The parallel system is illustrated in figure 67, where the lamps are connected between two main conductors cross-wise, like the steps of a ladder. The current is thus divided into cross channels, like water used for irrigating fields, and it is obvious that, although the circuit is broken at one point, say by the rupture of a filament, all the lamps do not go out. Fig. 68 exhibits the Edison three-wire system, in which two batteries or dynamos are connected together in series, and a third or central main conductor is run from their middle poles. The plan saves a return wire, for if two generators had been used separately, four mains would have been necessary. The parallel and three-wire systems in various groups, with or without accumulators as local reservoirs, are chiefly employed for incandescent lamps. The main conductors conveying the current from the dynamos are commonly of stout copper insulated with air like telegraph wires, or cables coated with india-rubber or gutta-percha, and buried underground or suspended overhead. The branch and lamp conductors or "leads" are finer wires of copper, insulated with india-rubber or silk. The current of an installation or section of one is made and broken at will by means of a "switch" or key turned by hand. It is simply a series of metal contacts insulated from each other and connected to the conductors, with a sliding contact connected to the dynamo which travels over them. To guard against an excess of current on the lamps, "cut-outs," or safety-fuses, are inserted between the switch and the conductors, or at other leading points in the circuit. They are usually made of short slips of metal foil or wire, which melt or deflagrate when the current is too strong, and thus interrupt the circuit.