H—34

Magnetic Amplifiers.—A more exhaustive study is now in progress into this method of power control by means of small direct currents. The method offers the most promising approach to the amplification of low powers from low impedance sources. A circuit now developed permits the full-scale deflection of a 0-1 milliamp-meter from a thermocouple with a temperature change of 20° c. A power-control circuit giving an output of 15 watts for an input of 0-05 watt has also been developed; this can be considerably improved by the use of better magnetic materials. There appears to be a wide field of useful application for both these types of circuit. Detection of Small Metal Objects.—A request was received from the Wellington Hospital for assistance in detecting small foreign bodies inside the eye during operations. A sensitive amplifier with suitable frequency response, used in conjunction with a small probe, was found to be effective and'was supplied to the hospital. Crystal Triodes. —Sample crystal triodes were received from the United Kingdom by the courtesy of the manufacturers. Tests, while not conclusive, indicated that these triodes have not yet reached sufficient repeatability and stability to be suitable for practical applications. Tests on P.V.C. Overhead Power Cables. —A sample of P.V.C. insulated aerial cable and two types of braid-covered aerial cables were subjected to 2,000 hours accelerated ageing. The electrical and physical properties of the P.V.C. cable compared favourably with those of the other cables. Routine Testing.—Testing of electrical equipment and commodities has formed a steady activity; these included switches, fuses, water-heater elements, hot-plates, switchboard materials, and motors. INSTRUMENTATION FOR NUCLEAR PHYSICS Geiger Muller Tubes.—An improved vacuum system has been constructed and put into operation. This enables more accurate filling of the tubes and also quicker location of leaks in the assembled G.M. tubes. Tubes with excellent characteristics can now be constructed in a small range of special types. Health Instruments.—Several laboratory radioactivity monitors have been placed in service in radio-isotope laboratories. These instruments are designed for checking the occurrence of any appreciable contamination of the laboratory or operator's clothing. A prototype ionization chamber monitor for beta and gamma radiation has also been built, and six more are to be made. This instrument measures the direct radiation from isotopes so that the operators can check that the radiation level they are receiving is always below the permissible tolerance dose. Field Monitor.—A portable beta-counting instrument has been designed and constructed for the Soil Bureau. This instrument is now being used for preliminary measurements in the field with radioactive phosphorus. Scintillation Counters.—Work has commenced on the technique of growing organic crystals, and on their use as detectors of radioactivity. Mass Spectrometer.—A start was made towards the construction of a general-purpose mass spectrometer in November, 1949, but some delay was experienced while waiting for the arrival from England of materials and for

35