A four-valve TRF (Tuned Radio Frequency) domestic receiver in a circular Bakelite case. TRF radios were usually battery powered, and as the batteries were costly, the radio was normally only turned on to listen to a specific programme. Tuning was a complicated process, so much so that listeners were advised to switch their sets on 10 or 20 minutes before the programme started to allow sufficient time to tune in. When combined with a wood flour filler, phenol formaldehyde, known by its trade name 'Bakelite' after its inventor Leo Baekeland, forms a useful mouldable plastic, with very good electrical insulating properties. It was the first plastic to be used for making radios, and was ideal for the Art Deco-style designs of the 1920s and 1930s.