In addition to the pressure there must also be a suitable path or channel provided for the water to flow through, or there will be no flow, however great the “head,” until something breaks down under the strain. In the case just cited, although there is full pressure in the water in the pipe, there is no current of water as long as the tap remains closed. The opening of the tap completes the necessary _path_ (the greater part of which was already in existence) and the water flows. [Illustration: FIG. 36.--Hydraulic analogy of the electric current. If, say 10 gallons of water flow in every second into a system of vessels and pipes of any shape, whether simple or more complicated as shown in the figure, and 10 gallons flow out again per second, it is evident that through every cross section of any vessel or pipe of the system 10 gallons of water pass every second. This follows from the fact that water is an uncompressible liquid and must be practically of the same density throughout the system. The water moves slowly where the section is large and quickly where it is small, and thus the quantity of water that flows through any part of the system is independent of the cross section of that part. The same condition holds good for the electric current; if in a closed circuit a constant current circulates, the same amount of electricity will pass every cross section per second. Hence the following law: _The magnitude of a constant current in any circuit is equal in all parts of the circuit._] For the production of a steady electric current two very similar conditions are necessary. There must be a steadily maintained electric pressure, known under different aspects as “electromotive force,” “potential difference,” or “voltage.” This alone, however, is not sufficient. In addition, a suitable conducting path is necessary. Any break in this path occupied by unsuitable material acts like the closed tap in the analogous case above mentioned, and it is only when all such breaks have been properly bridged by suitable material, that is, by conductors, that the effects which denote the flow of the current will begin to be manifested. The necessary electromotive force or voltage required to cause the current to flow may be obtained: 1. Chemically; 2. Mechanically; 3. Thermally. In the first method, two dissimilar metals such as copper and zinc called _elements_, are immersed in an exciting fluid or _electrolyte_.