AMPLIFIERS. The function of an amplifier is to increase the intensity of electrical or other fluctuations without seriously altering their wave form. Amplifiers are used extensively in elec trical communication and also for rendering perceptible sounds and other vibrational waves that could not otherwise be observed.
In telephony the carbon transmitter amplifies the effect of the sound waves at the same time that it converts them into electrical waves. Thermionic amplifiers are inserted at intervals in long telephone lines and are also used to actuate loud-speaking re ceivers. In radio telephony and telegraphy, as well as in carrier current systems, in which the high-frequency currents are trans mitted over wires, amplifiers are used in both the high and low f requency paths. Amplifiers also find application in the reception of submarine cable signals and in submarine and subterranean signalling. Outside the communication field they make possible refined electric and other measurements; they have been used to assist the deaf to hear, to render heart sounds audible, to detect and locate aeroplanes and submarines, and for a variety of other purposes.
The essential features of an amplifier are a source of energy such as a battery, and a means whereby the flow of energy from this source may be controlled by the wave to be amplified in such a way that the variations of the original wave are reproduced upon an enlarged scale in the wave set up by this flow. Almost all of the amplifiers in use up to 191 o were of the type of the telephone transmitter, which depends upon variations in the electrical re sistance of a mass of granular carbon which result from variations in mechanical pressure. When it was desired to amplify electrical variations as at an intermediate point in a long telephone line, a so-called "mechanical repeater" was used. Here the incoming current actuated a telephone receiver, the diaphragm of which agitated the carbon of a transmitter connected with the outgoing line. Various other devices based on changes in resistance with mechanical motion were developed to meet particular needs, notably those of submarine cable telegraphy.
With the beginning of the period under review the amplifying possibilities of electric discharge devices were coming to be recog nized. After a period of experimentation in which a variety of forms were developed, those involving arcs and gaseous discharges gave way to the three-electrode high vacuum thermionic (see THERMIONICS) type variously known as audion, valve, triode, vacuum tube and pliotron. In the thermionic amplifier the cur rent from the battery traverses a path between two electrodes in a highly evacuated glass bulb. The current between these elec trodes consists of a stream of electrons emitted by the hot cathode or "filament" which is heated by current from an auxil iary source. The magnitude of this electron current is controlled by the potential of a third electrode or "grid" situated between the filament and plate and having openings through which the electrons pass. The source of the wave to be amplified is con nected with the grid and filament so that the potential of the grid relative to the filament varies in accordance with the wave to be amplified. Similar variations are thus impressed on the current from the battery. Any external circuit included in the path of this current then receives an amplified copy of the im pressed wave. To secure maximum amplification the amplifier proper is generally connected with the external circuits through special connecting circuits. Transformers are used for this pur pose at voice frequencies and tuned circuits at radio frequencies.
BIBLIOGRAPHY.-E. H. Armstrong, "Some Recent Developments in Bibliography.-E. H. Armstrong, "Some Recent Developments in the Audion Receiver," Proc. I.R.E. (1915) ; I. Langmuir, "The Pure Electron Discharge and its Applications in Radio Telegraphy and Telephony," Proc. I.R.E. (1915) ; J. R. Carson, "A Theoretical Study of the Three-element Vacuum Tube," Proc. I.R.E (1919) ; B. Gherardi and F. B. Jewett, "Telephone Repeaters," Trans. A.I.E.E. (1919) ; N. W. Nichols, "The Audion as a Circuit Element," Phys. Rev. (1919) ; L. De Forest, "The Audion--its Action and Some Recent Applications," Journal Franklin Inst. (192o) ; H. J. Van der Bijl, The Thermionic Vacuum Tube and its Applications (1920) ; H. Bark hausen, "Die Vakuumrohre and ihre technischen Anwendungen," Jahrb. Draht. Teleg. u. Teleph. (1919, 1920, 1921) ; R. W. King, "Thermionic Vacuum Tubes and Their Applications," Bell System Technical Jour nal (1923) ; J. A. Fleming, The Thermionic Valve and its Develop ments in Radio-telegraphy and Telephony (2nd ed. 1924) .
(R. V. L. H.)