Patent GB-657277-A (GB)
Title: Improvements in or relating to instruments for the measuring of ionization currents
IPC: G01L21/00
Filed: 1949-02-03
Abstract: 657,277. Gas pressure and ionisation measurements. STRIKER, G. O. Feb. 3, 1949, No. 3050. Convention date, Sept. 27, 1948. [Class 37] The ionisation of a gas in a chamber 1, Fig. 1, is measured by providing a cathode 2 emitting a periodically varying flow of electrons, an electron collector 3, and an ion collector 4, and amplifying and indicating the A.C. component of the current from the collector 4. It is stated that the ratio m of the A.C. to the D.C. component is the same for both the ion current from 4 and the electron current from 3; further, that the pressure P of the gas is K/m I<SP>1</SP> 2 /I 1 , where I<SP>1</SP> 2  is the A.C. component of the ion current, and I 1  the D.C. component of the electron current, so that the A.C. component I<SP>1</SP> 2  of the ion current is a measure of the gas pressure if I 1  is kept constant. The cathode 2 is heated by A.C. so as to provide a varying flow of electrons, which are attracted to the positively charged collector 3, instrument 9 measuring the average current flow; ions are attracted to the negatively charged collector 4, and the A.C. component of the resulting current flow is applied over condenser 12 to a logarithmic amplifier 13 whose output is applied over filter 15 to a peak rectifier circuit 17, 18 feeding meter 21. An attenuator 14 adapts the circuit to gases having different constants K. In Fig. 2, the one instrument 21 is used to measure the A.C. component of both the electron and ion current; with switch 24 in its lower position, the heating current through the cathode 2 is adjusted to give a standard reading on meter 21, so that with switch 24 in its upper position, the meter reading is proportional to gas pressure or ionisation; switch 22 allows variation of the load for the electron current so as to alter the range and allow the operation of the cathode at as low a temperature as possible. The electron and ion currents may be amplified in separate amplifiers and measured on separate meters, Fig. 3 (not shown), the ion current being maintained constant and the variation of electron current being used to indicate variation in pressure or ionisation. In the arrangement of Fig. 4, the electron current is amplified at 13 and applied to meter 21 whose pointer 43 can make contact at 38 or 39, so that with switch 41 closed, motor 40 is operated to adjust resistance 27 in the cathode circuit to bring the pointer to a datum position; the ion current is then applied over switch 24, and switch 41 is opened so that the meter reading is proportional to pressure or ionisation; attenuator 36 permits range adjustment. In the modification of Fig. 5, the A.C. components of the electron and ion currents are opposed, and the magnitude of the electron current is adjusted at 35 until the meter gives a null indication; gas pressure or ionisation is indicated by the setting of resistance 35. The output of amplifier 13 may control a motor adjusting resistance 35, Fig. 6 (not shown). The cathode may be of photoemissive material exposed to light of varying intensity.