ELECTRICITY AS APPLIED TO RADIO

Graphic illustration of the wave length formula—Apparatus for producing electric waves—Analogy of the swinging door—Oscillations—Inductance and capacitance-—-Production of electric waves—Open and closed circuits—Harmonics—Theory of radio reception—Telephone receiver—Rectifying oscillations with a mineral detector—A complete receiving circuit—Radio frequency and audio frequency—Wave groups.

We have already, in a brief way, given some analogies of radio transmission and reception, and have taken up the more important principles of electricity, so that if we apply this little knowledge of electricity to the theory of vibrations which we have discussed in Chapter II, we shall be able to understand the principles of radio telegraphy and telephony and to operate the apparatus intelligently.

Science has computed the speed of electric waves at 186,000 miles per second, or 300,000,000 meters per second. Let us assume that we have a wave 300,000,000 meters long. Such a wave would pass between two points 300,000,000 meters apart in one second. The frequency of this wave would then be one cycle per second. A graphic description is shown in Fig. 22. Now, if the waves were 150,000,000 meters long, then two waves would be required to pass between two points 300,000,000 meters apart, and the frequency of the waves would be two cycles per second.

We described briefly before, the principle of the alternating current generator. The average alternating current generator that is used for house current has a frequency of 60 cycles. The alternator used for producing musical notes in spark radio transmission usually has a frequency of 500 cycles. The Alexanderson alter- nator has a frequency of as high as 200,000 cycles per second. Such an alternator is said to produce radio frequencies and is used as a direct source of ether agitation. All frequencies above 15,000 per second are called radio frequencies, and those below 15,000 are called audio frequencies.

When we press down key K, a current passes through primary P, inducing high voltage current into S. Condenser C consists of a piece of plate glass with a sheet of tin foil on either side. (See Fig. 27.) The high voltage produced in S causes a current to flow to both plates of the condenser, and .this charge is deposited thereon. By carefully studying Fig. 26, you will see that there must be a positive charge of electricity on one side of the condenser, and a negative charge on the other side. When the charge on the plates of the condenser becomes great enough, a severe stress is caused and the positive charge seeks to complete the circuit by flowing toward the negative charge.

 In the analogy of the swinging door, the straining of the spring, when the door is held open, corresponds to the stress in the condenser. Owing to the fact that the glass dielectric is a nonconductor of electricity, the current cannot flow through the condenser, so it seeks to go back through the primary in the opposite direction to which it came. But this it cannot do, for the lines of force that are produced in the secondary field set up a counter E.M.F. and resist the return flow of the current. However, the air between the electrodes in the spark gap G becomes ionized under the high potential. The air thus ionized becomes a conductor of electricity, and the circuit is completed by a spark discharge from the condenser across G and through L.

Here we have practically the same circuit as in Fig. 26, but we include an aerial A and a ground E connection. The aerial is really one plate of a condenser: the ground, the other; and the air between is the insulating material or the dielectric. Besides having capacity, the aerial also has inductance. (You will recall that on page 24, we found that when a current is passed through a wire, a magnetic field is formed about it.) This system of aerial and ground, acting as both capacitance and inductance, and covering a comparatively great area, acts as a radiator of the oscillations produced therein. The additional capacity and inductance, represented in the circuit by C and L, are to enable a varying of the oscillating value of the aerial, so that we are not dependent on the "natural period" of the aerial for our wave length.

On page 36, we likened an oscillation circuit to a swinging door. We noted that the capacity of an oscillating circuit was analogous to the spring in the door and that the inductance was analogous to the inertia. The inertia of the door is dependent upon its weight. The frequency of the swinging door is dependent on the physical dimensions of the spring and the door. Likewise, the number of swings or the frequency of the oscillating circuit in a given time is dependent upon its electrical dimensions, that is, the amount of capacity and inductance in the circuit. If either the ca- pacity or the inductance, or both, are increased, the frequency of the circuit will be reduced and conse- quently the wave length will be increased.

If, in Fig. 32, the capacity and inductance of circuit A, which is the same as the closed circuit in Fig. 26, is given a value, so that the circuit will oscillate at 1,500,000 cycles per second (200 meter wave length) and the aerial or open circuit B has the same value of inductance and capacity, then the oscillations set up in circuit A will cause similar oscillations to be set up in circuit B. (Note experiment with tuning fork on page 15.) Circuit B being a good radiator will radiate waves of 200 meters in manner shown in Fig. 30.

It is a well-known fact that, in music, few instruments produce absolutely pure notes; they all have overtones or undertones. These overtones and undertones are feeble vibrations set up, which produce harmonics of the fundamental note. Overtones and undertones add richness to the tone and are desirable up to a certain point. A pipe organ is a good example of overtones and the richness of tone so produced, and the tuning fork, of a pure note and the insipidness of the same.

Similarly in radio transmission, we have waves produced other than those to which our set is tuned. These wave lengths are multiples of the principal wave length. Thus a poorly designed transmitter having a principal wave of, say, 600 meters, will interfere with other stations near by on three hundred meters. A transmitting station sometimes radiates two wave lengths, one of which is not necessarily a harmonic of the principal wave. This occurs frequently when a circuit similar to Fig 29 is used. By using a loose-coupled circuit, such as Fig 32, this unwanted wave length can usually be eliminated. Using a circuit such as in Fig. 32 the wave radiated will be much sharper than if the circuit in Fig. 29 were used. This prevents undue interference with other stations.

In receiving, also, the sharper tuning can be had with the loose-coupled circuits. We will now take up the general theory of radio re- ception. A radio receiver is the counterpart of a transmitter, just described, except that it has means of making audible the vibrations received from the distant transmitter. A receiver has an aerial and a variable in- ductance and capacity, so that it can be tuned readily to any desired wave length.

If we are receiving the oscillations from a 200 meter wave, the surge back and forth at the rate of 1,500,000 per second is far beyond the range of audibility. This high frequency alternating current must be transformed into a direct current so that it will produce an audible signal in the telephone receiver. Certain minerals have the ability to conduct a current of electricity in only one direction. These minerals are called rectifiers and are used as detectors in radio. There are other rectifiers which are used in radio, which we shall discuss in a later chapter. Among the rectifying minerals are carborundum, molybdenite, silicon, galena and numerous others. Of these, galena is probably the most sensitive. In Fig. 34 we have a typical mineral detector re- ceiving circuit, and we shall follow the incoming signal and see how it is made audible.