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The paper describes a small portable equipment for use by the private prospector in the location and approximate assay of radioactive ores. Its features are cheapness in cost and maintenance, simplicity of operation, long battery-life (2000 hours), robustness, lightness of weight (6 lb) and smallness of size (8Â¿ in Ã 8 in Ã 3Â¿ in). A pair of headphones and a meter are supplied as indicators; one quarter of full-scale deflection of the meter denotes the presence of bed rock containing 0.03% uranium oxide, U3O8 (0.003r/h for Â¿-rays from radium), and full-scale deflection denotes that of 1% uranium oxide, U3O8 (0.018r/h). Cold-cathode valves are used in the instrument, one acting as a constant-amplitude oscillator feeding a miniature Cockroft-Walton voltage-quadrupler producing a stabilized voltage output. The moulded plastic container is completely weatherproof. The Geiger-MÃ¿ller counter tubes use bromine as the quenching agent, have thresholds of about 300Â¿330 volts, and have very long lives. The range of temperature over which they will operate is from Â¿50Â° C to +40Â° C, the temperature coefficient of threshold voltage being less than 0.2 volt/Â°C. Plateaux are 100 volts in length, with slopes from 0.03Â¿0.1 %/volt. The counters may be operated at Â±800 volts without damage. Their efficiency is not appreciably different from that of standard tubes.