The Receiver.— Several forms of receiver (hand telephone) employing' wood for the con taining case were produced and placed in pub lic use in the early days. Fig. 12 illustrates one of these. This type was subsequently improved in design and hard rubber replaced wood for the handle and ear-piece. (Fig. 13). Thus far the receiver had one pole-piece and coil placed on the end of a permanent magnet. In 1890 the first type of receiver having two pole-pieces and coils was developed. The bi polar receiver has passed through a long period of evolution, each successive step in the de velopment increasing its talking efficiency or improving the mechanical features of its design. Fig. 14 shows the bipolar receiver in use at the present time in the Bell System. Two per manent magnets are used. To their poles are welded soft iron pole-pieces which constitute the cores of the electromagnetic coils, The supporting cup, of non-magnetic material, is welded to the magnets and cores, thus form ing a rigid unit which is mount ed within a hard rubber case and ear cap. The lat ter clamps the diaphragm firm ly in place close to, but not touch ing, the pole pieces. Every part is made of carefully select ed material, manufactured to extremely accu rate dimensions, and the whole is assembled a n d tested with the utmost care in order that in struments of the greatest attain able efficiency and mechanical strength may be secured. In all, over 50 different types and styles of receivers have been developed for use in the Bell System. Practically all of these have been abandoned in favor of the latest improved type. While great improve ments have been made in the design and effi ciency of the telephone receiver, its funda mental electrical principle is the same to-day as when in vented by Pro fessor 11 e I 1 . Outside the Bell System several differ ent types of re ceivers operat ing on the same principle, but differing some what in details of construction, are used.
The Trans mitter.— In the beginning, the hand receiver type of instru ment was also used as a trans mitter. This was shortly de veloped into the magneto box transmitter. (Fig. 15). Except for very short lines, this magneto type of transmitter was not sufficiently power ful. The solution of the vitally important prob lem of improving this part of the telephone system grew out of the invention of the vari able contact resistance transmitter.
In 1877 Emile Berliner inverted a transmit ter (Fig. 16) having, in contact, two pieces of metal; one stationary, the other capable of be ing vibrated by the voice. The principle was this: with two bodies in contact and forming part of an electric circuit, if the pressure on the contact be increased, the electrical resistance of the circuit is diminished and more current flows, whereas, if the pressure be diminished the resistance increases and less current flows. In the figure, A is a metallic diaphragm capable of being vibrated by the voice. This diaphragm at its centre is in contact with a metal ball C. The essential contact is at a. Vibration of the diaphragm, as explained above, causes corres ponding undulations in the current flowing in a circuit including the contact, a battery, the line and the receiving instrument. In later forms
of the Berliner transmitter, carbon contacts were employed instead of metal.
In 1877 Edison invented a transmitter (Fig. 17) in which the current varying element con sisted of a small disc of carbon (lamp black) placed between two plates so arranged that the vibration of a dia phragm varied the con tact pressure.
In 1878 Francis Blake invented an im portant improvement in, contact or microphone 11 transmitters. He em-i] ployed a platinum point bearing against a hard carbon surface. Fig. 18 is a cross-section of a Blake transmitter. The platinum point K is borne by a light spring A. The hard carbon disc C, mounted in a brass cup W, is supported by a substantial spring S. The platinum and carbon are first brought into light contact with each other after which the platinum point is forced against the diaphragm centre under considerable pressure from spring S. The commercial form of this instrument (Fig. 19) displaced the Berliner and Edison types and was extensively used for a long time as the Bell System standard.
In 1880 an English clergyman, Henry Hun nings, invented a transmitter employing granu lar carbon to secure a large number of points of microphonic contact. The essential feature was a thin platinum diaphragm in contact with a mass of granular carbon enclosed within a chamber having at the rear a plate of metal or carbon in contact with the granular carbon. In 1885 Bell engineers improved the Hunnings transmitter (Fig. 20) by using a horizontal plat inum diaphragm A and a gold-plated electrode C dipping into the granular carbon. While this transmitter operated well when at its best, after being used for some time the large quan tity of granular carbon produced an insensitive condition that seriously affected its efficiency.
To overcome this, A. C. White, in 1890, working in the Bell laboratories, invented the solid back transmitter (Fig. 21) in which the contact element is in the form of a small cham ber (termed the °buttons) containing two car bon disc electrodes; one fixed, the other free to vibrate. These electrodes are separated by a small space partially filled with granular car bon. The fixed electrode is rigidly supported by the solid backing afforded by a stout metal bridge. The movable electrode is firmly at tached to the centre of the diaphragm. The function of the granular carbon is the same as in the Hunnings type but the defects of the latter are overcome. This instru ment has proved so ry perfect 111 115 operation and so in its general design that it has formed the basis of all the best types of trans mitter. A number of modifications from the original design have, how ever, been made to improve its elec trical and mechanical features. Fig. 22 shows the construction of the Mutton° of the modern common battery solid back transmitter. Over 70 different types and styles of trans mitter have been developed and placed in service by the Bell system. Practically all of them have been displaced by the type just described. Transmitters employed by independ ent companies operate on the same - general principle as the type just described and resemble it in appear ance.