GB708728A - Improvements in and relating to the electrode construction of electroscopes - Google Patents

Abstract

708,728. Discharge apparatus. BRITISH OPTICAL .& PRECISION ENGINEERS, Ltd., and MINISTER OF SUPPLY. July 15, 1952 [May 2, 1951], No. 10351/51. Class 39(1) [Also in Group XXXVI] Between the outer casing of an electroscope and the collector electrode, which forms the wall of an ionization chamber, is a body of solid dielectric material. Fig. 1 shows a portable electroscope used to indicate an X-ray dosage or an amount of gamma-radiation to which the person carrying the instrument has been subjected. The electroscope. comprises a metal case 4, Fig. 1, within which is a quartz fibre electrode assembly 5 maintained at the potential of the case by mounting it on the end of a viewing microscope tube 6. The fibre electrode is located in an ionization chamber 12 formed by an insulated collector electrode 1, which consists of a thin metal cylinder spaced from the case 4 by dielectric material 2 such as polystyrene moulded to the electrode 1. The ends 13, 14 of the dielectric material 2 are formed with grooves 15, 16 to increase the length of the surface leakage paths. In a modification, Fig. 2, the capacitance of the collector electrode 1 to earth is increased by forming the electrode assembly of three parts moulded together, viz. the collector electrode 1, encasing dielectric material 2, and an earthed metal casing 17 surrounding the dielectric material and bent inwards to form an inner cylinder which is shorter than the outer cylinder so as to form the ionization chamber 12. This assembly is mounted so that the outside of the metal casing 17 is in contact with the inside of the outer casing 4 and is thereby maintained at the same potential. The electrode 1 may be closed by a top part 18, Fig. 3, covered by the dielectric material. A small portion 19 of the top 18 is cut out to form a window to admit light, and a small portion 20 of the dielectric material 2 is removed to permit the charge to be applied to the electrode 1. In a modification, Fig. 5, the assembly is made in two parts, A, B, fitting over one another. The part A forms a removable top cap and comprises the upper part 18 of the electrode 1 and the top of the outer cylinder of the metal casing 17. This fits over the part B which comprises the tubular part of the electrode 1 and the lower portion of the casing 17 and dielectric material 2.