The Birmingham age-herald., 1922-04-23
Age-Herald's
 Sunday
CRYSTAL ACTS AS
 VALVE IN RECEIVER.
Calena Performs Important Function by Allowing Cur rent to Flow in One Di-
 rection Only

The simplest radio receiving set is the crystal detector. capable of re- ceiving music and speech for a dis tance of 25 miles under practically all conditions and wireless telegraph code over very great distances.
 In previous articles on this page have described aerials of various types, protective devices for them, and the manner is which electro magnetic waves are produced and pro pagated through the ether. In this article l propose to give a descrip- ton of the action of the crystal de- tector in the receiving set.
 In the first place there are a num- Der of ways in which the crystal de- tector'can be used: the simplest is by placing it in series with the aeral and the ground, with the telephone reeeiv- CTS shunted across 1t. This Is very unsatisfactory method, because 1t does not permit one to tune out any unwanted signals, and the eonse- quence is that a great deal of inter- ference Is obtained.
 Double Circuit Best
 The remaining methods of utilizing the crystal can be roughly summar- ri1ed under the general heading of tuning apparatus" This may con sist of what is called a single cir cuit' or double circuit' the latter being the best of all. The single cir cult may consist of tuning coll, with one or two sliders, or Variometer, or honeycomb coil. In the latter case condenser in series is required d in order to obtain tuning. A double circuit may consist of loose coupler or a Vario- o-coupier.
 Now, this tuning apparatus is re- quifed in order to put the entire re- ceiNing equipment into electrical res onance with the incoming wave This can best be illustrated by using the analogy of two tuning forks. If you get two tuning forks of exactly the same pitch and place them at different ends of room and then strike one, the other will Yibrate in unison with it. If, however. you take tuning fork which has higher or lower ptteh 1t will not respond in any way to the vibrations of the other. This is exactly what happens in the tuninE circuit, and the adjustments that you make put that circuit into the same
 pitch or resonance as that of the incoming wave from the broadeasting station.
 It can be readily seen. therefore that with single circuit which must necessarily include the aeral as close tuning cannot be obtaned as with a double circuit. for this reason: the aerial receives all waves that pass over 1t and the current received is passed to the ground. Those currents that are in the syntony with the aerial circuit will be recorded through the detector in the telephones. Of course IL Is impossible to get 100 per cent efficiency in such tuning, and some of the undesired waves passing over the aerial will be recorded also, despite the fact that they are not of the same wave length as that to which the aerial circuit is attuned.
 Secondary Is Adjustable \ double circuit means the addition of secondary oseillating circuit. Tn the case Of loose coupler. this m cans the secondary inductance and a yaria- ble condenser shunted across it. The secondary circuit Is put into resonance with the primary or aerial circut by adjusting the number of turns in the fnductance and the amount of ca pacity in the condenser.
 It will thus be seen that while the aerial circuit may respond to some of the unwanted waves, the chances of