The Induction Coil.— Coincidently with the introduction of the variable contact (micro phone) transmitter came the application of the induction coil to the telephone. Fig. 23 shows the original telephone circuit and two important steps an the evolution of telephony whereby it became possible to talk over greater distances than formerly. In the simple magneto tele phone circuit A, the amount of electrical energy transmitted depended on the ability of the telephone, when spoken into, to generate a current independent of any outside source. This current was limited in strength to the small amount obtainable from the vibration of the diaphragm by the voice. B in Fig. 23 shows a circuit in which the variable contact transmit ters, the receivers, the line and the source of capabilities of the transmitter. In this circuit the transmitter acted as a valve to vary the energy furnished by the battery. C in Fig. 23 shows the use of the induction coil, the latter consisting of a bundle of iron wires surrounded by two separate wire windings. Each transmit ter is placed in series with a battery and the primary winding of an induction coil consisting of a few turns of large wire. The secondary winding of the induction coil, consisting of many turns of relatively fine wire, is connected in series with the line and the receivers. Plac ing the transmitter in a local circuit, independ ent of the line circuit, permitted the use of a low voltage battery and allowed the variation in resistance produced by the transmitter to be large in comparison with the total resistance of the primary circuit. The induction coil operates as a transformer. The relatively large variable current at low voltage in the primary circuit is transformed into a small current at high vol tage in the secondary circuit by reason of the large ratio of the number of turns in the sec ondary winding to those in the primary wind ing. The secondary current traverses the line with less loss than would he the case if the current were of lower voltage but larger.
Signalling Systems.— When the box tele phone was first employed, the custom was to tap on the diaphragm with a pencil as a method of calling the party at the other end of the line. Later this was superseded by a hand-operated hammer mechanism which gave a vigorous blow on the diaphragm that was transmitted telephonically to the station at the other end of the line (Fig. 11). Tap bells were also em energy (battery) were connected together in series. With this arrangement better results were obtained, the extent cif improvement de pending on the voltage of the battery, the con ductivity of the line and the current changing ployed for signalling as were ordinary electric vibrating bells and telegraph sounders. The need for a reliable and efficient method of sig nalling was realized at an early date and Wat son developed a ringer and magneto-generator to meet this need. The ringer (Fig. 24) con sisted of an electro-magnet having a centrally pivoted polarized armature carrying the striker which played between two gongs. Polarizing
the armature made it responsive to the rever sals in polarity of the magnet when alternating current passed- through the coils of the latter. To furnish the alternating current, a small magneto-electric generator (Fig. 25) was pro vided, consisting of a powerful permanent magnet carrying an armature capable of being rotated by a wheel which the calling party turned by means of a crank. Figs. 26 and 27 show, in diagrammatic form, the principles of the latest types of generator and ringer.
Automatic Svvitchhook.-- It was early ap parent that some means should be provided for switching, to and from the line, the signalling and talking circuits at the substation whenever it was desired to do so. At first, hand-operated switches were used for this purpose but it was or ((ringer,x' it was the practice, on party lines, to connect the sets in series, as shown in Fig. 33. To enable the ringer to operate effectively it was necessary to provide it with a large num soon found desirable to make the switching operation automatic, as users would frequently leave the hand-operated switch in the wrong position. To attain this end, a lever switch upon which to hang the receiver was invented by H. L. Roosevelt. The switch was operated by the weight of the receiver. Fig. 28 shows an early set arranged to accomplish the desired purpose. The hook-switch, in its downward position, left the circuit free for the incoming signal; in its upward position, which it assumed when the receiver was removed from it, the necessary contacts for talking were closed. Figs. 29 and 30 illustrate the .switchhook ar rangement for the present telephone. Figs. 31 and 32 show the circuits controlled by the switchhook for a standard magneto and com mon-battery substation circuit respectively.
Series and Bridging Bells.— In the early days, after the development of the magneto-bell, ber of turns of wire. As a result these ringers possessed considerable impedance, i.e., they fered substantial obstruction to the telephone current, seriously affecting the service between any two talking stations. The more ringers in the circuit, the poorer was the talk. To improve this condition, in 1889, John J. Carty, who at that time was known as one of the chief cal experts, and who has since become the rec ognized engineering leader in telephony, in vented the bridging bell. In this arrangement (Fig. 34) the bell, or uringer,P has a high impe dance winding which, while responsive to the relatively low frequency ringing current, does not permit the talking current to pass over the stalled at New Haven, Conn., in January 1878. (Fig. 36). This small board accommodated only a few lines. It did away with the block and plug method of connection, substituting, for the latter, pivoted strips of metal adapted to be rotated into contact with metallic buttons, all ringer circuit, but does allow it to pass over the line from one station to another. It was• this invention which made possible the successful use of party lines.