shown in fig1 is a portion of a sea body of water , between bottom a and surface b , waves c , forming peaks c 1 and valleys c 2 . it is the deferential hydrostatic pressure prevailing near the bottom a ( or anywhere else below the surface b ), namely the head balance ( h 1 - h 2 ) that is harnessed to produce useful energy according to the principles to be explained below . the energy generator generally denoted 10 , is comprised of a barrel - shaped casing 12 , with circular side wall 12 a , bottom 12 b and cover 14 . the generator 10 is self - contained in the sense that it works in a closed loop and need not to cooperate with any other system ; the functions of certain sub - systems are automatically controlled by feedback from relating other sub - systems , as will be seen later on . this stand - alone feature of the generator is regarded as one of the uniques of the present invention . furthermore , the generator 10 need not to rest on the sea bottom a , but can be held in buoyancy thereabove using suitable anchoring means ( not shown ). as further schematically seen in fig1 the cover 14 of the casing 12 is made as a membrane 16 , namely a rigid plate connected to the side wall 12 a of the casing 12 intermediate a yeildable sheet 16 a , thus allowing the displacement of the plate 16 a up and down following changes in the differential pressures applied thereto ( between the interior and the outer pressures as will be explained in detail below ). the general sub - systems of the generator 10 will be now identified for better understanding of the more detailed description given in conjunction with fig2 ; these include : turbine 24 drivingly coupled to electrical power generator 26 ; liquid supply source 28 for varying the volume of the cylinder of system 20 ; and interim fluid ( air ) supply vessel 30 , for supplying the cylinder of system 18 , and connected via conduit 32 to turbine 24 discharge port , thus closing the working loop of the fluid . referring now for more details to fig2 it should be first noted that in order to function properly , the pressure prevailing within the casing 12 of the generator 10 , denoted pg , must always be kept less than the minimum hydrostatic pressure applied by the waves b . namely under the water head h 2 ; otherwise , the membrane cover 16 will not respond to the differential pressure ( h 1 - h 2 ), i . e . become displaced up and down as desired . therefore , outer pressure gauge 40 and inner pressure gauge 42 are provided for constantly measuring these pressures , and to govern the operation of an electrically operated , reversible suction pump 44 for lowering the internal pressure ; relief valve 46 is associated with the high pressure vessel 22 for increasing the internal pressure , as the case may be . a central computerized unit cpu is included , which controls the various operational parameters of the generator sub - systems as will be explained below . the first cylinder - and - piston system 18 comprises cylinder 50 and piston 52 with piston rod 54 , which extends upwards where it is rigidly connected to the plate 16 , as well as downwards out of the cylinder 50 , where it becomes the rod of piston 56 of the second cylinder - and - piston system 20 , provided with cylinder 58 . thus defined are upper and lower effective spaces denoted s 1 and s 2 . the system 18 acts as a double - stroke air pump , compressing air supplied from vessel 30 to vessel 22 . there are provided two unidirectional inlets 60 , 62 connected to the vessel 30 on the one hand , and two unidirectional outlets 64 , 66 , leading to the vessel 22 , as shown . reciprocation of the piston 52 in either direction will therefore pressurize air into the vessel 22 . referring to the second cylinder - and - piston system 20 , it will be noted that the stroke of the piston 52 is opposed by that of the piston 56 , both being mounted to a common rod 54 . the cylinder 58 is of a variable effective volume ( space s 3 ), achieved by filling it partly , to a controlled amount , with liquid 70 , such as oil , through pump 72 from container 28 . the variable space s 3 within the cylinder 58 underneath the piston 56 is also adapted to be charged with pre - determined , variable pressure to be supplied from pressurized air vessel 22 via control valve 80 and pressure regulator 82 . the turbine ( or air motor ) 24 is operated by the pressurized air stored in vessel 22 , via control valve 84 and pressure regulator 86 . as already explained , the discharge port of the turbine 24 is connected by conduit 32 and control valve 88 to the interim air supply vessel 30 . finally , a rechargeable battery 90 recharged by the generator 26 ( through voltage regulator 92 ) is included for supplying electric power to operate the cpu , the suction pump 44 , the oil pump 72 and all the control valves and other devices as apparent from the foregoing description . the operation of the generator 10 proceeds as follows . as already mentioned , the internal pressures pg is pre - set and maintained to a value less to a certain extent , than the value of the external hydrostatic pressure to which the membrane cover 16 is subjected . otherwise , should the internal pressure exceed the external pressure , the membrane cover would not respond to , i . e . become displaced downwards under the external pressure represented by the water head h 1 ; and if the internal pressure is too low , again the membrane 16 will not function , but remain stationary at its lowermost position , irrespective of a reduced water head h 2 . regulation of the pressures is maintained by the suction pump 44 , or the relief valve 46 ( controlled by pressure gauges 40 , 42 ), in accordance with the actual working conditions , taking into account , among other parameters , the height of the sea waves b at any given time . supposing that the internal pressure has been properly adjusted , the piston 52 is at its uppermost position , and the generator is first subjected to the increased hydrostatic pressure proportional to a wave peak c 1 , then , under such elevated pressure the membrane cover 10 will descend . the piston will move down and a quantity of air ( space s 2 ) will be compressed into the vessel 22 . simultaneously , a pressure will be built - up in the space s 3 . this counter - pressure is essential in order to achieve the upwards stroke of the piston 52 along with the lifting of the membrane cover 16 , after the wave peak c 1 has passed away and a lower hydrostatic pressure , related to head h 2 , prevails . the appropriate adjustment of the counter pressure , which is of major importance for starting and maintaining the cyclic operation of the unit , is achieved by adjusting at least one of the following variants : changing the effective volume s 3 , and / or charging extra pressure thereinto . the first variant is accomplished in the present example by the filling / evacuating the oil 70 into / out of the lower part of the cylinder 58 ; and the second variant is adjusted by partly directing compressed air from the vessel 22 , though pressure regulated valve 82 into the space s 3 . the combination of the two variants , each being individually controllable by the cpu , along with suitable calculation of the area of the piston 56 relative to that of the piston 52 ensure the availability of a wide range of changeable factors required for achieving the desired result , namely , effectively , lifting the piston 56 when a relative relief of force is sensed by the membrane 16 , caused by the decreased hydrostatic pressure h 2 , with minimum loss of energy . as the compression cycles continue , the pressure will be built - up within the vessel 22 . upon reaching a level sufficiently high , pressure regulator 86 , by a command of the cpu , will open and the compressed air will drive the turbine 24 for as long as the pressure remains effective for that purpose . again controlled by the cpu , the valve 84 will close and a new cycle will be started . low pressure air is directed through conduit 32 from the outlet port of the turbine 24 to the vessel 30 , and therefrom to the cylinder 50 . the air is therefore recycled in a closed working loop ( except for a portion either expelled from the relief valve 46 , or sucked by suction pump 44 — as already mentioned above ). it goes without saying that a plurality of generators as herein described , are readily adapted to work in parallel , thus compensating for the inherent operating pauses of each one of them . once installed , no maintenance or servicing is requested for a long period ( say , for replacing the batteries 90 ). it is thus suitable for use along coasts of deserted areas , where the supply of conventionally produced electricity is too expensive . those skilled in the art will readily understand that various changes , modifications and variations may be applied to the invention as above exemplified without departing from the scope of the invention as defined in and by the appended claims .