Certainly wouldn't have been practical without smaller, faster, more reliable relays, and once it got there the applications were immediately understood. What

Certainly wouldn't have been practical without smaller, faster, more reliable relays, and once it got there the applications were immediately understood. What I meant was that it seems surprising, from a modern perspective, that it never seems to have even been proposed until much later. There are some early hints: I recalled a mention in Gardner's "Logic Machines and Diagrams" of an early (1880s) electromechanical design by Allan Marquand [0], who had built several mechanical logic machines. But Gardner wrote that there wasn't anything published on switching circuits as logic until 1910 in a review by Ehrenfest [1], and it wasn't developed until several independent publications in the 1930s, most influentially by Shannon. By then switching circuits had already gotten very complicated in practice. [0] pg 112 https://archive.org/details/logicmachinesdia00mart/page/112/mode/1up https://archive.org/details/logicmachinesdia00mart/page/112/... Alonzo Church uncovered a diagram by Marquand of a logic machine using relays [2], but it seems to have been sequential, using the relays as memory of what hadn't yet been disproven [3]. In 1886 Charles S. Peirce wrote to Marquand mentioning that electrical circuits could be used for Boolean addition and multiplication [4], though it doesn't seem he was thinking about relays, and I don't know that this was ever applied. [1] pg 138, reference 2 https://archive.org/details/logicmachinesdia00mart/page/138/mode/1up https://archive.org/details/logicmachinesdia00mart/page/138/... [2] Mays "The first circuit for an electrical logic-machine" https://archive.org/details/sim_science_1953-09-04_118_3062/page/281/mode/1up https://archive.org/details/sim_science_1953-09-04_118_3062/... [3] Patterson's review of Mays, https://doi.org/10.2307/2964205 https://doi.org/10.2307/2964205 [4] https://electronics.stackexchange.com/a/321534 https://electronics.stackexchange.com/a/321534 , it seems clear to me that Peirce was talking about manual switches, not relays as the answer seems to suggest.