The Crystal Set

If you go into a wireless shop and ask bravely for a crystal for your receiver, the assistant will immediately ask you what kind you require. If you say, “What kind do you keep?” he will reel off a list of names as long as you arm. Some of these names you will recognise as those of mineral substances with which you are already acquainted such as silicon, galena, copper pyrites, iron pyrites etc.

You will find, however, that many of them have such names as hertzite, lionite, electronite, and what –not. If you are not experienced in using crystals you can do no worse than choose one with a fancy name, for they are practically all specially treated galena, itself a very good rectifying substance. The special crystals are greatly superior to ordinary galena in one very important point – the number of sensitive spots to be found on their surface.

Sensitive Spots

If you take an ordinary cubical crystal of galena you will find that is very bright and shiny and smooth. On the smooth surface you will be able to find two or three exceedingly sensitive places, but they will take a good deal of finding. The specially treated crystals on the contrary are sensitive on practically every part of their surface. If you examine such a crystal under the microscope, using a low power, you will see that the surface consists of a very large number of tiny crystals, and as each of these tiny crystals has one or two sensitive spots on its surface, you can easily see why the whole substance must have a very large number of sensitive places.

Try Gold Wire

A Note about Carborundum

A crystal which gives the best results when signals are weak is not always the best when we are so situated that our local broadcasting station gives very strong signals, even with a crystal detector.

When strong signals are the rule, carborundum without any battery applied to it (as is usually necessary) will often be found particularly good. The carborundum crystal should be made to press against a steel or brass plate and trials should be made to see which way round in the circuit it works best.

Try the plate connected to the aerial and the crystal to the phones first of all, and then compare the signal reception this way with that given by connecting the crystal to the aerial and the plate to the phones, do not make the mistake of connecting the ‘phones to the aerial side and the crystal to the earth side, for although the circuit may look the same the result will be very different. It is always necessary to place the crystal on the aerial side.

The headphones were then connected to the headphones terminals. Before any signal could be heard, the cat's whisker had to be adjusted so that it touched the crystal in the position which produced a loud click in the headphones. The programme was then tuned in by rotating the tuning knob, and when fixed in on a station the cat's whisker was further adjusted to obtain the loudest sound. The basic four tuning circuits were the slide coil circuit, the tapped inductance circuit, the variometer, and the loose coupled circuit.

The slide coil control was one much favoured. The tuning consists of a slider making contact with the coil which is, wound around a cardboard tube which also acts as the body of the set.

The coil of the Brownie tube crystal receiver consists of eighty turns of twenty-two gauge enamelled copper wire. The diameter of the coil is 3.2 inches and the maximum wavelength which can be received is 530 metres. This simple circuit is however very inefficient due to the so-called ‘dead end' effect whereby part of the coil is not being used for reception, with the result that energy is wasted at the ‘dead end'.

Radial tapped inductance crystal receiver circuit used by Gamage Ltd, for their early- 1922 crystal set. The coil consists of a hundred turns of nineteen gauge double cotton covered copper wire tapped off at various intervals to give a maximum wavelength of approximately 440 metres. For longer wavelengths, a loading coil was put in series with the variable condenser; and when not in use the loading coil terminals were connected together by a short lead, otherwise the set would not function.

For 1600 metres, a No 150 loading coil was used; and for 2000 metres, a No 200 loading coil was used. To tune the set to medium wave band, the switch was slowly moved over the studs until the station required was heard. Then for finer tuning the variable condenser was adjusted.

Perhaps the most popular form of crystal tuning circuit was the variometer. A variometer consists of two series-connected coils where one of the coils is capable of rotating within the other. As they are rotated with a current flowing through them, their magnetic fields either assist or resist one another. This variable magnetic field is known as a variable inductance. The standard variometer circuit was employed in the Western Electric Company crystal set; the aerial was connected to terminal A1 to receive stations whose wavelength was under300 metres, and to the terminal A2 to receive stations whose wavelength was between 350 and 500.

This passed the aerial current through either of the fixed condensers C1 and C2 before entering the crystal detector circuit. Another fixed condenser C3 was placed across the headphone terminals R , as this usually improved reception by cutting out any sudden pulsations of high frequency current.

The typical standard type of valve wireless equipment on sale at this early period consisted of a number of separate items, linked to form a wireless “set”, which in many cases was very complicated to operate because of the large number of controls.

In order to receive a programme, it was necessary to erect an aerial between two masts, either in the garden or fixed on the roof. It was a common site to see a sea of aerial masts swaying high above the ground, especially if one was in a town. 235 / 705 Metres Medium Wave If the listener had no garden or space to Coil erect his aerial, an indoor frame aerial could be used as an alternative.

The aerial connected directly to the receiver whose dials, valves and other components were generally left exposed and mounted on a vertical, horizontal or sloping panel, an arrangement which gave the set an appearance of something out of a science fiction than of a domestic object.

To supply the power, high tension batteries of about ninety volts were needed, and also an accumulator to operate the filaments of the valves. The accumulators had to be kept well charged and this usually meant carting them down to the local garage every two or three months.

If a receiver having two or more valves was used, then there was enough power to drive a horn loudspeaker, and this meant that the whole family could gather around and listen. In those days everybody sat facing the wireless when it was on!) With a less powerful receiver employing only one valve, headphones were connected in place of a loudspeaker. The valve of this time were known as “bright emitters” as their filaments lit up like electric lamps.

Because of the great heat needed to drive them, they were liable to burn out after only a short life span, and as they cost around £1.1s each, they had to be treated with special care (it is known that some amateurs used them to see their way into bed and read by). There was virtually only one type of bright emitter, the “R” valve, whose function was general purpose high frequency amplifier, detector, or low frequency amplifier.

The silvering which occurred on the inside of valves from the mid – 1920s onwards is oxide of magnesium burnt to absorb any traces of gas in the bulb.

Marconi valves made at the Osram lamp works are not a product of the Broadcasting era. They were made long before wireless reached its popularity – and they have lost none of the lead they thus obtained.

However, two companies, neither of them large, had already designed valves differing significantly from the R valve, and they were to be richly rewarded for their farsightedness.

A C Cossor Ltd, of Highbury, North London , descended from a firm founded in 1859 as makers of thermometers and barometers. Since 1895 Cossor had built up a high reputation making specialized electrical glassware, from cathode-ray and X-ray tubes to low vacuum thermionic valves, before playing their part in the production of R valves towards the end of the war. Anyone could see that the valve they launched in the Autumn of 1922 was nothing like an R valve, its electrodes being of totally different shape. Moreover, each electrode was credibly claimed to have a specific advantage: the anode collected the current more effectively, the grid was more rigidly supported so that the valve was less microphonic, and the arched filament did not sag as the valve aged. It sold well, and established Cossor in the first division of contemporary valve makers.

Stanley Mullard's early history was distinctly unpromising. 1910 he landed a good job at a large well-run lamp factory in France , by 1920 the Mullard Radio Valve Co Ltd was registered.

Mullard now applied his unparalleled expertise in the manufacture of R valves to producing a variant of the basic design that would be better engineered yet cheaper.

He made the electrode assembly more rigid by mounting its axis vertical instead of horizontal; this also allowed the envelope to be cylindrical instead of spherical, making the valve easier to pack and less liable to damage in transit.

By the early autumn of 1921 the valve was ready to be launched, and a name had to be found that would commend it to amateur constructors. Mullard consulted Laurence Harley, a young graduate he had just recruited as his first technical assistant, Harley suggested that as the valve amplified, rectified, and oscillated it might be called ‘ARO'. Mullard liked the concept but rearranged the initials , and launched the valve as the ‘ORA'.

The principal advance in receiving valves during the 1920s

Changing the ‘bright emitter' to the ‘dull emitter. The bright emitters tungsten filament was very similar to that of a light bulb. The dull emitters filament needed to reach only a dull red heat to emit copious electrons, so it consumed only a quarter or an eighth as much power.

To the listener, having to get filament-heating accumulators re-charged was both a nuisance and an expense, so manufacturers were under great pressure to produce dull emitter valves. Initially, their filaments were made from tungsten containing a small percentage of thorium oxide; late, these were superseded by still-more efficient filaments coated with the oxides of various metallic elements.

1922.

Competing valves soon appeared that took 0.06A at 3V and ran happily off a pair of dry cell, replacing those that had current 0.25A punishingly high for a dry cell, and the voltage of 1V wastefully low for an accumulator.

A further advance in design was also demonstrated at an exhibition mounted by the National Manufacturers and Traders at the Albert Hall in September 1925, when decorative and cabinet loudspeakers were shown for the first time Sterling's Mellowvox cone loudspeaker was one step away from the horn type. Although still on a stand, it was hornless, using a paper diaphragm instead, which came in ‘four artistic colour combinations of brown, black or mauve, with a gold floral design'. Price £4.4s. Cabinet speakers had highly decorative fretwork grilles, usually in a floral or musical motif, a feature which wasto be carried on well into the following decade.

The General Strike of 1926 caused a nationwide scramble for sets, since newspapers were not being printed, and did much to show the public the importance of broadcasting, and the value of a wireless set as a conveyor of news and information.

In the same year, the first of the self-contained mains sets had appeared on the market. Made by Gambrell Bros. Ltd., they were simply plugged into the electric light socket. It was inevitable that they should attract much public attention since there were no batteries or accumulators to buy, no aerial to erect in the garden.

The whole set was enclosed in an attractive wooden case with internal aerial, speaker and components, and only a few control knobs visible. The set was so simplified that no technical knowledge was required to use it.

The type of speaker employed in these mains sets was of the ‘moving coil' variety, which had been in limited use for a few years but know came into its own.

The moving coil speaker was capable of much better quality than the other types, provided that it was fed with large input of signal energy, and was at the same time capable of handling much larger volumes of sound without becoming overloaded.

It was therefore ideal for use with the comparatively powerful mains receivers.

At about the same time , the introduction of battery eliminators meant that battery sets could be successfully worked off the mains, thus cutting down the constant expense of replacing batteries and charging accumulators.

In many ways 1927 was an important year in the history of wireless . The old British Broadcasting Company became the new British Broadcasting Corporation. The Corporations licensed listeners now numbered over two million, and were served by ten main stations and ten relay stations all over the country.

An industry had been firmly established, so it was inevitable that with this responsibility the smaller Company had to give way to the Corporation.

By this time the crystal set was fast disappearing, as was the horn type loudspeaker. For outings and picnics, portable sets using newly introduced two volt battery triode valves were beginning to take foothold, and the mains sets with their stylish wooden cabinets were being accepted as something for the home, a piece of furniture, and not just a scientific toy.