one of very low resistance, connected directly between the two supply pipes, thus making a shunt, or by-passage, round the water-wheels. There would be nothing to limit the flow of water through this large pipe except the slight resistance to the passage of the water, and the ability of the pump to supply it. With an electric circuit on the "constant potential," or multiple system, there is an analogous effect when the wires are connected by a conductor of low resistance, or when they themselves come together. The dynamo is so constructed that it tends to keep the pressure constant. A tremendous current passes through the wires forming the short circuit, and if the dynamo is of sufficient size and there are no protective devices in circuit, the wires will heat until they actually melt.

It is commonly said that the electric current chooses the path of least resistance, but this needs a qualification to make it exact. With two paths more current will go through the path of low resistance than through the path of high resistance, but the total current will divide in exact inverse proportion. In Figure 5, a large pipe may be connected between A and B, and the flow of water in it will be proportional to the difference of pressure between A and B, and inversely proportional to the resistance of the connecting pipe. But the current through any other pipe

between A and B, no matter how small, will also be proportional to the difference of pressure between its ends, and inversely proportional to its resistance, just as if the larger connecting pipe were not there. If the large pipe carries so much water that the pressure between A and B cannot be kept up by the pump, then of course the amount that will flow through any other pipe will be smaller than before the large pipe was connected. This, however, is because of the effect on the pressure and not because the water chooses the large pipe to the exclusion of the smaller.

The electrical multiple system may be elaborated by running from the mains, branches, and from the branches, taps, just as in a water or gas system branches or taps are taken from the main pipes. The dynamo is maintaining a pressure between the wires that lead from its terminals, and any branches that are taken from these wires are simply extensions of the mains, proportioned to the current they will have to carry. With a water or gas system, the free space outside, the air, may be looked upon as one of the pipes, and the devices used to keep up the pressure on the system are keeping up a pressure between the pipes and the free space. Electricity cannot flow in this space and it is thus necessary to provide the two wires.

Figure 6 shows an extended electrical multiple system. The mains, AA, will have to carry all the current passing through the lamps, or, if there are twenty lamps each taking ampère, the mains will

have to carry 10 ampères. The branches, BB, carry only the current required by the lamps they feed, or, as

the system is laid out in the figure, they will carry five ampères each. The taps, CC, will carry 21 ampères each. In this way the current may be distributed throughout a building, branches being led to various points and the taps taken off as lamps are required.

With what is called the "closet" system the taps are

all taken off at a few centrally located places, and in recesses in the wall may be set all the safety-fuses and the switches, so that the controlling may all be done from these few points. Figure 7 shows a system arranged in this way. This is the system used almost exclusively where the wiring is concealed. It requires

more wire, but it does away with the safety-fuse blocks scattered about in unsightly and inconvenient places, and provides a special place for them in a receptacle that is unobtrusive.

The three-wire system is a special form of the mul

tiple system and is largely used because it effects a great saving of copper. Figure 8 will serve to illustrate the principle. Two dynamos having the same voltage are used, and are connected as shown in the diagram. Suppose the voltage of each to be 110 volts. Then A is creating a difference of pressure between c and d of 110 volts, and B is creating a difference of pressure between d and e of 110 volts, so that there is

a resulting difference of pressure between c and e of 220 volts. The "positive side" (or the side the current starts from) of A is the negative side (or the side the current returns to) of B just as it might be with two pumps, one creating a certain pressure and passing the water to the next, which adds a like pressure. the lamps were connected directly between c and e, the

If now