Radio broadcast .. (1922-30)

5OO Radio Broadcast One of the most common forms of crystal detector is a piece of galena and a steel point just touching it. The steel point is adjusted until signals are heard best in the receivers. When this is done, the perfect turnstile is more nearly realized. How near this is will be understood when tests have shown that none of the known detectors has an efficiency over 1 5 per cent. (This is eliminating any amplification or regenerative effects accomplished with vacuum tubes.) Just how the turnstile effect is produced in a crystal detector has never been clearly shown. If it were, it might be possible to raise this percentage for detectors. The fact that many investigators have spent years of study on the problem, and the fact that many thousands of conceivable combinations have been tried would seem to indicate that new investigators would have a rather difficult job improving on the work that has already been done. Late experiments indicate that the crystal detector action is caused by the thin film of moisture present on the crystal surface which makes a small battery cell out of the combination of the two dissimilar substances. This small cell has, of course, very small capacity, and the electron current in one direction will aid this cell action, and in the other direction the electrons will practically stop the cell action. As a result, they work on together in one direction and they practically stop in the other. Other investigators consider the effect to be one of heat. The rushing electrons heat the small contact which gives rise to "thermal electricity." Thermal electricity is caused by heating the contact of two dissimilar substances. This thermal current is in one direction only so that the rapid variations across the contact will heat it; and this heat will develop the thermal current which going in one direction will work around through the telephone receiver and give a reproduction of the changes in intensity. CARE OF CRYSTAL DETECTORS SOME practical points may be mentioned in the care of crystal detectors. The first and most important is that the crystal be kept clean. Fingers are the worst offenders. Do not touch the crystal surface. A crystal may be washed with alcohol, or even soap and water. The soap should be thoroughly washed off. The steel point or "cat whisker" should always be clean and fairly sharp. It must not be too sharp, however, as this will cause it to " burn" off too quickly. THE "BY-PASS" CONDENSER MUCH unnecessary mystery surrounds the use of the by-pass condenser used to improve the efficiency of detectors, connected around the telephone receiver. A condenser consists of two conducting metal plates pressed close together with an insulating substance between them. The metal plates will allow electrons to gather on them, but the insulation will keep the electrons from crossing from one plate to the other. There are electrons everywhere. Therefore, in what might be called the state of rest, or "zero" electric pressure, there are still a large number of electrons present on everything, including the two sides of a condenser. Electrons are quite human in not liking to be crowded too much. If more electrons are then forced upon one side, the density of the electrons will, of course, be increased on that side, which means more electric pressure, and there will be a rush of the electrons off the other metal side. For very rapid movements back and forth of electrons, the condenser will therefore act exactly like a straight wire connection, as there will be a motion of electrons into it on one side and out of it on the other. So such a condenser will let such electron vibrations pass through it easily. A good value for a by-pass condenser is one or two one-thousandths of a microfarad. This condenser is connected across the telephone receiver, making it possible for the rapid electron impulses to pass through the turnstile detector quickly and on through the by-pass condenser to ground. They would have much more difficulty in getting through the telephone receiver at this rapid rate, due to the many turns of fine wire of which it is made. " But doesn't this lose all the benefit of the electron action as fall as the telephone receiver is concerned?" No, because although the effect is such as to have a quick motion of electrons along this path through the by-pass condenser, there is no actual motion of electrons from one side of the condenser to the other across the insulation put there to keep them from crossing. So they will pile up on one side. When the impulse has reversed, the trapped electrons cannot return through the turnstile set against them. So, this extra number of electrons has but one