urgent project for the South African Chamber of Mines to provide a means of radio communication underground for rescue purposes. After a feasibility investigation Wadley wrote a report indicating that it could be done and detailing his recommendations. The Chamber did not pursue the matter for more than a decade. Wadley retired in 1964 (aged 44) and lived on the south coast of KwaZulu-Natal until his death from cancer in 1981 (aged 61). Wadley loop It was in 1948 at the CSIR that Wadley invented the Wadley Loop receiver, which allowed precision tuning over wide bands, a task that had previously required switching out multiple crystals. The Wadley Loop was first used in the Racal RA-17 a 1950s top-of-the-range British military short wave receiver and later in the South African made, commercially available "Barlow-Wadley XCR-30" radio. The Wadley Loop is more widely used today in spectrum analysers, where the noise sidebands of the analyser's tunable oscillator are cancelled due to the spectrum analyser having a sideband noise much lower than the signals being measured. This device was even more useful to the SABC, SAPO, the South African Military and British Government agencies. A Wadley receiver (circa 1952) is on display at the South African Institute of Electrical Engineers historical collection in Observatory, Johannesburg. Tellurometer In the early 1950s the CSIR was asked to develop a portable measuring device that could measure distances with an accuracy of 1 in 100 000. In 1954 this project was given to Wadley. Colonel Harry A. Baumann (Rhodes Scholar, engineer and Land Surveyor) of the South African Trigonometrical Survey had already come up with the invention and Wadley developed it further. The Tellurmeter could measure up to a distance of 80 km by measuring the time delay in microwave transmissions. It was used in land surveying but has been mostly replaced with laser-based systems. The replacement to the microwave tellurometer was also developed by a South African, H.D. Hölscer. Miniaturised versions are still used in some surveying instruments and mine lift-shafts. One of the first test of the tellurometer involved a measurement of the distance between Brixton Hill in Western Johannesburg and Fort Klapperkop in Pretoria, this being the most accurately known survey baseline in South Africa at the time. It successfully proved the accuracy of the tellurometer but, over time, Wadley noticed a bias to the measurements he was obtaining. He ascribed these to