TAM meter for calculating TV viewer numbers

From 1963 to 1974, Infratam (a society for TV audience research) conducted a survey on television usage. The sample consisted of 825 households in which a Television Audience Measurement (TAM) meter detected the voltage of the TV's program switch and, at minute intervals, recorded on a gauge whether the TV was on, and if the TV was on the channel ARD or ZDF. The recording tapes were collected at the end of each week and evaluated at the institute.