CRYSTAL SETS

Sun (Auckland), Volume II, Issue 463, 19 September 1928, Page 6

CRYSTAL SETS

The crystal receiver employs the simplest of all wireless circuits. It is easy to build and simple to operate, and its upkeep costs are practically nil. It will give good headphone strength up to 20 or 30 miles away from a high-powered broadcasting station providing that a good aerial is used. Its chief disadvantage as compared with the valve receiver may be summed up as follows: It will not successfully operate a loud speaker, and it is rather difficult to tune out undesired stations. Furthermore, most types of crystals require frequent adjustment, which, in addition to the aforementioned interference, rather tends to mar the enjoyment of the programme. Interference, however, may be greatly reduced by carefully following these instructions: Pay particular attention to the aerial and earth, as recommended. Bear in mind that both signal strength and interference increase in proportion to the height of th© aerial. Decreasing the height will reduce interference to a greater extent than signal strength. Therefore, city dwellers should keep their aerials as small as possible, consistent with comfortable reception. Before building a receiver, select a circuit employing both aerial and secondary coils tuned by variable condensers. These two coils should be kept as far apart as possible when interference is experienced. This will make the tuning sharper, and interfer-

ence from undesired stations will be reduced. The crystal proper plays a very important part in the reception of wireless signals. Briefly, it converts the feeble alternating currents into unidirectional impulses, permitting these to have an audible effect in the headphones. Crystals may be divided into two distinct classes. The first type, commonly known as “perikon” combinations, requires a second crystal to be in contact with it. The second class are those crystals requiring a metal contact. Crystals such as tellurium and zincite; zincite and copper pyrites; zincite and bornite come under the first class. The perikon combinations are not popular among broadcast listeners, so we will pass on to the second type without further mention of them. In the second class there are galena, iron pyrites, carborundum, molybdenite and silicon. Both silicon and carborundum require a steel contact; galena will work equally well with gold, silver, brass or copper contacts. Carborundum is the most reliable crystal, but it requires a three-volt battery and a potentiometer for successful operation Galena, whether synthetic or natural, is one of the most popular crystals. It is extremely sensitive, but it requires a very light contact with the cat’s whisker, and therefore is liable to be easily thrown out of adjustment. However, the synthetic types of galena do not suffer from this defect to the same extent as do the natural types. Crystals should on no account be handled with the fingers. If they become insensitive, they may be cleaned by immersing them in alcohol for about ten minutes, and then allowing them to dry.

Headphones used in conjunction with crystal sets should be of the high resistance type (3,000 or 4,000 ohms), and when two or more pairs are used they should bo joined in parallel. In order to join two or more pairs in parallel, take one phone tip from each pair of phones and securely fix them to one of the phonfe terminals of the receiver. The remaining tips are connected to the other phone terminal. Finally, when using several pairs of headphones, make quite sure that they are all of the same resistance, otherwise the lower resistance telephones will have more than their share of the current.