This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
1. An electric current may be produced commercially by means of either a primary battery or a dynamo-electric machine. The elementary form of battery is simply a vessel A, Fig. 1, containing alkaline or acidulated water, into which dip two plates, one of which C may be copper, and the other Z, zinc. If two wires be secured to these plates and their ends brought together, a current will flow through them from the copper to the zinc, and will be interrupted when they are separated. The copper is then said to be the positive element, and the zinc, the negative element, while the liquid is called the electrolyte. The path of the current from one plate through the liquid to the other plate, and through the wire to the starting point, is called the electrical circuit.
2. A circuit is broken, or opened, when its conducting elements are disconnected in such a manner as to prevent the current from flowing.
A circuit is closed, or completed, when its conducting elements are so connected as to allow the current to flow.
A circuit in which the earth, or ground, forms part of the conducting path is called an earth, or a grounded, circuit. The external circuit is that part of a circuit which is outside or external to the electric source.
The internal circuit is that part of a circuit which is included within the electric source.
In the case of the simple voltaic cell mentioned in Art. 1, the internal circuit consists of the two metallic plates, or elements, and the electrolyte; an external circuit would be a wire or any conductor connecting the free ends of the electrodes.
3. Conductors are said to be connected in series when they are so joined together as to allow the current to pass consecutively through each. For example, Fig. 2 represents a closed circuit, consisting of a simple voltaic cell B, and four conductors a, b, c, and d connected in series.
A circuit which is divided into two or more branches, each branch transmitting part of the main current, is a derived, or shunt, circuit, and the separate branches are said to be connected in parallel, or multiple arc. An example of a derived circuit, of two branches in parallel, is shown in Fig. 3. The main current flows first through the conductor a, then divides between the branches c and b, and finally unites, completing the circuit through the conductor d; the two branches c and b are the conductors which are connected in parallel, or multiple arc.
4. In every circuit containing a battery, there is at all times a tendency for a current to flow, even when the circuit is open, which is caused by the fact that there is always an electrical pressure acting. It is as though we had a tank full of water, and a stop-cock at the bottom communicating with a pipe. The pipe represents the wire of a circuit. There is always a pressure on the stop-cock, and as soon as we open it, a current of water flows through the pipe. The amount of water which would run out in a given time, say 1 minute, would depend on the size of the pipe; if it were of large cross-sectional area, more water would flow than if it were small, because the larger pipe offers less resistance. Another condition would also influence the rate of flow, namely, the height of water. If the tank were very high above the outlet, the pressure would be great. The flow, then, of water depends on the resistance offered, and on the pressure applied; the less the resistance or the greater the pressure, the more water will be forced through. The same holds good with respect to electricity. If the resistance of the circuit is low and the pressure high, a large current will flow; if the resistance is great, less current will pass, unless the pressure be raised to such a point as to overcome the increased resistance. This is the explanation of the fundamental law in electrical calculations, known as Ohm's law.