GB289837A - - Google Patents

Abstract

289,837. British Thomson-Houston Co., Ltd., (Assignees of Shaw, G. R., and Tellkamp, B. F.). May 3, 1927, [Convention date]. Vacuum tubes.-In the treatment of metal filaments by exposure while incandescent to a gas containing carbon or other non-metal, the duration of the exposure is controlled by the increase in resistance of the filaments, and means are provided to prevent the filaments from being heated outside the gas chamber. The filament mounts are supported by their exhaust tubes in holders 26, shown in plan in Fig. 3, adapted to be raised and lowered on vertical posts 12 by pedal action to bring the filaments into and out of gas chambers 35, the parts being duplicated to enable one filament mount to be inserted while another is being treated. The leading-in wires of the mounts are inserted in clips 27, 28 connected to the current supply. Current is switched on to the filaments by a lever 52 pivoted at 56, and the sliding brackets 49 of the mount holders 26 are provided with interlocks coacting with an interlock bar 55. carried by the lever 52 to permit current to be switched on only when a filament is in a chamber 35. The main switches 57, 62 are closed by the lever 52 through springs 59 which retract it when it is released, during which action a controlling circuit is closed and again broken by fingers 65 on the lever bridging and leaving contacts 63, 64. Current is supplied to the filaments 19, Fig. 5, by a constant-current transformer 86 controlled by an induction regulator 81. When the fingers 65 close the controlling circuit, a magnetic relay 84 closes the primary circuit of the transformer 86, and when the controlling circuit is again broken a. resistor 90 allows sufficient current to pass to keep the relay switch 84 closed, although this current is insufficient to close it initially. Constant current being maintained through the filaments 19 the voltage across them increases as their resistance rises. This voltage is impressed on the grid 97 of a three electrode thermionic valve. A predetermined negative bias is applied to this grid by a battery C and a potentiometer 102. When the potential on the grid becomes positive owing to a filament 19 having absorbed a predetermined amount of the non-metal, grid current flows and operates a relay 96 which closes a circuit through a relay 100 from a point 101 of the potentiometer. This reduces the grid bias applied to the valve and thus increases the grid current operating the relay 96. The relay 100 breaks the circuit of the transformer 86, thus terminating the treatment of the filament. A mixture of benzol and hydrogen may be supplied to the chambers 35 by the apparatus shown in Fig. 4. Benzol is forced to a desired height in a mixing chamber 68 by the pressure of hydrogen acting through a pipe 71 and controlling valve 72. Hydrogen passing by a pipe 74 and controlled by a valve 76 carries benzol vapour into the chamber 35.