Density control for photographic enlargers

Claim 1

A CONTROL DEVICE FOR PHOTOGRAPHIC PRINTERS HAVING AN ADJUSTABLE ENLARGER HEAD CARRYING A LIGHT SOURCE AND A LENS, AND OPERABLE TO PRESELECT THE LIGHT FLUX EMANATING FROM SAID LENS TO PRODUCE A PRINT OF PREDETEMINED DENSITY COMPRISING (A) A HOUSING, (B) A MEMBER WITHIN SAID HOUSING ADJUSTABLE BY AN OPERATOR TO PRESELECTED POSITIONS CORRESPONDING TO ENLARGER HEAD LENSES OF DIFFERENT FOCAL LENGHT, (C) TIMER CARRIED BY SAID MEMBER AND ADJUSTABLE TO PRESELECT VARIOUS EXPOSURE TIMES AND PROVIDED WITH A MOVBLE SWITCH-ACTUATING ARM OPERABLE TO CAUSE TERMINATION OF AN EXPOSURE, (D) A POTENTIOMETER CARRIED BY SAID MEMBER HAVING AN ADJUSTABLE CONTACT OPERABLE TO CAUSE SAID POTENTIOMMETER TO PRODUCE VOLTAGES OF A DIFFERENT MAGNITUDE, (E) MEANS OPERATIVELY CONNECTING THE ADJUSTABLE CONTACT OF SAID POTENTIOMETER AND SAID TIMER TO SAID ENLARGER HEAD WHEREBY THE ADJUSTMENT OF THE LATTER SIMULTANEOUSLY MOVES SAID ADJUSTABLE CONTACT AND SAID TIMER SETTING TO CAUSE SAID POTENTIOMETER TO PRODUCE A VOLTAGE IN ACCORDANCE WITH THE MAGNIFICATION RATIO AS SELECTED BY THE ADJUSTMENT OF THE ENLARGER HEAD AND FOR THE TIME PERIOD DETERMINED BY THE SETTING OF SAID TIMER, (F) A SECOND MEMBER AFFIXED TO SAID HOUSING AND ADJUSTABLE BY THE OPERATOR TO PRESELECTED POSITIONS CORRESPONDING TO NEGATIVE AREA, (G) AND AN ADJUSTABLE RHEOSTAT IN SAID HOUSING ELECTRICALLY CONNECTED TO SAID POTENTIOMETER AND PROVIDED WITH A CONTACT OPERABLE BY THE ADJUSTMENT OF SAID SECOND MEMBER TO RENDER THE TOTAL RESISTANCE OF THE ELECTRICAL SYSTEM WHICH INCLUDES SAID POTENTIOMETER AND RHEOSTAT PROPORTIONAL TO THE SQUARE OF THE FOCAL LENGTH DIVIDED BY THE AREA OF THE NEGATIVE AT ALL ADJUSTED POSITIONS OF SAID CONTROL DEVICE.