GB840780A - Improvements in or relating to installations for making x-ray photographs - Google Patents

Abstract

840,780. Measuring X-rays. NEDERLANDSCHE RONTGENAPPARATENFABRIEK N.V. Oct. 29, 1957 [Oct. 31, 1956], No. 33685/57. Class 40 (3). [Also in Groups XX and XXXV] The tube voltage is increased throughout a radiographic exposure so that the longer the exposure the higher the voltage at the end of the exposure. The mean voltage during exposure thus increases with increasing duration of exposure and the exposure timing means can thus automatically make the mean voltage equal to the voltage which would previously have been selected for an exposure of constant voltage. In the form shown, a photo-cell 40 receives light from a fluorescent screen to be photographed. A high secondary resistance in the transformer 1 enables the voltage of the X-ray tube 7 to increase when the cathode 5 is allowed to cool during the exposure. Prior to exposure, variable resistances 16 and 17 are included in the circuit of the cathode heater transformer 14. On moving exposure switch 26 to a pre-exposure position, contacts 24 and 27 are connected and relay 19 is operated to short-circuit resistor 17 and increase the heater current. The exposure is then initiated by closing timer contacts 28, and 29 and moving contact 24 to engage both 25 and 27. Relay 12 immediately closes contacts 11 to energize X-ray tube 7 and open contact 31 to deenergize relay 20 and decrease the cathode heater current by reinstating resistor 17. The exposure is terminated by firing of thyratron 42 and the operation of relay 22 to break contacts 21 and thus de-energize relay 12. Operation of relay 20 is prevented by holding relay which is not affected by contacts 21 so that the cathode heater current is not increased. On moving switch 26 back to the open position, relays 22 and 35 are de-energized and the circuit is ready for the next exposure operation. A clockwork timer may replace photo-cell 40 and condenser 41 which fire thyratron 42. The apparatus is stated to be operable without modification from a fluctuating mains voltage having a variable resistive load and the circuit component can be chosen so that the X-ray tube can never be over-loaded. Known means may be provided to prevent fluctuations in the voltage of the cathode heater circuit.