TEE MAGNETO TELEPHONE 129 



short rings, and, although the bells at all stations on the line ring whenever the genera- 

 tor is turned at any one of them, yet, by employing a code, only that station responds 

 whose code signal is sounded. The polarized bell consists of two coils of fine wire, 

 (see L,Ll Fig. 96) with soft iron cores held between -the poles of a U-shaped per- 

 manent magnet, NIS by being attached to S. This has the effect of prolonging the S 

 pole so that the opposite ends of the coil cores have S polarity. Across one end of 

 this pair of coils is a soft iron armature F pivoted at its centre by the screw E. To 

 its centre is attached a slender rod H ending in a small metal ball between the two bells 

 K,K1. Armature F under the influence of the adjacent magnet acquires S polarity 

 at the centre and N polarity at each end. The armature in this condition is attracted 

 equally to both cores and rests against one or the other as it may happen to stop. The 

 coils, however, are connected in series and wound in opposite directions. If, then, a 

 current passes through them it will tend to increase the strength of one pole, and to 

 decrease the strength of or change the polarity of the other. Thus, if L is strengthened, 

 LI will be weakened and the armature F will be attracted to L, causing the clapper 

 II to strike the bell Kl. However, the current being an alternating one, the changes 

 in the strength of the poles will take place first on one side and then on the other alter- 

 nately as the current moves, first in one direction and then in the other. As already 

 stated this takes place fifteen or more times per second, according to the frequency of 

 the alternations of the current from the generator. The clapper H therefore is caused 

 to vibrate very rapidly, striking the bells at each vibration and causing an almost 

 continuous ringing sound. 



For bridging service as used on forest lines, ringer coils are wound to a resistance 

 of 1,000, 1,600 or 2,500 ohms. The standard Forestry Branch instruments and exten- 

 sion bells have 2,500-ohm ringers. Such high-resistance ringers are employed on 

 heavily loaded lines for the purpose of preventing the escape of current through the 

 ringer circuit of the numerous instruments attached to the lines. These high-resis- 

 tance coils, which are generally bridged permanently across the line, are wound so 

 as to give a very high magnetic retardation, and, although readily operated by the 

 low-frequency currents from magneto generators they are effectual blockades against 

 the high-frequency current of the talking circuit. The higher resistance also is a con- 

 siderable aid in signalling, since it operates to reduce the amount of current that is 

 diverted to the ground at each instrument and therefore increases the number of 

 instruments that may be rung on a given line. It must be noted in this connection, 

 however, that all ringers on the same line must be wound to the same resistance, as 

 otherwise the low-resistance ringers will tend to prevent the others from receiving 

 the proper amount of current. 



In some instruments the ringer is thrown out of the circuit while the telephone 

 is being used for talking. This is not the case, however, in the standard Forestry 

 Branch instruments. But, although the ringers in these instruments remain perman- 

 ently bridged across the line at all times, their high resistance effectually bars out the 

 talking current, so that this current passes through the receiver to the ground in the 

 instruments in use and not through the other path offered through the ringer coils. 



It will also 'be noted in the diagram of the wiring of the standard telephone 

 (Fig. 97) that when the generator of any station is being used to ring another station, 

 two paths are available to the current, one through the line wire to the distant station, 

 thence to the ground, and back to the generator whence it came; the other through 

 the ringer of the calling station itself. By this means a portion of the ringing current 

 is shunted through its own bell and this bell is rung the same as are all other bells 

 on the line. The advantage of this arises from the fact that the ringer is particularly 

 liable to trouble due to several causes. It may, for instance, fail to ring owing to 

 improperly adjusted bells or armature, or to coils being burned out by lightning, or to 

 faulty connections in the ringer wiring, or to a short circuit on the line .or in the 

 protector. Such trouble is immediately noted as soon as an attempt is made to ring 

 a distant Nation when the bells are permanently connected across the line: 



792119 



