A common mistake in biomagnetic experimentation is the assumption that Helmholtz coils provide uniform magnetic fields; this is true only for a limited volume at their center. Substantial improvements on this design have been made during the past 140 years with systems of three, four, and five coils. Numerical comparisons of the field uniformity generated by these designs are made here, along with a table of construction details and recommendations for their use in experiments in which large volumes of uniform intensity magnetic exposures are needed. Double-wrapping, or systems of bifilar windings, can also help control for the non-magnetic effects of the electric coils used in many experiments. In this design, each coil is wrapped in parallel with two separate, adjacent strands of copper wire, rather than the single strand used normally. If currents are flowing in antiparallel directions, the magnetic fields generated by each strand will cancel and yield virtually no external magnetic field, whereas parallel currents will yield an external field. Both cases will produce similar non-magnetic effects of ohmic heating, and simple measures can reduce the small vibration and electric field differences. Control experiments can then be designed such that the only major difference between treated and untreated groups is the presence or absence of the magnetic field. Double-wrapped coils also facilitate the use of truly double-blind protocol, as the same apparatus can be used either for experimental or control groups.

Uniform magnetic fields and double-wrapped coil systems: Improved techniques for the design of bioelectromagnetic experiments

Magnetite, the only known biogenic material with ferromagnetic properties, has been identified as a biochemical precipitate in three of the five kingdoms of living organisms, with a fossil record that now extends back nearly 2 billion years. In the magnetotactic bacteria, protoctists, and fish, single-domain crystals of magnetite are arranged in membrane-bound linear structures called magnetosomes, which function as biological bar magnets. Magnetosomes in all three of these groups bear an overall structural similarity to each other, which includes alignment of the individual crystallographic [111] directions parallel to the long axis. Although the magnetosomes represent only a small volume fraction in higher organisms, enough of these highly energetic structures are present to provide sensitivity to extremely small fluctuations and gradients in the background geomagnetic field. Previous experiments with elasmobranch fish are reexamined to test the hypothesis that gradients played a role in their successful geomagnetic conditioning, and a variety of four-turn coil designs are considered that could be used to test the various hypotheses proposed for them.

Magnetite biomineralization and geomagnetic sensitivity in higher animals: An update and recommendations for future study

Seven new primary poles upgrade the apparent polar wander path for northern Australia for the period 1730–1590 Ma. This brings the total number of primary poles to sixteen, of which nine have well‐defined SHRIMP U–Pb zircon ages. The upgraded path provides a tool for dating and correlation of tectonic events as well as fluid migration episodes for one of the most important periods of base‐metal mineralisation in Australia. Other new primary and overprint poles allow the path to be extended tentatively forward in time to ca 1500 Ma and back to ca 1770 Ma. The 1730–1590 Ma path comprises five quasilinear segments or tracks, each terminated by a sharp bend. The bends can be related to major (supersequence) boundaries in the northern Australian superbasins and to the initiation of sub‐basins that host the base‐metal deposits. Most bends are also associated with magnetic overprints related to episodes of fluid circulation, and three appear to be associated with the expulsion of metal‐rich brines that formed th...

Towards a high resolution Late Palaeoproterozoic – earliest Mesoproterozoic apparent polar wander path for northern Australia

WE have carried out the first archaeomagnetic investigations of aboriginal fireplaces in Australia. Evidence of human occupation in Australia extends back to 25,000 to 32,000 BP, and is based on the remains of a human cremation found in the ancient lakeshore dune of Lake Mungo1, a dried out lake in western New South Wales (33.7° S, 143° E). We report here a study of some ancient fireplaces from an equivalent horizon around Lake Mungo; these fireplaces have preserved a record of a recent excursion of the Earth's magnetic field.

Evidence of a geomagnetic excursion 30,000 yr BP.

Archaeomagnetic studies have been made of prehistoric aboriginal fireplaces occurring along the ancient shore of Lake Mungo, a dried out lake in southeastern Australia. Directions of magnetization preserved in ovenstones and baked hearths show that wide departures of up to 120° from the axial dipole field direction occurred about 30000 years ago. The determination of the variation in geomagnetic field strength from the baked material is complicated by non-ideal behaviour during Thellier’s double heating method. The problem appears to arise from the subsequent (postfiring) formation of iron oxyhydroxides during a period in which the water level in the lake rose. During laboratory heatings these oxyhydroxides dehydrate causing the non-ideal behaviour observed. The ancient field strengths deduced are therefore probably minimum values. The geomagnetic excursion recorded between at least 30780 ±520 and 28140 ±370 years b.p. on the conventional radiocarbon time scale is associated with very high field strengths between 1 and 2 Oe (1 Oe » 79.6 A m_ 1 ). The field strength subsequently decreased to between 0.2 and 0.3 Oe after the excursion. This main excursion is referred to as the Lake Mungo geomagnetic excursion. There is evidence that a second excursion associated with low field strengths of 0.1-0.2 Oe occurred around 26000 years b.p. A review of geomagnetic excursions less than 40 000 years in age shows that it may be premature to assume that these are world-wide synchronous features. The range of ages and their groupings in different parts of the world may indicate they are temporary non-dipole features of continental extent. However, the duration of most excursions (order of 103 years) is very similar to that of polarity transitions and this could indicate they are aborted reversals.

The Lake Mungo geomagnetic excursion

Three orthogonal coil systems have been constructed within a cube of side 2.5 m to cancel the geomagnetic field to less than 1 gamma over a large volume. The north-south and vertical components are cancelled by systems of four square coils whilst a square Helmholtz pair cancels the east-west component. Cancellation to 1 part in 105 (0.5 gamma in Canberra) is achieved within a sphere of diameter 35 cm, more than 20 times the field-free volume which could be obtained using conventional Helmholtz coils within the available space. Three fluxgate detectors monitor the residual field and their output, taken through a feedback circuit, allows compensation for field variations. The residual field can be maintained to less than 0.5 gamma over the working volume of about 22,000 cm3 during the critical part of a laboratory experiment.

G. R. Luck

Papers

A large volume magnetic field free space for thermal demagnetization and other experiments in palaeomagnetism