It is sometimes necessary to create a rotating magnetic field in a pre-defined region of space. This is typically to cause movement of an element within the defined region-e. g. a motor-or to cyclically change the magnetisation state of an element within the defined region in order to allow some characteristic of the element to be explore. Certain remote magnetic sensing applications fall into this second category.

An example of such remote magnetic sensing is described in patent application No. GB 9625561.7 filed 9th December 1996 and in International Patent Application No. PCT/GB97/03389 (WO 98/26312). In this system a rotating magnetic field is used to interrogate a coded marker tag made which carries a radial disposition of high permeability strips or wires. The magnetic field strength is such that each strip or wire only comes out of magnetic saturation for a small range of field angles, during which a subsidiary a. c. field excites the strip, causing harmonic re-radiation. Using a suitable antenna coil and tuned receiver, the harmonics can be detected at a distance. By synchronising the received signals with the field rotation, the angular placement of the strips or wires within the tag can be determined. A simple coding scheme relating strip angles to a data sequence enables a low-cost data passive tag to be implemented.

Another example is described in patent application No.

GB 9717574.9 whera a rotating field combine with a lower amplitude high frequency field is used to interrogate a marker tag attache to a surgical catheter. In this case the single high-permeability element in the marker comes out of saturation when the rotating field is orthogonal to its"easy"axis, i. e. its axis of easy magnetisation. At this point the marker is excited to produce harmonics by the low amplitude high frequency field, and re-radiated harmonics can be detected at a distance. Since the angle of the field is known, the marker orientation can be easily reduced.

A rotating magnetic field may be generated by driving alternating currents though appropriately positioned coils. The principle is well-known and widely employed. For example, if a pair of coils are set with their axes at 90 degrees to each other, and the coils are driven with currents L. sin (ot and L. coswt, then in the region where the field from the two coils overlap the resultant field direction will rotate with a rotation frequency.

While this technique is simple to employ, it becomes inefficient or expensive when significant field levels are required. This is because the coils used to create the orthogonal fields have losses due to finite electrical resistance. If the coils use fine wire, they have high resistance, and dissipate power uselessly as heat; if the coils use thick wire they are bulky and costly.

The invention makes use of a rotating permanent magnet to create the main rotating magnetic field. The magnet can itself be rotated by a low-amplitude rotating field created by the prior art method, the field created by

the permanent magnet easily being ten or more times stronger than that required for its rotation. For a well balance magnet, with low friction bearings, the additional power required to rotate the magnetic is only that necessary to overcome air resistance, and is thus very low.

Accordingly, one aspect of the present invention provides a device for generating a magnetic field characterised in that the device comprises a permanent magnet, for example a bar magnet, mounted for rotation and dispose within the windings of a coil.

Preferably, the magnet is a bar magnet mounted so as to be able to rotate with low friction; the magnet may, for example, be mounted on a shaft which passes through the centre of the magnet; or it may be supporte between a pair of opposed pivot points; or it may be supporte in a low friction bearing.

The magnet is preferably a high-field alloy magnet, for example a Ne-Fe-B (neodymium/iron/boron) magnet. The magnet is preferably mounted within a hollow casing which also serves as a former for the coil windings.

There are preferably two sets of orthogonally dispose coil windings carried by the casing.

A typical arrangement for producing a rotating magnet field at a distance from the generating source will now be described, by way of example, with reference to Figure 1. In this example the permanent magnet 1 is mounted on a shaft 6 which is inside a pair of orthogonal solenoid coils 2 and 3 wound on a rectangular former 4 with sides 16 cm x 16 cm x 8 cm.

The shaft 6 is pivotally mounted inside former 4 for low friction rotation.

Using an arrangement as shown in Fig. 1 to generate a rotating magnetic field at a frequency of 16 Hz, and using a bar magnet five cm long made from Ne-Fe-B, we have found that the electrical power required to produce a field level of one gauss at an external distance of ten cm from the surface of the coils was less than 10% that required without the magnet present.

In order for rotation of the magnet to commence, it is necessary to provide an initial impulse. Since the drive currents for coils 2 and 3 can be independently controlled, this impulse can easily be effected by an appropriate start-up sequence of drive currents to the different excitation coils.