TELEPHONE No. 72
Strowger Automatic System
Pedestal table telephone made by Automatic Electric in Chicago, probably assembled over here from piece parts at the Edge Lane factory of ATM in Liverpool. It is the phone used by the GPO on the two auto exchanges introduced in 1912 (Epsom, Surrey and the Official Switch [i.e. Post Office Headquarters] in London).
Telephone No. 72 was used on ATM's automatic systems and the Telephone No. 72A was used at Epson and the GPO Official automatic system only. Both phones used a Bellset No. 8.
The very first phones used had no numbers on at all, but from 1913 the phones had 72 stamped on them.
The Telephone No. 72 has a Mercedes dial, a body with a stepped bottom part made of brass and an electro-magnetic receiver. Circuitry is pure Chicago and not customised for Britain. It has no induction coil and just uses a simple DC series circuit with the transmitter and receiver in series with the dial pulse springs. The receiver is shunted to stop tinkling, but NOT the bell! It has a bias spring to stop this, and the resistive part of the bell in series with a capacitor, acts as the spark quench for the pulse contacts.These are the component parts (taken from the GPO Rate Book 1928)
Telephone No. 72:-
Transmitter No. 1 (Mouthpiece No. 4)
Receiver Bell No. 4A
Dial No 1 or Dial No. 8FA
Hook Receiver F
Telephone No. 72A:-
The Receiver Bell No. 4A has no permanent magnet. The American make has a Diaphragm No. 9 and an Earpiece No. 4, whilst the Mark 234 has a Diaphragm No. 12 and an Earpiece No. 2.
The diagram for the Telephone No. 72 (Issue C) of the diagram (AT12) is dated November 1918 (the N172 diagram is dated December 1919). The Telephone No. 72 was superseded by the Telephone No. 124 when the Dial No. 8 was adopted as standard in Nov 1922 (according to Engineer in Chief's diagram 'Misc 623 - Automatic Telephones, Existing and Proposed Dial Spring Assemblies and Telephone Circuits' - dated 17.9.24). The diagram states 'used at all ATM Co's exchanges until adoption of Telephone No. 124 in Nov 1922.
Used with a Bellset No. 8.
Superseded by the Telephone No. 150.
Known in Australia as the Geelong phone because exactly the same phones were used at Geelong in 1914 (first auto exchange in Australia).
Last revised: December 31, 2017