seventy-eight-rpm

A phonograph record designed to be played at 78 revolutions per minute. 78 (seventy-eight) is the natural number following 77 and followed by 79. 78 is a triangular number, and its factorization makes it a sphenic number. As a multiple of a perfect number, 78 is itself a semiperfect number. 77 and 78 form a Ruth-Aaron pair under the second definition in which repeated prime factors are counted as often as they occur.