The telephone system of the British post office. A practical handbook (1901)
Herbert, Thomas Ernest · London, New York, Whittaker and Co. · leaf 89
he short spring is in contact with the other inner spring. These switch-springs are termed five-point switch-springs, the fifth point being thebody of the switch-spring, which is occasionally employed and serves toconnect the body of the switch-spring to the long spring.. 74 Switch-Springs, The… … Figure 64. The central spring carries an ebonite projection with a steel ball—restingin the body of the switch-spring. The insertion of a peg raises the ball,thus lifting the central spring from the lower to the upper spring. The Telephone System of the British Post Office. 75 CHAPTER XII.…
Telephone · Postal service
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