What is "electrical transcription"

Electrical transcriptions, "recordings made exclusively for radio broadcasting" were widely used in the era of old-time radio, They provided material—from station-identification jingles and commercials to full-length programs—for use by local stations.

Physically, electrical transcriptions look much like long-playing records that were popular for decades. They differ from consumer-oriented recordings, however, in that they were "distributed to radio stations for the purpose of broadcast, and not for sale to the public.... The ET had higher quality audio than was available on consumer records" largely because they had less surface noise than commercial recordings,