This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Except by a mistake, no lamp is likely to be so badly treated as the one mentioned, but it is of daily occurrence for lamps made for (say) a 100-volt circuit, to be- submitted to 105 or 110 volts for a short time, owing perhaps to imperfect governing of the engine driving the dynamo, thereby allowing the latter to run too fast and so give too great a pressure.
The connection of incandescent lamps to the circuit will now be described. W e will suppose that we have a compound-wound dynamo of 100-volts pressure, which, of course, being compound-wound, will, at a constant speed, give a practically constant pressure, irrespective of the load, and we will further suppose that we have some 16-C.P. and some 8-C.P. incandescent lamps. Immediately we connect the two ends of the filament of a 16-C.P. lamp, which protrude through the glass, to the two terminals of the dynamo, we have provided for it a complete circuit, and the current will flow through the lamp and render the filament incandescent. There need be no doubt as to the result, since we have a 100-volt circuit, and have attached a lamp constructed to allow neb a current to go through it, at a pressure of 100 volts, as will give a light of sixteen candles, the current in this case being .6 of an ampere, so that the lamp takes (1000 x 6 - ) 60 watts.
We will now hang another lamp on, in exactly the same manner. It also will immediately light, and we shall have two lamps, each giving a light of sixteen candles, and taking 6 ampere at the common pressure of 100 volts, since each offers a distinct and independent path for the current from the dynamo. For every lamp we so attach, the dynamo will produce just sufficient current and no more.
If we now in an exactly similar manner, and as if the other lamps did not exist, attach an 8-C.P. lamp, we shall have two of 16 and one of 8 C.P. burning simultaneously, the last-mentioned taking, if it is a 30-watt lamp, only 3 of an ampere, in consequence of being of less power.
We can continue to attach lamps until so much current is required from the dynamo that the wires on its armature become excessively hot, and the cotton, or other insulation employed on it, is impaired. I put it in this way to make it clear, but of course a dynamo is known to have a certain capacity, such as 100 volts and 50 amperes, which would not in practice be exceeded.
Just as the lamps were attached one by one, they can be disconnected one by one; if we wish to extinguish any particular lamp, it can be disconnected without affecting the others.
These lamps, it will be noticed, are connected to the dynamo in "multiple" or "parallel", an expression which has been already explained.
In practice it would he very inconvenient to have a bunch of lamps attached to the terminals of a dynamo, but if the copper wires for each of the lamps were many yards long, the dynamo could be in one position, and the lamp- as far off, from it and each other, as desired. The copper wires, as already mentioned, should be of such a size as to absorb practically none of the energy. Two main wires or cables, insulated suitably, are attached to the dynamo-terminals, which are usually two copper screws, and the wires are thence carried through the building as required. To any point along the wires lamps can be attached with the same effect as if they were attached actually to the dynamo-terminals, provided that these mains are of such a size as not to absorb any appreciable quantity of energy.
This arrangement can be elaborated by connecting at any distance from the dynamo along the two cables, two more cables, one to each of the first, and then carrying these two (to which lamps can be attached) in another direction. In the same way, from any point on the two latter, a further pair of cables can be taken, and so on. This arrangement is called the "Tree" system of wiring, and is illustrated in Fig. 623; the current from either terminal of the dynamo
through any lamp, and back to the other terminal, must, of course, be uninterrupted.
Fig 623. - Diagram of Tree Wiring.
As it would be most inconvenient to detach a lamp, when the light from it is not required, an arrangement called a switch has been designed to give the same effect It does nothing more than break the circuit to which it is attached, and so prevent the current passing. Cutting the wire, either before or after it the lamp, would serve the same purpose as the switch, but the switch has the advantage of severing the connection by a simple piece of mechanism, the obvious part of which is simply the rotation of a button, or pressure upon it. A switch can be so located that it will cut a wire feeding one lamp, a dozen, or any other number, and to make this clear, 1 may mention that at least one of the wires leading from the dynamo is always passed through a switch within a few feet of the machine, that by use all the lamps can be extinguished or lighted. I say "at least one wire", because the best method with a large current would be to have a switch on each main cable, close to the dynamo. These two switches are in such a case combined on one base, and controlled by one handle, when they are called a double-pole switch, since they break both electrical poles of the circuit.
Fig. 624 - View of small Switch.
Fig. 625 -View of Small Switch with the Outer Case removed.
As it would be inconvenient to attach lamps by the small wires protruding from their glasses to copper wires in order to connect them to the main cables, "landholders" are provided, which vary considerably in design, but which do nothing more than hold the lamps on to the wires leading to them. One variety is illustrated in figs. 627 and 628, and a lampholder combined with a switch in Fig. 629.
Fig. 630- View of Doable-pole Main Switch.