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.
The simplest method of obtaining current in practical form for electric lighting is, no doubt, the primary battery, but owing to the small amount of energy obtainable from a battery of reasonable dimensions, the unpleasant process of cleaning and renewing, and the somewhat great expense of maintenance, I do not purpose to consider it in detail. Many forms of batteries have been put upon the market, especially during the last few years, some of them showing very great ingenuity, but all with which I have come in eontact have at least one of the above-mentioned disadvantages to a sufficiently marked extent to condemn them for electric lighting on an extensive scale.
The dynamo, which has made such rapid strides towards perfection since it became recognized as a commercial article about eighteen years ago, is the means now almost invariably adopted for obtaining electric current. It can be best explained by reference to a steel magnet, which is merely a piece of tempered steel magnetized (originally by a lodestone), and is generally either straight or of horseshoe pattern. Between the poles of a magnet there are always a number of " lines of force ", which the accompanying illustration will make clearer than any description. This illustration was obtained by placing the small magnet shown, upon a table, and then covering it with a piece of cardboard having upon its surface a quantity of iron filings, which immediately placed themselves as seen. Any magnetical metal coming within the range of these lines of force will be attracted to the matter, and will itself carry some of the lines and become a magnet. Any non-magnetical metal will, of course, be quite unaffected as far as magnetism is concerned.
But supposing we inserted and rotated in the lines of force between the poles a magnet, a ring of copper (such as a key ring), there would be induced in the copper ring an electric current of minute quantity. This phenomenon of induction would be produced without any friction, or any treatment beyond its rotation within the limits of the lines of force. It will at once be seen that, by having a powerful magnet and many large coils of wire, a powerful current could be produced, and this fact has been taken advantage of in the dynamo, which is nothing more than a magnet, with coils revolving in the lines of force created.
A dynamo, however, has not a magnet such as that just descrilted (which is known as a permanent magnet), but has what is known as an electro-magnet
- shown in Fig. 611, - as this can be produced more strongly and cheaply than a permanent magnet, and offers other advan tages which will be seen hereafter. There is, however, no difference between a permanent magnet and an electro-magnet, as far as the lines of force are concerned, and so the principle remains the same. An electro-magnet differs from a permanent magnet in that it is not made of hard steel, but of soft iron, - and sometimes soft steel - and has wound upon it many turns of insulated copper wire, through which a current is passed, which renders the iron, as long as the current passes, strongly magnetic The coils which revolve in the lines of force of such a magnet are Mined the armature. This armature generally consists of a number of discs of thin
Fig 610 - Magnet and Lines of Force.
Fig 611. - Electro-magnet charcoal iron, - each with a hole in its centre concentric with its circumference, - which are slipped upon a steel shaft, and upon the periphery formed by these plates are laid the insulated copper wires forming the coils, which are to revolve between the poles of the magnet.
Fig. 612 - Shaft, with Discs in Position, forming Armature.
Fig. 612 - Armature Partially Wound.
These coils are connected to a number of copper plates on edge, radiating from the shaft, as shown in Fig. 614, and separated from one another by insulation of some sort, preferably mica. The copper plates form what is known as the commutator. Upon this commutator, at diametrically opposite points. brushes are brought to bear, which collect the current as it is generated in the coils revolving between the pole-pieces.
As I have mentioned, the electromagnets of a dynamo require a current to be passed round them, to enable them to give the required lines of force. This current is obtained from the armature, and is very small when compared with that which the armature is capable of giving out of force to create a small current in the armature, which then passes through the winding upon the magnets, and strengthens them sufficiently to induce more current in the armature; thus the machine on being "started up", - that is to say, driven by an engine, - gradually "builds up". With dynamos having a Urge quantity of iron in their magnets, the time required to magnetize the poles and "build up" the machine is very appreciable.
A question often arises in the student's mind as to how it is possible for the dynamo to commence work, seeing that the magnetism of the poles is dependent on the current in the armature, and the current in the armature on the magnetism of the poles; for, as above described, to become magnetic, iron must have a current passed round it. How then is the dynamo to start itself?