Abstract :
The paper deals with two recent developments in the design of audio-frequency metal locators. The first is the introduction of discrimination against low-conductivity magnetic bodies for the purpose of suppressing unwanted signals. A solution is described which forms the basis of a number of these locators. The principle is to utilize the existence of a phase difference between desired and undesired signals. The need for a more radical departure from the established principles is then examined and the second development (locator No. 7) is discussed. In this locator an alternating magnetic field is set up over a large area, and local field distortions, due to metallic bodies, are observed directly by means of a balanced-coil system. In this way it is possible to attain high sensitivity and operating speed, to eliminate interference between operators, and, for war-time applications, to safeguard the user from booby-traps sensitive to an alternating field.
