DEVELOPMENT IN TELEPHONE SERVICE 321 



UMi 



Fig. 27. 



A person is shown (Fig. 28) talking to a box telephone, keeping a hand 

 telephone pressed against his ear. It is evident that he can talk or listen with- 

 out removing either instrument, and consequently can carry on a conversation 

 with as much ease and rapidity as if in the presence of the other person. If he 

 is in a noisy place he can, in listening, turn his other ear to the box telephone, 

 thereby hearing what is said with increased loudness, and at the same time 

 shutting out external sounds. All the telephones described above do not require 

 any battery whatever, and for ordinary purposes are all that can be desired, 

 both for loudness and distinctness. 



By reason of its simplicity of operation, the ' push-button mag- 

 neto' (Fig. 29) type of instrument was popular during its brief 

 existence. In construction and operation it materially differed from 

 the crank instrument. In the latter, the current followed the re- 

 volving of an armature within a magnetic field; in the former, the 

 current was produced by pushing the button on the face of the instru- 

 ment, thus ' forcibly detaching a soft iron armature from the poles 

 of a permanent magnet surrounded with coils of insulated wire.' The 

 following instructions were sent with this instrument in 1880: 



To signal the central office, press the black knob firmly twice, turn the 

 switch so as to cut out the stations on the same line beyond, then place the 

 telephone to the ear. If there are two black knobs on the instrument, one above 



vol. lxx. — 20. 



