What you see here is called a Western Electric Type 60 AP Selective Signaling System or sometimes called a Model 60 AP Selector.
It was a gift from a friend of mine in Illinois after he saw my fascination with the device. He has an extensive knowledge of mechanical telephone systems and a fully working vintage telephone system.
In a nutshell, it was used on railway dispatching telephone circuits for a couple of purposes.
The first was to allow the dispatcher to “selective signal” a particular station by sending a sequence of “reversals” down the telegraph line. The wheel on top is “programmed” by inserting a pin at certain positions that correspond to the unique code or number of that station.
In the picture above, the code is 4-6-7. To reach this device, the dispatcher issues four reversals and then pauses which causes the wheel to rest on the fourth pin. If the pin isn’t there, the wheel spins back to the home position.
Then the dispatcher issues six reversals which causes it to rest on the next pin. Again, if the pin isn’t there, the wheel spins back to the home position.
Finally the dispatcher issues seven reversals causing it to rest on the final pin and thus making the connection to just that selector.
When the transmission is complete, another reversal will cause all of the units on the line to reset to their home position.
Another very important use for the AP selectors was to synchronize all of the stations in the line to the exact same time. By sending a sequence of 22 reversals will set the wheel on a special pin from which one pulse transmitted would ring a buzzer.
Just before the top of the hour, the dispatcher would send this sequence and ring the buzzer to get the operators attention and then ring the buzzer at the top of the hour. The operator would make sure their clocks are all precisely set to the top of the hour thus ensuring that the entire line is now synchronized.
Check out the link below on the Telephone Tribute web site for a far more detailed description of the AP Selector.
Link on the Telephone Tribute site: http://www.telephonetribute.com/railroad_phone_equip.htm
The reason I am fascinated by the AP Selector in that it is a compact and reliable piece of mechanical engineering.
An engineer (or team) had to develop every aspect of every start in the process and do it in the mechanical domain. A pulse pulls this arm, and pushes this wheel and when the pulse stops, if the pin is there, let the spring pull it back and so forth.
Today, the solution would likely be interface some software on to the telegraph, count pulses and do all sorts of things with just a few lines of code.
However many software developers today can get things done by dragging and dropping a module and writing a bit of interface code. Many have forgotten HOW to make things happen.
Maybe the solution is to teach programmers how to do things at the lowest levels so that they understand what is happening at the highest level.