Patent GB-681728-A (GB)
Title: Improvements in or relating to control apparatus for chemical reactions
IPC: G03D13/00
Filed: 1949-12-29
Abstract: 681,728. Photographic developing apparatus. WOOD, M. C. Dec. 29, 1950 [Dec. 29, 1949], No. 33283/49. Class 98 (ii). [Also in Groups XXXVIII and XL (c)] Apparatus for the control of chemical reactions comprises a time-actuated signal device to denote when a particular reaction should cease and means automatically responsive to the temperature in the reaction vessel for varying the time at the end of which the signal device will be actuated. The apparatus is particularly applicable to photographic development, especially of X-ray films. Preferably, the signal device is actuated by the charging of at least one condenser, the charge applied to the condenser being automatically varied in response to any fluctuations in the temperature of the developer bath or reaction vessel, e.g. by charging the condenser through a resistor variable with temperature. Such a resistor may comprise a resistance thermometer, or a resistor having connections to a mercury-inglass thermometer which short-circuits part of the resistor. As shown, the resistor 3 is tapped at various points by wires which are sealed into the glass of a mercury thermometer. As the temperature increases, the wires are brought into contact seriatim with the rising mercury column, whereby an increasing portion of the resistor 3 is short-circuited. A power supply including a transformer 1 and a neon stabilizer 11 charges a condenser 2 through resistor 3, the junction of the condenser 2 and resistor 3 being connected to one grid of a double-triode trigger valve 7. To start the timing sequence, a pressbutton B is pressed to discharge condenser 2. One portion of valve 7 is then conducting heavily and the other portion conducts very little, so that a relay 6 is energized, moving a switch 5 to the position shown so that a condenser 4 is charged, and closing switch 15 to energize a bulb 16 which indicates that the apparatus is in operation. When the voltage across condenser 2 reaches a given value, the trigger valve 7 conducts heavily on the previously non-conducting side and only slightly on the previously conducting side, so that relay 6 is de-energized. Switch 5 moves into its second position so that condenser 4 energizes relay 12, causing a switch 13 to close and a buzzer 14 to operate. The junction of resistor 3 and condenser 2 is also connected to the grid of a triode-connected pentode 9, the current through which varies during the timing period and is indicated on a meter 10. The reading on meter 10 thus gives an indication as to the length of time due to elapse before the end of the timing period. A pilot lamp may be connected to the thermometer leads to enable the temperature to be read, relays may be connected in circuit to indicate by means of a buzzer or lamp that the temperature has risen or fallen above or below pre-determined levels, and the doubletriode valve 7 may be replaced by a thyratron. In a modification, one terminal of a source of electrical energy may be connected through a lamp or buzzer to a lever which is slowly moved, as by a clockwork mechanism, over a series of contacts. The other terminal is connected to the lowest wire of the above-described mercuryin-glass thermometer, the other wires being connected to the contacts. Thus the level of the mercury will determine at which of the contacts the lever will complete the circuit.