This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by H. Y. Margary)
It is advisable that a man who wires a building for electric-lighting purposes should have a very fair knowledge of the properties of the forces called magnetism and electricity. The more extensive the wire-man's knowledge of these properties is, the more likely is his work to be thoroughly sound. Many men, however, wire buildings with a very limited knowledge of the theoretical requirements of such work, being guided by simple rules of thumb, their own experience, and the knowledge and experience of those from whom they learnt their trade. It is here purposed to treat wiring from the theoretical as well as from the practical point of view, and for the purpose of facilitating description in the subsequent chapters a few of the simple properties of magnetism and electricity are given.
Simple Magnetic Phenomena To the more practical-minded reader some of the simple phenomena set forth in this chapter may appear foreign to the practical requirements of electric wiring. It must be remembered, however, that the majority of our readers may have practically no knowledge of the properties of electricity whatever or an opportunity of "picking up" such knowledge. The architect, for instance, is rarely possessed of a knowledge of electricity, yet he must from time to time write Specifications to govern the wiring of his buildings, and lacking experience he can only fall back on theory. It is also necessary to mention the various units used in electrical measurements, so that a knowledge of a few elementary facts concerning electricity must be gained before these can be explained.
The peculiar force known as magnetism was first discovered by mankind to be possessed by a mineral form of iron oxide known as magnetite, or more popularly as lodestone. This material possesses the property of attracting small particles of iron, and if a piece of steel is rubbed with a piece of lodestone the magnetic properties of the lodestone are imparted to the metal. If a symmetrical bar of steel be stroked in one direction with a piece of lodestone, and suspended from a fine silk thread, as shown in Fig. 125, and allowed to oscillate, it will be found that it always comes to rest in a direction approximately north and south, for which reason the end of the magnetised bar of steel which, when suspended, turns towards the north is called the north or north-seeking end. The other end is called the south or south-seeking end.
If another piece of steel be magnetised and then suspended near the first magnet, it will be found that the north-seeking end of the former will be attracted by the south-seeking end of the latter, while it will repel the north-seeking end. This last fact is sometimes expressed as follows:- Like ends repel, unlike ends attract one another.
If a magnet formed as described above be placed on a table beneath a sheet of glass, as shown in Fig. 126, and upon the glass fine iron filings be sprinkled, it will be found that these filings arrange themselves in definite lines curving from one end of the magnet to the other. These lines represent the lines of force created by the magnet in the particular plane of the sheet of glass. It can be shown that similar lines exist in every plane passing through the axis of the magnet.
It should be noticed that these lines of force appear to emanate principally from two points P, P, near the ends of the magnet. These points are called poles, the north-seeking end being called the north pole and the south-seeking end the south pole.
The lines of force due to a magnet of the horseshoe form are shown in Fig. 127. The lines of force are regarded as emanating from the north ends of magnets. The space around a magnet in which the lines of force exist is called a magnetic field.
Simple Electric Phenomena If a plate of zinc and a plate of copper be immersed in a vessel containing dilute sulphuric acid (see Fig. 128), and if the plates are connected above the acid by a piece of wire, a circulation of the liquid will be observed from the zinc to the copper plate, while bubbles of gas will collect upon the copper plate. If now a light piece of magnetised steel, suspended from a fine silk thread, be placed near the wire which connects the two plates, it will be seen to set itself in some definite position relatively to the wire, indicating that the wire has become possessed of a magnetic power. This is indeed the case, for apart from the mechanical and chemical action which has been observed, a physical action also occurs, namely, a current of electricity passes through the wire. If the wire be wound into a spiral, as in Fig. 129, it will be found that on the passage of a current of electricity a magnetic effect is produced in the neighbourhood of the wire similar to that produced by the magnet in Fig. 126.
A piece of soft iron placed in the axis of the spiral of wire has the effect of increasing its magnetic properties. The magnetic effect of a spiral is also increased with the number of turns of wire, the increase being in direct proportion to the number of turns.
If the ends of a coil of wire are connected to the -terminals of a galvanometer (i.e. an instrument for indicating the flow of a current of electricity), and a magnet be suddenly placed in the centre of the coil, the needle of the galvanometer will give a swing in one direction, indicating that a current of electricity has been created in the coil by the movement of the magnet. If the magnet be suddenly withdrawn from the coil the indicating needle of the galvanometer will swing in the opposite direction, indicating that a current of electricity has been produced flowing through the coil in the opposite direction to the current produced by inserting the magnet in the coil. It will be observed that the more rapidly the magnet is inserted or withdrawn from the coil the more pronounced is the movement of the galvanometer needle, and therefore the greater is the current causing this movemerit. So long as the magnet is at rest, or, in other words, so long as the coil is not cutting the lines of force of the magnet, no current flows through the coil, but so long as the magnet moves within the coil a current is created. A current may also be created by keeping the magnet stationary, and moving the coil anywhere within the magnetic field in such a manner that lines are cut by the turns of wire. A current produced within a circuit without actual contact with another circuit to which a current is flowing, or by a magnet as just described, is called an induced current, and it is upon this principle of induced currents that innumerable practical appliances such as the dynamo and the transformer depend.