MAGNETIC DATING: THE SPINNING MAGNETOMETER RY M . R. HAROLD

INTRODUC~ION

HERE are various types of apparatus f o r measuring weakly magnetized specimens, many of them being used in the study of rock magnetism. This T type of work auows one to use very bmall samples (about 10 c.c.) and

hence smaller apparatus. Archaeo-magnetic dating. however, involves the measure ment of much larger samples (IOofl c.c. and above) and this fact immediately complicates matters as far as the design of a magnetometer is concerned. One type of apparatus commonly used is the astatic magnetometer; this measures the direction of magnetization by the effect of the specimen's magnetic field on a freely suspended magnet, the etfect of the earth's tield being annulled by a second magnet rigidly attached to i t but more remote from the sample. On the other hand Pro- fessor E. Thellier, of the Institut de Physique du Globe, has developed the use of an inductometer. or spinning magnetometer: here the specimen is continuously rotated inside a set of coils, thereby generating an alternating voltage in the manner of a dynamo. This is the type of apparatus used in the Archaeological Research Laboratory. and the way we employ the dynamo principle in determining the direction of magnetization of a sample will be explained later. The great advantage that a spinning magnetometer has over the astatic type is that it can be used in a normal laboratory in the presence of a relatively large amount of local magnetic interference. It can also accommodate rather larger samples.

Sample

I1 1 I ,Rotor slip-rings

- 3 O f t

I I / ~ kp l , 5a to r b a t t e r y

Moqsl~p

Stator adjustable by hand

Synchronous m o t o r

50 c/sec -

Block diagram of spinning magnetometer used at the Archaeological Research Laboratory.

Reproduced from Physirp m d Archorology by courtesy o f In t r rwence Publishers Ltd.

411 A R C H A E O M E T R Y

THE COILS The apparatus is shown in diagrammatic form in Fig. 1 . The synchronous

motor which spins the table is situated 30ft. away from the coils in order to remove the magnetic effects of the moving metal parts of the engine. Transmission is by means of a nonmagnetic tufnol shaft, connected through a right-angled gear and a vertical dural shaft to the sample table. The speed of rotation is only five times a second, this being considered to be the maximum safe speed at which to spin a large and heavy sample. This, mounted in a box. has to be placed on the table with its centre of gravity accurately aligned with the axis of rotation, in order to avoid excessive shear on the shaft. For this purpose a balance has been constructed with which one may adjust the centre of gravity of a 6Kg. specimen to within 0.5 mm. of the centre of the table.

The coils consist of three coaxial x t s of square windings, each set being a cubic system of six coils. The number o f turns on each coil are such that uniform sensitivity to internal magnetic field change is provided over a maximum volume; in this respect they are similar to Helmholtz coils, but are in fact more economical in space. For smaller samples only the inner two sets are used; for larger samples the inner coil system is removed and the outer two employed. The system is designed so that whichever pair is in use. the two sets are of equal area-turns. but wound in opposition. This has the effect o f eliminating external magnetic fluctua- tions due to passing cars, etc.. but since the sample is nearer to the inner than the outer coils there is a net e.m.f. generated by any magnetic moment rotating within the coils. The size of the coils is dictated by the volume of the objects to be studied, and because they are so large ( I . 2 and 3 feet respectively) they arc wound on square formers, which are much easier to construct than circular ones. The volume over which the sensitivity changes by only I(;!> is 4000 c.c. for the small set, and 24,000 C.C. for the large set of coils.

When deciding on the number of turns to be put on the coils, account had to be taken of the gauge of wire used, the amount of space available for it, its weight (tending to distort the formers) and its total resistance. The object was to obtain an optimum signal-to-noise ratio for any given sample, and the resistance of the two sets of coils turned out to be lo00 ohms and 2300 ohms respectively. Thus for the small coils the minimum detectable magnetic dipole. as deter- mined by the Johnson resistance noise of the coils, and assuming the band- width A f to be I c/s. is I O F e.ni.u. This is equivalent to a 5 c/s alternating magnetic field of lW3 oersted. The micropulsations of the earth’s magnetic field in the 5 c / s region have an amplitude of the order of 1W7 oersted, so that the compensation coils are absolutely essential when measuring weakly magnetized samples. In addition, special precautions must be taken to prevent spurious signals being generated in the coils; these include the protection of all coils from dampness. and a rigid supporting system for the shaft and table which does not transmit the shaft vibrations t o the coils. Furthermore, the shaft is made of dural in order to disperse the very considerable amount of static electricity that can be generated at the bearings. These are of graphite-impregnated nylon.

PHASE MEASUREMENT The direction of magnetization is determined by measuring the phase of the

voltage induced in the coils with respect to the rotation of the shaft. This is done indirectly. but very simply, by allowing the coils to pick up additional voltage of the

A K C H A E O M L I K Y 49

same frequency and amplitude but which can be adjusted to be exactly 180" out of phase. Consequently. at balance there is zero output from the coils.

Thellier accomplishes this hy mounting on the table itself, with the sample, two small horizontal coils at right angles to one another. These are fed with direct current via slip-rings attached t o the shaft, and the magnitude and direction of the current in each is adjusted so that the resultant of the two magnetic moments produced by the coils is exactly equal and opposite to that of the horizontal moment of the sample. The magnitude and direction of the specimen's magnetism are thus calculated in absolute units from the values of the direct currents and the dimen- sions of the coils. This method suHers from the disadvantage that "brush-noise" generated in the sliprings has to be overcome by careful design, and by making the shaft long with the slip-rings at the end remote from the coils. In the system used at Oxford this difliculty is overcome by feeding a stationary auxiliary coil with a 5 c / s alternating voltage. This i s obtained by passing a direct current through the stator of a magslip and tapping off the resultant 5 c / s signal from the rotor, which is attached to the rotating shaft. A simple attenuator serves to vary the amplitude, and rotation of the stator by hand alters the phase.

The induction coils pick up the signals from the sample and from the auxiliary coil and the resultant sinuwidal voltage is fed into a low-noise transistor amplifier. Further amplification allows the signal t o be viewed on a cathode ray oscilloscope. The amplitude and phase of the reference voltage are then adjusted so as to obtain complete cancellation. By inserting a 5 c /s filter in the amplifying chain noise is largely excluded. and for specimens of medium strength (0.1 e m u . ) the phase measurement i s accurate to 0.1". For very weak samples, however, the noise at 5 c j s (external electromagnetic disturbances, amplifier noise, etc.) which cannot be rejected by filtering alone will become indistinguishable from the actual signal, and a different technique must be used. After the normal filtering to get rid of the wide-band noise, the signal is synchronously rectified; this rectitied voltage is then electronically integrated over a period of about a minute. Any noise voltage, which will be random in phase, will tend to integrate to zero, while any signal voltage, which is phased, will on integration build up a D.C. voltage proportional to the time of integration. In principle, no matter what the signal-to-noise ratio is, any desired accuracy can be achieved in this way merely by prolonging the intergration time; the longer this is. the more nearly will the integrated noise tend to zero. However, the practical limit to this observation time would Seem to be about a minute. Again, very few specimens are so weak as to need this elaborate measurement; most kiln samples are 0.1 e m u . or stronger.

This system of phase measurement has the great advantage of allowing the intensity of magnetization, as well as its direction, to be accurately determined. This is most important in view of the measurements shortly to be undertaken in this Laboratory to determine the past intensity of the earth's field, as well as its past direction.