Telephones

Radio Record, Volume V, Issue 9, 11 September 1931, Page 9

Telephones

PEAKERS and phones are essential parts of the radio man’s apparatus because they have to perform the very important function of bringing the electrical impulses back to sound impulses. Before sound ean be transmitted it must be converted into an electrical form, and this is the function of the

This is how the change is brought about. From the plate of the last valve in your set flows a current of direct electricity, but, as explained when we were dealing with chokes, mixed with this is the alternating current from the radio station.. If you remove the cap of your telephone, and I dare say every wireless enthusiast has, you would see a strip of thin iron (often stalloy), known as the disphragm. Underneath this are what appear to be two pieces of iron, around which are wound coils of very fine wire. In reality it is a one piece iron magnet bent into a horseshoe shape, and around each leg or arm, whichever you may term it, is a large number of turns of very fine wire. These two coils are connected together, so that in each earphone we have only one big coil. The coils are connected and the two free ends taken to the set. One side will connect with the plate of the valve and another with the battery, so that the battery current must flow through the phones before it can get to the valve which it has to energise. Likewise the alternating current ean get to earth (it is always trying to get there) only by going through the phones and the battery to earth. When this alternating (audio) current passes round the coil of fine wire the alternations of electricity affect the pulling power of the magnet. The result is that as different impulses go through, varying strains are put upon the diaphragm, which moves

in response. The impulses travel so quickly that one cannot see the diaphragm moving. The diaphragm pushes and pulls the air, and so we hear. If we do not want the battery current to be passed through the phones we could, so to speak, short-circuit it by means of an audio choke. You will remember that when we were explaining radio chokes, we said that radio or audio frequency eurrent could not pass through a radio or an audio choke, as the case may be, but it was capable of passing through a condenser. Battery

current, on the other hand, can pass through the choke, but not through the condenser. Now, if we connect up an audio frequency choke, a condenser and phones as shown in the sketch, it ean be seen that the battery current can get to the valve through the choke, but cannot pass throngh the phones because of the condenser. The audio frequency current coming from the last valve cannot pass through the choke, so it is forced through the condenser, through the phones, and to earth. The virtue of this system is that the direct current is prevented from flowing through the phones, damaging the wiring. You will remember‘that the wire is very thin, and your valve, if it is a fairly big one, requires a large amount of current to energise it, and this current passing through the fine wire in your phones would be burnt out. Now you understand why your speaker refuses to go after you change your ordinary general purpose valve for a power valve without installing a filter. A good pair of phones has a high resistance, say 4000 ohms. They shouk be light and comfortable, and should fit closely to the head. Uncomfortable phones take away much of the joy of listening.