of the Sun's highest position. In this example, 12 noon was determined while the ship was anchored in port at Greenwich, England.

For example, on the voyage, if the shipboard measurement of the highest Sun position was observed to occur at 1 pm according to the ship's clock, then the ship's location was 1 hour west of the original port or 15° west because 360° around the world means 24 hours, so 15° equals 1 hour (Figure 2.5). With the development of superior clocks that could accomodate the unstable motion at sea, longitude determinations improved in the time of Captain James Cook's voyages of discovery. Between star sightings and in overcast conditions, navigation directions were set with the ship's magnetic compass together with a chart of the declination values for that region of the ocean. In later years, fine adjustments were added for small variations in the North Star's location and for the seasonal change in Sun's position.

Today, in an age of modern electronic navigation systems, small airplanes, as well as many freighters and passenger ships, still identify their course in magnetic directions from their onboard compasses. By international agreement, airport runways are identified with their eastward magnetic declination value to the nearest degree (Figure 2.6). This method of naming runways accommodates the numerous small planes that fly on compass directions only.

FIGURE 2.6 ► Airplane runway number 87 is named for its magnetic compass alignment, 87° east of geographic north.

For many years, scientists have known that the Earth's field in space influences the behavior of the charged particles populating that distant region and call it the magnetosphere. Field-line navigation in the magnetosphere has found a place in the modern space age. Man-made satellites have to be aimed to properly use their solar panels and to align their Earth sensors and communication systems. In the beginning years of space exploration, satellite alignment was initiated using a magnetic sensor. An onboard device compared the observed and the desired values of the Earth's magnetic field to orient the satellite. Subsequent adjustment with star sensors then trimmed the final position. Although more sophisticated systems are now used for alignment, magnetospheric field measurements are consistently near the top of all lists of exploratory satellite equipment for mapping the main field and investigating the important physics of particle disturbances in space (Figure 2.7).

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