A Herschel circuit is a series of Herschel conduits or other components, connected by placing them so that the output Herschels from early conduits become the input Herschels for later conduits. Often the initial component is a converter accepting some other signal type as input -- usually a glider, in which case either a syringe or bronco is most commonly used. The Silver reflector is a well-known early Spartan Herschel circuit from before the syringe was discovered, where the initial converter is a Callahan G-to-H.
Sometimes a direct connection between two conduits is not possible due to unwanted gliders that destroy required catalysts, or wanted gliders that are not able to escape. In this case, small "spacer" conduits such as F116, F117, Fx77, R64, L112, or L156 can be inserted between the other conduits to solve the problem.
Some converter or factory conduits do not produce a Herschel as output, instead generating other useful results such as gliders, boats or MWSSes; see Herschel-to-glider, demultiplexer, and H-to-MWSS respectively for examples of these. For those conduits which do produce an unwanted Herschel, an eater such as SW-2 can be added to delete it.
If the first and last conduits of a chain connect to each other in a loop then there is no need for a G-to-H to generate the first Herschel, or an eater to consume the last one. The circuit becomes a self-supporting Herschel loop. A loop is also formed by a G-to-H connected to a Herschel-to-glider converter, with the glider reflected back to the G-to-H's input with glider reflectors of the appropriate colour, usually Snarks. In either case, if the loop has a surplus glider output, it becomes a gun; if no output is available it is an emu.