Ferrofluids are suspensions of ferromagnetic particles, which behave macroscopically as strongly magnetizable fluids. It is well known that a magnetic field can move magneti- zable fluids, especially ferrofluids, cf R. E. Rosensweig, 1985, Ferrohydrodynamics (Cambridge University Press); R. E. Rosensweig, J. Poppelwell and R. J. Johnston, 1990, J. Mag. Mag. Mat. 85,171; and Mario Liu, Physical Review Letters 74,4535, 1995.

US-A-4, 808,079 (issued 28 Feb 1989) discloses a magnetic pump for ferrofluids and a number of advantages connected to that magnetic pump. It is also disclosed that travelling magnetic field gradients are capable of moving magnetizable fluids. However, travelling magnetic fields are hard to realize, and need to be produced by numerous coils. This renders the structure of the device and its control rather complicated, and severely confines its possible geometry.

The objective of the invention is to simplify the action of pumping and moving magneti- zable fluids.

This objective is achieved by only partially filling the vessel with the magnetizable fluid, leaving a free surface, and by the magnetic field, which possesses both a normal and a perpendicular component to the free surface, at least one of those components being time dependent.

This field exerts a force, or a pressure head, of the magnitude (ale) x B. x dB/dt,

where i is magnetic permeability of the vacuum, and a a dissipative Onsager coefficient [cf. Mario Liu, Physical Review Letters 74,4535,1995], of the order a = x x T, where X is the magnetic susceptibility and i the relaxation time of the magnetization.

A more detailed consideration, including the off-diagonal dissipative Onsager coefficients, yields (ale) x (B. x dB/dt-B. x dBn/dt) as the force.

To achieve continuous pumping in a given direction, the time dependence of the above expression for the force needs to be either always positive or always negative. Because Bn and Bt cannot increase without bounds, a periodic time dependence must be adopted for them.

Preferably the magnetic field is spatially uniform, at least in the region of the free surface of the magnetizable fluid.

One example for the magnetic field is B. = B x cos cot, Bt = B x sin cat, which corresponds to a magnetic field rotating around an axis that lies in the free surface of the magnetizable fluid. For this special field, the pumping is proportional to the square of the field magnitude and hence time independent.

In its simplest form, the pump possesses one coil (oblique to said free surface) or two perpendicular coils, all fed with electric currents to produce a time dependent magnetic field. A vessel positioned within the field is partially filled with a magnetizable fluid. The rest may be empty or filled with a nonmagnetic fluid. The coil or coils are oriented such

that there is a Bt, tangential to the surface of the magnetizable fluid, and a B perpendicular to it. These components of the field may be generated with one or more coils.

The vessel contains at least two connecting tubes submerged in the magnetizable fluid.

Both lines are equivalent, may even be equal. It is the time dependence of the magnetic field which determines which of the tube is the feed line, and which the outlet. The time dependent magnetic field moves the magnetizable fluid, sucks it from the feed line and pushes it into the outlet. Reversing the time dependence of the magnetic fieid, the roles of the two tubes are also reversed, and the fluid moves backwards. Valves are not necessary but may still be employed.

The advantages of the invention are especially: -Low wear and tear: There are no moving parts, little wear and tear, and no need for frequent maintenance.

-Easy control : The pumping force can be exactly administered, as it is controlled by varying the electric current. The force may be applied in either direction, forwards or backwards. The time needed to alter an action is the brief time of an electrical switch, and there is no inertia of moving parts to contend with.

-Low costs: The very simple structure implies low prizes. And it invites miniaturiza- tion, or the construction of cascades of pumps.

Possible applications of the invention are: -Maintenance-free and reliable cooling devices to be employed in less accessible or dangerous locations such as nuclear reactors or satellites. (The cooling and fuel supply in satellites are reputed to have in fact been the motivation for the development of ferrofluids. Unfortunately, no efficient means were then found to pump ferrofluids.) -Remote-controlled, miniaturized hydraulics, switched by electric currents.

-Completely enclose (and hence clean) ferrofluid circuits, which may administer medicine to specific areas of the body.

An example on how to realize the invention is rendered in two figures, and considered in greater details below.

Fig 1 shows schematically a device for magnetic pumping.

Fig 2 shows the time dependent variation of the tangential and normal component of the magnetic field.

As shown in Fig 1, the device consists of a vessel 10, which has two connecting tubes, 12 and 14, situated opposite to each other. The vessel 10 is not completely filled with ferrofluid FF, leaving a free surface 16.

The current carrying coil or coils are not rendered in Fig 1. They produce a magnetic field with two components: B is normal to the free surface 16 of the ferrofluid FF, and Bt is parallel to the free surface and along the connecting line between the two connecting tubes, 12 and 14.

It is practical to have two specialized coils, each generating one of the components, Bn and Bt. The field producing coils may be in-or outside the vessel 10. If outside, the material of the vessel need to be nonmagnetic.

The pressure head created in the ferrofluid (FF) is of the order of (B. x dB/dt). Since the B, cannot grow without bounds, its temporal derivative needs to change sign from time to time. A steady pumping may nevertheless be achieved by switching the sign of Bn at the same time. The same holds for (Bt x dBn/dt), where Bn and Bt have reversed roles.

Together, both terms generate the pressure head that is proportional to (Bn x dBt/dt Bt x dB./dt Fig 2 shows an example of steady magnetic pumping, from the combined effect of both terms. Both components of the magnetic field vary sinusoidally, with a phase lag of Ti/4,

such that the maxima of B coincide with points of strongest ascent of B.. The time dependence rendered in Fig 2 represents a rotating field, with B. oc cos cot, Bt oc sin c3t, The axis of rotation lies in the free surface of the magnetizable fluid and along the connecting line between the two connecting tubes, 12 and 14. For the above field, the pumping is time independent. If the phase lag is reversed, the sense of rotation and the sign of the pumping are also reversed.

Should one aim to achieve an oscillating or pulsating pumping, one may combine a static B with an oscillating Bt, or vice versa, a static Bt with an oscillating B.. A valve in one or both connecting tubes will suffice to maintain the unidirectional pumping.