A very important source of sustained high frequency oscillations in wireless telegraphy and telephony of more recent origin is the three electrode high vacuum tube to which more ex tended reference will be made herein.
A great advantage of sustained oscillations in wireless telephony and wireless telegraphy also is that the property of resonance may be more fully availed of than when more or less intermittent oscillations are employed. It may be noted that in wireless telegraph and wire less telephone operation the receiving appa ratus is practically the same. In wireless teleg raphy the full effect of the radiated waves from maximum to zero is available, whereas in wireless telephony only a small portion of i the emitted wave energy is available, namely, that due to the modulation of the wave energy caused by the action of the telephone trans mitter, estimated to he about 5 per cent of the total energy radiated. This, however, in view of the amplifying powers of the high vacuum tube detector is not now of so much import ance. See TELF.GRAPHY, WIRELESS.
De Forest Wireless Telephone.— In Fig. 2 is outlined an arrangement of the De Forest wire less telephone system employed in 1909 and pos sibly prior thereto. A singing arc a is em ployed as the source of sustained high fre quency oscillations. C is a variable condenser, L is a transformer or tuning coil, the secondary soil s of which is in series with the aerial wire A, and a microphone transmitter M. a is an arc with copper-carbon electrodes, burning in the flame of an alcohol lamp m. The arc is supplied with a 220 or 440 volt direct current supplied by a generator indicated at G. K K are choke coils, or inductances, in the supply circuit, used to shut out the high frequency oscillations of the arc from the generator. The arc a sets up in the circuit a, C, p sustained oscillations that are radiated as electric waves from the aerial wire A. Speech spoken into transmitter Al modulates the amplitude of these waves practically in a manner equivalent to the action of the voice currents in the Van Rysselberghe simultaneous system of telegraphy and telephony upon the telegraph currents therein. There is, of course, the difference that in the latter case it is the slow telegraph impulses of current that are modulated by the voice currents, whereas in the case under con sideration it is the high frequency radio cur rents that are modulated by the voice currents. It is now the practice to term the modulated high-frequency waves employed in wireless telephony, and it may be added, in wire tel ephony also, carrier waves, since they virtually carry the voice waves.
The receiving circuits of the said De Forest telephone system are shown in Fig. 3. A is the aerial, L is the tuning coil, C is a variable condenser. The detector employed is the De Forest audion. (See TELEGRAPHY, WIRELESS). comprises a tantalum lamp filament F, a grid G of No. 22platinum wire and a small platinum plate P. all within an exhausted bulb
b, resembling a six-volt incandescent lamp. The filament is lighted or heated by a three-cell storage battery B, the current strength of which is regulated by a variable resistance r. P B is a potential battery of 10 to 30 volts, adjustable as to potential by a potentiometer, not shown. One terminal of the oscillation circuit L, C is connected to the grid, the other terminal to the filament, as in the figure.
When the heating battery and the potential battery are properly adjusted the audion is very sensitive and produces amplified sounds in the telephone receiver. The incoming high frequency, or radio sustained oscillations, as stated, do not perceptibly affect the telephone receiver owing to their regularity, uniformity of amplitude and high frequency, but the modu-L lations of the amplitude of the incoming audio oscillations due to the microphone transmitter, being within the range of the human ear, are heard in the telephone receiver as articulate speech.
Wireless Telephone Transmitters.— The transmitters for this work necessarily require high current strength in the transmitter circuit. This develops heat in the carbon granules of the microphone transmitter, which leads to packing of the carbon. This impairs or stops the transmission of voice waves.• The packing can he broken up by tapping the transmitter at suitable intervals, and De Forest provides a special device for this purpose. In other cases a device consisting of a number of tubes leading, from a single mouthpiece to a numb9r of transmitters, all acting upon the one circuit, is employed. Fessenden uses a special trans mitter for this work, by means of which a current strength of 15 amperes is. modulated successfully. For aeroplane service during the war transmitters of special design were found to be necessary if successful results were to be obtained. In this service, besides difficulties due to packing, precautions had to be taken to annul engine, propeller and other external noises. To this end the diaphragm of one type of transmitter is so constructed as not to re spond readily to air vibrations below 200 or above 2,000 per second. In another type a perforated plate is interposed between the speaker's mouth and the diaphragm of the transmitter, which device baffles the movement of extraneous sound waves toward the dia phragm while the voice waves spoken directly into the mouthpiece are not impeded. Still another device employed in this service con sists in leaving the back of the transmitter open in such a way that exterior noises impinge on the back and front of the diaphragm of the transmitter with equal strength, effecting no result, whereas the voice waves hit the dia phragm in one direction only and thus set tip the required variations in the transmitter cir cuit, even although at such times the pilot or observer cannot hear his own voice.