As explained mostly by Astounding Science Fiction editor John W. Campbell in late 1950s and early 1960s editorials, Hieronymous machines were mockups of real machines which worked by analogy, being directed by psi or ESP powers.
As an example, one could create a receiver or similar device, with prisms and vacuum tubes represented by their cardboard or even schematic representations. Through the use of mental powers, such a machine would function as would its "real" equivalent. Campbell claimed that such machines actually did perform this way, but the concept was never taken seriously elsewhere or followed through on in later years.
