The magnetic field that envelops earth and makes compasses point north has weakened over the last 250 years but it appears to have held steady during the 250 years before that, scientists reported last week.

Experts have been able to measure magnetic field strength only since 1837, when the German scientist and mathematician Carl Friedrich Gauss invented the technique. Before that, compasses could show only the direction of the field. Since Gauss’s day, measurements have shown the magnetic field, which deflects high-speed particles from the sun, quickly weakening, 5 percent per century. At that rate, it would disappear in about 2,000 years.

To infer the field strength of earlier times, scientists look to lava flows and shards of pottery, which preserve a snapshot of the field at the time they hardened. The lava-encased trees at Lava Tree State Park in Hawaii, for instance, tell the field’s strength there in 1790 after a volcanic eruption. But these data are not precise, with errors of 10 percent or more.

David Gubbins, a geophysicist at the University of Leeds in England, has even studied ships’ logs, which recorded the inclination of the field, the angle relative to the surface. The inclination varies based on the location and the strength of the field.

A look at many measurements should allow for a more accurate and global picture, with many of the errors canceling out. In the current issue of the journal Science, Gubbins and his colleagues analysed 315 published estimates from lava and artifacts to show the magnetic field between 1590 and 1840 declined more slowly, 0.75 percent per century. The quicker decay rate began around 1860, Gubbins said, coinciding roughly with the beginning of scientific measurements of the field strength.

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