u,,, ,, Resler 



I would like to discuss now some variations of the propulsion system which 

 would allow smaller fields. In the preceding discussion we have assumed that 

 the magnetic field must necessarily act on the sea water and also that the elec- 

 tric field came from a battery-type source or possibly a generator. We also 

 assumed that we would use electrodes, always a source of possible difficulty in 

 MHD-type devices. 



Consider first the possibility of avoiding the electrodes. To operate without 

 electrodes the currents would have to close in the fluid, but then the electric 

 field would have to be induced in the fluid. K the fluid flow was in an annulus 

 the electric field could be induced by a coil and the fluid in the annulus would 

 act like the secondary of a transformer. More appropriate for our purposes, 

 consider a coil surrounding an annulus, a conducting fluid in the annulus, and a 

 single-turn coil moving with speed Vp, carrying a current and thus producing a 

 magnetic field, as depicted in Fig. 3. The radial component of the magnetic 

 field B passing through the fluid at a relative velocity Vp - u will induce a cir- 

 cumferential electric field proportional to B and the relative velocity. So in 

 this case v/s in our formula is 



V/S « (Vp - U) B . , , ,-v . (20) 



Fig. 3 - Magnetic field of a 

 conducting fluid in an annulus 



The moving magnetic field can be likened to a screen that is dragged 

 through the fluid. The higher the conductivity of the fluid the less porous the 

 screen. The field tends to drag the fluid with it, and the interaction is the same 

 as already described except for the reinterpretation of the electric field term. 

 Of course, it is possible to energize a solenoidal winding electrically so that a 

 magnetic field configuration will travel along the winding, thus making it un- 

 necessary to move the coil physically. In this case, Vp would be the phase ve- 

 locity of the electromagnetic configuration along the winding. 



To propagate magnetic fields of the size of 20,000 gauss and larger along 

 the coil is not at all practical, so that to use this scheme a fluid of larger con- 

 ductivity is required. Consider for a moment other fluids such as the liquid 

 metals which possess conductivities from 10'' to 10^ mho/cm. K a- were 10'* 

 in our example instead of 0.045, the required field would be reduced by 



1442 



