RI (Radio Instruments) Ltd,
PM (Permanent Mineral) Detector
Introduced 1925. Original cost 6 shillings. Cylindrical body with brass end fittings. There is no cats whisker, instead an adjustable arm alters the movement of one mineral against another. Marketed as the alternative to glass enclosed cats whisker detectors and “the discovery of 1925″.
The first advert I have found for this detector is in the June 1925 edition of The Wireless Constructor magazine (page 787). A later advert taken from the Popular Wireless and Wireless Review magazine dated Oct 2nd 1926 (back cover) is shown below.