Patent GB-577043-A (GB)
Title: Electrical apparatus for chemical analysis
IPC: G01N27/48
Filed: 1943-12-01
Abstract: 577,043. Determining physical qualities of materials ; potentiometers ; thermionic valve ammeters and voltmeters. COLEMAN, E. de W. Dec. 1, 1943, No. 20084. [Class 37] A test apparatus utilising a dropping mercury electrode includes means for momentarily importing a high potential to a droplet on the electrode to reduce the surface tension and cause it to be released from the electrode and initiate the formation of a new droplet, means for thereafter applying a substantially constant potential to the forming droplet, and means for measuring the current supplied to the forming droplet resulting from the constant potential. A cell 14 contains the sample 15 under test with a mercury pool 22 forming the reference electrode. The mercury pool may be covered with calomel to assist depolarization and a low concentration of potassium or lithium chloride may be dissolved in the liquid 15. Suspended above the liquid 15 and with its lower end submerged is a capillary tube 16 connected by a rubber tube 18 to a mercury reservoir 19. Conductors 23, 24 make contact with the mercury pool 22 and the mercury in the reservoir 19. Small drops 25 of mercury spontaneously form at the end of the capillary tube 16 and fall at the rate of about one drop in three seconds. A cell circuit is pirovided for impressing on the cell any of a plurality of potentials and includes a potentiometer 26 comprising a slide wire 28, slider 29, battery 27, and variable resistance 32. The conductor 23 from the reference electrode 22 is connected to the potentiometer at the point 31, and the conductor 24 from the mercury reservoir is connected to the slider 29 through a resistance 33 and normally contacting switch blades 34, 35. A desired potential is thus impressed on the cell and the current flowing through the cell is measured by measuring the voltage drop across the resistance 33. A condenser 36 and resistance 37 are connected in parallel with the resistance 33 through switch blades 38, 39 which are normally open but momentarily closed to charge the condenser 36 in accordance with the potential across the resistance 33 so that the charge on the condenser may be measured to measure the current through the cell 14. A potentiometer 41 including a battery 42, resistance 43, a slider 44 is connected to the conductor 24 through normally open switch blades 45, 34 and a limiting resistance 46. Closing of the switch 45, 34 applies a high cathode potential to the drop 25 and thereby reduces its surface tension so that it is released from the capillary tube and a new drop begins to form, the cell circuit being open during the closing of the high potential circuit. The switch blades 38, 34 are actuated by means of a motor 46<1> driving a cam 47 having a cam face 48. This closes the blades 38, 39, thereby charging the condenser 36. These blades then open and the blade 34 breaks contact with the blade 35 and makes contact with the blade 45, opening the cell circuit and closing the high potential circuit so that the drop 25 is released. As soon as the blade 34 is released a new drop is allowed to form while the face 48 rotates through a major portion of a revolution. The charge on the condenser 36 is measured by a potentiometer 56, the slider 61 of which is adjusted until no change is produced in the charge on a condenser 65 when a switch 62 is closed alternately on contacts 63, 64. No change is therefore produced in the grid potential of a valve 51 and no response is given by a meter 54 in the anode circuit. The potential applied to the cell is less than that measured by the potentiometer owing to the voltage drop in the resistance 33. This may be avoided by a second potentiometer which applies to the cell circuit an additional potential equal to that lost in the resistance 33. The corrective voltage may alternatively be supplied from the anode circuit of an amplifier. In addition to the ionic current through the cell there is also a condenser current representing the energy required to charge the constantly forming surface of the mercury drop. This may be compensated for by introducing into the cell circuit the voltage drop across a resistance in series with the potentiometer 26.