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A hand-held pipette includes a piston driven by an actuator assembly contained within a housing having first and second mutually spaced non-adjustable stops. The actuator assembly is subdivided into first and second sections respectively provided with first and second contact surfaces. The actuator

A hand-held pipette includes a piston driven by an actuator assembly contained within a housing having first and second mutually spaced non-adjustable stops. The actuator assembly is subdivided into first and second sections respectively provided with first and second contact surfaces. The actuator assembly is arranged to reciprocate between a rest position at which the first contact surface is in contact with the first stop and the second contact surface is spaced from the second stop by a control distance, and an advanced position at which the second contact surface is in contact with the second stop and the first contact surface is spaced from the first stop, with the stroke of the actuator assembly and the stroke of the piston being equal to the control distance. A motor-driven mechanism is arranged to displace one section of the actuator assembly relative to the other section, resulting in a corresponding change to both the overall length of the actuator assembly and the control distance.

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I claim: 1. A hand-held pipette comprising: a housing provided with a chamber and internal mutually spaced first and second stops; a pipette tip in fluid communication with said chamber; a reciprocating piston arranged to coact with said chamber in aspirating and dispensing fluids into and out of s

I claim: 1. A hand-held pipette comprising: a housing provided with a chamber and internal mutually spaced first and second stops; a pipette tip in fluid communication with said chamber; a reciprocating piston arranged to coact with said chamber in aspirating and dispensing fluids into and out of said pipette tip; an actuator assembly for reciprocating said piston, said actuator assembly having an overall length subdivided into first and second sections respectively provided with first and second contact surfaces; at least one resilient member configured to urge said actuator assembly into a rest position at which said first contact surface is in contact with said first stop and said second contact surface is spaced from said second stop by a control distance, said actuator assembly being arranged to reciprocate between said rest position and an advanced position at which said second contact surface is in contact with said second stop and said first contact surface is spaced from said first stop, the stroke of said actuator assembly and the stroke of said piston being equal to said a control distance; and said actuator assembly including an adjustment mechanism configured to displace one section of said actuator assembly in relation to the other section thereof, said displacement resulting in a corresponding change in both the overall length of said actuator assembly and said control distance. 2. The hand-held pipette of claim 1 wherein the second section of said actuator assembly includes a plunger projecting externally from said housing, and wherein the displacement of one section of said actuator assembly with respect to the other section thereof results in a corresponding change in the extent to which said plunger projects from said housing. 3. The hand-held pipette of claim 1 wherein the first section of said actuator assembly is mechanically coupled to said piston. 4. The hand-held pipette of claim 3 wherein the first and second sections of said actuator assembly and said piston are arranged coaxially. 5. Then hand-held pipette of any one of claims 1-4 wherein said adjustment mechanism comprises an electrically powered motor included as an integral component of one of said sections, and wherein said motor is mechanically coupled to an integral component of the other of said sections. 6. The hand-held pipette of claim 5 wherein said electrically powered motor has an output shaft extending along the length of said actuator assembly, said output shaft providing a connection between said first and second sections. 7. The hand-held pipette of claim 1 wherein said first stop is fixed within said housing, and wherein said at least one resilient member comprises at least one first spring for applying a biasing force to said actuator assembly. 8. The handheld pipette of claim 7 wherein said at least one first spring is in tension. 9. The handheld pipette of claims 7 or 8 further comprising a pair of first springs, wherein said pair of first springs are arranged in parallel. 10. The hand-held pipette of claim 7 wherein said housing includes an internal abutment against which said second stop is resiliently urged by a biasing force exerted by a second spring, said actuator assembly being movable beyond said advanced position in response to an increased operating force of a magnitude sufficient to overcome the biasing forces exerted by both said at least one first spring and said second spring. 11. The hand-held pipette of claims 1, 7 or 10 wherein said first and second stops are defined, respectively, by first and second collars surrounding said actuator assembly. 12. The hand-held pipette of claim 6 wherein said motor is connected by a flexible connector to a battery also contained within said housing. 13. The hand-held pipette of claim 1 wherein said adjustment mechanism comprises an electrically powered stepper motor incorporated as an integral component of one of the sections of said actuator assembly. 14. The hand-held pipette of claim 13 further comprising a control for automatically controlling the operation of said stepper motor in response to command signals. 15. The hand-held pipette of claim 14 wherein said control includes a feedback loop configured to generate position signals indicative of each step of rotation of said stepper motor. 16. The hand-held pipette of claim 14 wherein said control includes an encoder wheel rotatably fixed to an output shaft of said stepper motor, and an optical sensor having a mutually aligned light source and photocell arranged on opposite sides of said encoder wheel, said encoder wheel having peripheral teeth circumferentially spaced by slots, said photocell being operative to generate position signals in response to both exposure to said light source via said slots and isolation from said light source by said teeth. 17. The hand-held pipette of claim 16 wherein at least one of said teeth and slots of said encoder wheel is configured to be different in shape in comparison to said other teeth and slots provided on said encoder wheel. 18. The hand-held pipette of claims 16 or 17 wherein the combined total of said teeth and slots is equal to the rotational steps of said motor. 19. The hand-held pipette of claim 2 wherein the plunger of said second section projects from an upper end of said housing, and said chamber projects from a lower end of said housing. 20. The hand-held pipette of claim 1 wherein said chamber, piston and first and second stops are arranged coaxially. 21. The hand-held pipette of claim 1 wherein said actuator assembly is shiftable in response to an operating force applied to the plunger of said second section. 22. The hand-held pipette of claim 1 wherein said adjustment mechanism further includes a key pad for receiving manual input commands. 23. The hand-held pipette of claim 1 wherein said adjustment mechanism further includes a voice recognition unit for receiving audible input commands. 24. A hand-held pipette comprising: a pipette tip in fluid communication with a chamber; a reciprocating piston arranged to coact with said chamber in aspirating and dispensing fluids into and out of said pipette tip; an actuator assembly for reciprocating said piston, said actuator assembly including a first contact surface that is separated from a second contact surface by an actuator length, said actuator length defining an associated stroke distance of said hand-held pipette; and said actuator assembly including an adjustment mechanism and including a motor and configured to displace one of said first and second contact surfaces of said actuator assembly with respect to the other of said first and second contact surfaces of said actuator assembly to change the actuator length and thereby change the stroke distance of said hand-held pipette. 25. The hand-held pipette as claimed in claim 24, wherein said motor is integrally formed with said actuator assembly. 26. The hand-held pipette as claimed in claim 24, wherein said first contact surface is urged against a first stop within a housing of said hand-held pipette in a rest position of the piston, and said second contact surface contacts a second stop within the housing of the hand-held pipette in an actuated position of the piston. 27. The hand-held pipette as claimed in claim 26, wherein said first contact surface is integrally formed with the motor. 28. The hand-held pipette of claim 24 wherein said adjustment mechanism further includes a key pad for receiving manual input commands. 29. The hand-held pipette of claim 24 wherein said adjustment mechanism further includes a voice recognition unit for receiving audible input commands. 30. A method of using a hand-held pipette, said method comprising the steps of: providing a pipette tip in fluid communication with a chamber; reciprocating a piston that is arranged to coact with said chamber in aspirating and dispensing fluids into and out of said pipette tip; providing an actuator assembly for reciprocating said piston, said actuator assembly including a first contact surface that is separated from a second contact surface by an actuator length, said actuator length defining an associated stroke distance of said hand-held pipette; and activating a motor to displace one of said first and second contact surfaces of said actuator assembly with respect to the other of said first and second contact surfaces of said actuator assembly to change the actuator length from a first actuator length to a second actuator length and thereby change the stroke distance of said hand-held pipette from a first stroke distance to a second stroke distance. 31. The hand-held pipette of claim 5 wherein said electrically powered motor is operable in response to command signals input manually on a key pad located externally on said housing. 32. The hand-held pipette of claim 31 wherein said electrically powered motor is also operable in response to audible commands received via a microphone and processed by a voice recognition system embodied in a microprocessor.