Patent GB-553593-A (GB)
Title: Polarograph
IPC: G01N27/48
Filed: 1942-03-05
Abstract: 553,593. Current measurements; determining physical qualities of materials. STEGHART, F. L. March 5, 1942, No. 2903. [Class 37] [Also in Group XL] Apparatus for polarographic analysis comprises a polarographic cell, a source of supply of fluctuating current, an electronic amplifier with negative feed-back, and a movement of small time constant which is operated by the amplifier output to make a record on a paper chart. A battery 1 feeds a potentiometer 2 the sliding contact 3 of which is moved by a motor 4 for slowly varying the voltage across the cell 5. In series with the cell 5 is a high resistance galvanometer 6 which reflects light from a source 7 on to a photoelectric cell 8. This controls the grid of a gasfilled valve 9 operated by alternating current from a transformer 31. An automatic switch 32 prevents the anode current from being switched on before the cathode is hot. A condenser 33 and choke 34 smooth the interrupted rectified current passed by the valve 9. A resistance 35 limits the anode current and a condenser 36 gives the grid voltage a correct phase in relation to the cell 8. Start of flow of anode current is thus prevented over part of the positive half wave, so that the anode current starts at such a point that the mean anode current which is fed back to the input compensates the input current. The galvanometer 6 is also connected through a high resistance 10 to a small standard resistance 11 in the anode circuit of the valve 9. The anode current is fed to a movement 12 which operates a pen 13 on a chart 14 moved by a motor 15. Across the terminals of the movement 12 are condensers 16 which can be connected by a switch 17. The range of measurement may be altered by changing the amplification factor of the amplifier by varying the resistance 10. The time constant of the movement 12 is low enough to give a record of the current at any moment when the condensers 16 are switched off and a mean current showing only small oscillations when the condensers are switched on. Alternatively the inertia of the movement may be changed by positioning weights on and off the movement. The effect of condenser current is eliminated by sending through a second winding 18 in the galvanometer a counter current which varies proportionally to the voltage across the cell 5. This counter current is derived from the voltage between the sliding contact 3 and a fixed point 2A on the potentiometer 2 and is adjustable by means of a rheostat 19. For carrying out a quantitative analysis of a small amount of a comparatively negative element, such as nickel or zinc, in the presence of a large amount of a more positive element it is necessary to suppress a big wave to measure the small wave that is built up on its top so that a record of the small wave may be taken with high amplification. To effect this a small current from a battery 37 is sent through the galvanometer coil 18 and adjusted by rheostats 22, 23. A switch 26 cuts out this device when not required, and a further switch 27 allows the condenser current device to be cut out. The current from the battery 1 is standardized by measuring the potentiometer current through a resistance 28 when a two-throw switch 29 is pressed upwards. A rheostat 30 is adjusted until a standard deflection is obtained on the movement 12. The main potentiometer 2 is electrically or mechanically synchronized with the paper drive 15. In a modification, the standard resistance 11 is omitted and a resistance is put in series with the cell 5. The amplifier amplifies the voltage drop across this resistance which is counterbalanced by the voltage drop of the anode current through the standard resistance. If the two voltage drops are not equal and opposite a current passes through the coil and the galvanometer 6 varies the illumination of the photo-cell 8 until a balance is obtained.