By reason of its simplicity of operation, the 'push-button magneto' (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 revolving of an armature within a magnetic field; in the former, the current was produced by pushing the button on the face of the instrument, 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:
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DEVELOPMENT IN TELEPHONE SERVICE
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 without 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.
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