Basic facts you need to know about electricity and magnetism

Electricity from batteries

There are two kinds of electricity, alternating current (AC), the kind that comes from the electricity mains, and direct current (DC), which is the kind that comes from batteries. Every battery has two poles or connections, one positive and one negative. Current drawn from a battery flows continuously in one direction only, hence the old alternative name of ‘continuous current’. Batteries may be subdivided into two classes known as primary and secondary types.

Primary batteries produce electricity from chemical action within ‘cells’ which continues until the chemicals become exhausted and the battery ‘runs down’. There are various combinations of chemicals capable of producing electricity at various voltages per cell, but for our purpose we need only consider the ‘Leclanche’ type of cell used for torch and transistor radio batteries which produces 1.5 V. Two or more cells may be joined in a chain, positive to negative, to obtain any desired voltage for various jobs. Vintage radio sets used batteries of up to 165 V to provide power for the valves. Although they are not ‘dry’ (except when totally run down!) primary cells and batteries are commonly known as dry types to distinguish them from secondary batteries which employ acid or alkaline solutions.

Secondary batteries also employ chemical action within cells to produce electricity but in this case the chemicals have to be activated (‘charged’) first by connecting the cell to an external source of DC voltage for a certain time. When charging is complete the cell will deliver voltage until the chemical action ceases, when it may again be recharged. This process may be repeated many times before side effects within the cell cause it to fail completely. The lead-acid type of cell used in vintage radio sets produces 2 V, and once again two or more may be series connected to provide higher voltages, although most of the sets you are likely to encounter use only a single cell.

Alternative names for a secondary battery are storage battery or accumulator, the latter being the one most generally used in vintage radio.

Vintage radio sets made to operate on batteries usually employed a combination of one or more primary batteries and one accumulator to provide high tension (HT) and low tension (LT) supplies for the valves. Earlier sets also required another source of voltage for the valves to provide grid bias (abbreviated to GB, and of which more later) of up to − 16 V. From 1939 the introduction of a new range of valves known as ‘all-dry’ types made it possible to have receivers working from just two dry batteries without the need for accumulators. However, by far the greatest number of vintage receivers you are likely to encounter were made to operate from mains electricity and to understand how they worked it is first necessary to know the basic facts about how electricity is produced and its connection with magnetism.

Electricity and magnetism

Magnetism was discovered some thousands of years ago when someone tripped over a strange piece of rock, picked it up and found that for some unknown reason it attracted to itself anything made of iron. What was more, if this rock was rubbed along a strip of iron it too started to attract other pieces of iron. Although this was interesting, no practical use for magnetism was found for some time – until around the year 1200 AD, in fact, when someone else discovered that if a magnetised needle was hung on a piece of twine it would spin around and settle into a position where one end pointed north and the other south. Now, this really was useful, because it enabled sailors to travel the world with a sporting chance of reaching their intended destinations and eventually returning home. It also brought about a rather contradictory piece of terminology. Not unnaturally the end, or pole, of the needle that pointed north was called the north pole and the one that pointed south the south pole. However, if two magnets are held close to each other the north pole of one is attracted to the south pole of the other and vice versa, hence the rule that unlike poles attract, like poles repel. Logically, this ought to mean that the pole of a magnet that points north isn’t really its north pole but its south, but as things then start to become rather confusing that part is conveniently overlooked.

As for the rock which started the whole thing off, the old name given to it was lodestone or loadstone; lode/load in this instance coming from a old word meaning a way, hence a rock that shows the way. Nowadays it is more prosaically referred to as a form of iron oxide called magnetite.

For the next seven hundred years or so not much else happened with magnets until electricity began to be studied in the early nineteenth century. It didn’t take long for scientists to find links between electricity and magnetism and thus unwittingly start the journey that would lead to electric light and power, radio communication, party political broadcasts on television and other advantages which we enjoy today. In fact, it’s a fascinating story that is worth investigating in specialist reference books but here we shall have to content ourselves with enough of the gist of it to give the basic principles on which vintage radio sets depend.

What changes an ordinary piece of iron into a magnet is that when stroked with magnetite groups of its molecules called domains, normally pointing in random directions, are lined up to point all the same way. This effect can also be obtained by a different means, as we shall see in a moment.

Magnets produce what is called a magnetic field. An experiment to demonstrate this, once familiar to school boys (perhaps it is still), was to place a magnet under a sheet of thin card, onto which iron filings were scattered, whereupon the latter would obligingly form themselves into exact patterns tracing out what are called the lines of force from the magnet, as in Figure 1.3(c).

The sort of magnets we have been talking about are known as permanent magnets because once they have been magnetised by some means they remain so more or less indefinitely. Now for another type of magnet. If a length of wire is wound around a cardboard tube to make a coil, and its two ends connected to a battery, a magnetic field is set up in and around it. This can be demonstrated by placing a compass alongside the coil, when its needle will be deflected when the battery is connected and will remain so until it is disconnected. If a six-inch nail is placed inside the coil, when the battery is connected it will become magnetised because the lines of force from the coil will have lined up its molecules. The nail will remain magnetised for as long as the current flows through the coil but will revert to just an ordinary piece of iron when the battery is disconnected. This type of magnet is known as an electromagnet and it too produces lines of force that may be plotted with the aid of iron filings.

Because a coil will induce magnetism in this way it is said to possess inductance, the unit of which is the henry, taken from the name of a scientist as are so many electrical and radio terms. Coils of any desired inductance may be produced by varying the number of turns and the gauge of wire used – and the material on which the coil is wound, as we shall see a little later.

That is one aspect of the relationship between electricity and magnetism. Here is another. If that same coil of wire is passed at right angles through the magnetic field of a permanent magnet a voltage of brief duration will be induced into it, sufficient to be indicated on a sensitive electrical instrument called a galvanometer, which will respond to very small voltages or currents. If some means is found of maintaining constant movement of the coil the voltage will be maintained. In practice this is carried out by having the coil wound on what is called an armature which is mounted on bearings and free to rotate within the poles of a powerful magnet. This is the basis of the dynamo or generator which, when driven mechanically, will produce a steady current of electricity.