Here are my latest BBC Transcription disc purchases. These ones are some of the older ones around. The smaller discs look like they are 78's - though unusually this is not shown on the labels, while the larger ones are 16 inch (yes 16!) All are from the 1940-50s.
The 78s were the first type of disc used by the BBC to record shows. For a 30 minute broadcast, a DJ would need to swap discs every 3-5 minutes! During the 1950s, the BBC moved to using 16 inch discs. These could have around 13-15 minutes, meaning a DJ would only need to swap discs once in a 30 minute slot - much less active duties!
Later, these discs became 10 inch discs with a similar length, but much smaller and therefore easier to store.
Then these became 12 inch discs but could store an entire 27-28 minutes on one side! Hour programmes still required a disc change.
These days, shows are recorded on compact disc, no disc changes required!
Just a few notes about these 16 inch discs, they apparently play from the centre rather than from the outside on one side, then the more normal outside to inside on the other. They are quite thick and heavy. One of these weighs around 600g! No I do not have any equipment to play the 16 inch discs ...