Letter 24 By Wire and by Radio
Dear Boy:
The simplest wire telephone-circuit is formed by a transmitter, a receiver, a battery, and the connecting wire. If two persons are to carry on a conversation each must have this amount of equipment. The apparatus might be arranged as in Fig. 129. This set-up, however, requires four wires between the two stations and you know the telephone company uses only two wires. Let us find the principle upon which its system operates because it is the solution of many different problems including that of wire-to-radio connections.
Imagine four wire resistances connected together to form a square as in Fig. 130. Suppose there are two pairs of equal resistances, namely R1 and R2, and Z1 and Z2. If we connect a generator, G, between the junctions a and b there will be two separate streams of electrons, one through the R-side and the other through the Z-side of the circuit. These streams, of course, will not be of the same size for the larger stream will flow through the side which offers the smaller resistance.
Half the e. m. f. between a and b is used up in sending the stream half the distance. Half is used between a and the points c and d, and the other half between c and d and the other end. It doesn’t make any difference whether we follow the stream from a to c or from a to d, it takes half the e. m. f. to keep this stream going. Points c and d, therefore, are in the same condition of being “half-way electrically” from a to b. The result is that there can be no current through any wire which we connect between c and d.
Suppose, therefore, that we connect a telephone receiver between c and d. No current flows in it and no sound is emitted by it. Now suppose the resistance of Z2 is that of a telephone line which stretches from one telephone station to another. Suppose also that Z1 is a telephone line exactly like Z2 except that it doesn’t go anywhere at all because it is all shut up in a little box. We’ll call Z1 an artificial telephone line. We ought to call it, as little children would say, a “make-believe” telephone line. It doesn’t fool us but it does fool the electrons for they can’t tell the difference between the real line Z2 and the artificial line Z1. We can make a very good artificial line by using a condenser and a resistance. The condenser introduces something of the capacity effects which I told you were always present in a circuit formed by a pair of wires.
At the other telephone station let us duplicate this apparatus, using the same real line in both cases. Instead of just any generator of an alternating e. m. f. let us use a telephone transmitter. We connect the transmitter through a transformer. The system then looks like that of Fig. 131. When some one talks at station 1 there is no current through his receiver because it is connected to c and d, while the e. m. f. of the transmitter is applied to a and b. The transmitter sets up two electron streams between a and b, and the stream which flows through the Z-side of the square goes out to station 2. At this station the electrons have three paths between d and b. I have marked these by arrows and you see that one of them is through the receiver. The current which is started by the transmitter at station 1 will therefore operate the receiver at station 2 but not at its own station. Of course station 2 can talk to 1 in the same way.
The actual set-up used by the telephone company is a little different from that which I have shown because it uses a single common battery at a central office between two subscribers. The general principle, however, is the same.
It won’t make any difference if we use equal inductance coils, instead of the R-resistances, and connect the transmitter to them inductively as shown in Fig. 132. So far as that is concerned we can also use a transformer between the receiver and the points c and d, as shown in the same figure.
We are now ready to put in radio equipment at station 2. In place of the telephone receiver at station 2 we connect a radio transmitter. Then whatever a person at station 1 says goes by wire to 2 and on out by radio. In place of the telephone transmitter at station 2 we connect a radio receiver. Whatever that receives by radio is detected and goes by wire to the listener at station 1. In Fig. 133 I have shown the equipment of station 2. There you have the connections for wire to radio and vice versa.
One of the most interesting developments of recent years is that of “wired wireless” or “carrier-current telephony” over wires. Suppose that instead of broadcasting from the antenna at station 2 we arrange to have its radio transmitter supply current to a wire circuit. We use this same pair of wires for receiving from the distant station. We can do this if we treat the radio transmitter and receiver exactly like the telephone instruments of Fig. 132 and connect them to a square of resistances. One of these resistances is, of course, the line between the stations. I have shown the general arrangement in Fig. 134.
You see what the square of resistances, or “bridge” really does for us. It lets us use a single pair of wires for messages whether they are coming or going. It does that because it lets us connect a transmitter and also a receiver to a single pair of wires in such a way that the transmitter can’t affect the receiver. Whatever the transmitter sends out goes along the wires to the distant receiver but doesn’t affect the receiver at the sending station. This bridge permits this whether the transmitter and receiver are radio instruments or are the ordinary telephone instruments.
By its aid we may send a modulated high-frequency current over a pair of wires and receive from the same pair of wires the high-frequency current which is generated and modulated at the distant end of the line. It lets us send and receive over the same pair of wires the same sort of a modulated current as we would supply to an antenna in radio-telephone transmitting. It is the same sort of a current but it need not be anywhere near as large because we aren’t broadcasting; we are sending directly to the station of the other party to our conversation.
If we duplicate the apparatus we can use the same pair of wires for another telephone conversation without interfering with the first. Of course, we have to use a different frequency of alternating current for each of the two conversations. We can send these two different modulated high-frequency currents over the same pair of wires and separate them by tuning at the distant end just as well as we do in radio. I won’t sketch out for you the tuned circuits by which this separation is made. It’s enough to give you the idea.
In that way, a single pair of wires can be used for transmitting, simultaneously and without any interference, several different telephone conversations. It takes very much less power than would radio transmission and the conversations are secret. The ordinary telephone conversation can go on at the same time without any interference with those which are being carried by the modulations in high-frequency currents. A total of five conversations over the same pair of wires is the present practice.
This method is used between many of the large cities of the U. S. because it lets one pair of wires do the work of five. That means a saving, for copper wire costs money. Of course, all the special apparatus also costs money. You can see, therefore, that this method wouldn’t be economical between cities very close together because all that is saved by not having to buy so much wire is spent in building special apparatus and in taking care of it afterwards. For long lines, however, by not having to buy five times as much wire, the Bell Company saves more than it costs to build and maintain the extra special apparatus.
I implied a moment ago why this system is called a “carrier-current” system; it is because “the high-frequency currents carry in their modulations the speech significance.” Sometimes it is called a system of “multiplex” telephony because it permits more than one message at a time.
This same general principle is also applied to the making of a multiplex system of telegraphy. In the multiplex telephone system we pictured transmitting and receiving sets very much like radio-telephone sets. If instead of transmitting speech each transmitter was operated as a C-W transmitter then it would transmit telegraph messages. In the same frequency range there can be more telegraph systems operated simultaneously without interfering with each other, for you remember how many cycles each radio-telephone message requires. For that reason the multiplex telegraph system which operates by carrier-currents permits as many as ten different telegraph messages simultaneously.
You remember that I told you how capacity effects rob the distant end of a pair of wires of the alternating current which is being sent to them. That is always true but the effect is not very great unless the frequency of the alternating current is high. It’s enough, however, so that every few hundred miles it is necessary to connect into the circuit an audion amplifier. This is true of carrier currents especially, but also true of the voice-frequency currents of ordinary telephony. The latter, however, are not weakened, that is, “attenuated,” as much and consequently do not need to be amplified as much to give good intelligibility at the distant receiver.
In a telephone circuit over such a long distance as from New York City to San Francisco it is usual to insert amplifiers at about a dozen points along the route. Of course, these amplifiers must work for transmission in either direction, amplifying speech on its way to San Francisco or in the opposite direction. At each of the amplifying stations, or “repeater stations,” as they are usually called, two vacuum tube amplifiers are used, one for each direction. To connect these with the line so that each may work in the right direction there are used two of the bridges or resistance squares. You can see from the sketch of Fig. 135 how an alternating current from the east will be amplified and sent on to the west, or vice versa.
There are a large number of such repeater stations in the United States along the important telephone routes. In Fig. 136 I am showing you the location of those along the route of the famous “transcontinental telephone-circuit.” This shows also a radio-telephone connection between the coast of California and Catalina Island. Conversations have been held between this island and a ship in the Atlantic Ocean, as shown in the sketch. The conversation was made possible by the use of the vacuum tube and the bridge circuit. Part of the way it was by wire and part by radio. Wire and radio tie nicely together because both operate on the same general principles and use much of the same apparatus.