These symbols are known as detached contact symbols. On schematic diagrams, the relay contacts were moved away from the relay coils and placed where they were functionally utilized in the circuit, which greatly simplified the analysis of the circuit. This approach was developed by the Bell System Labs in the early 1950's. You can read all about it in an article in the Bell Labs Record (BLR) of July 1954, titled "Detached Contact Schematics". You can search online for the Bell Labs Record. Thanks for watching.

Excellent article. Thank you for this reference! 😊

Yes, you are correct. An "in use" trunk grounds its sleeve lead back to the selector bank terminal. This ground operates the E relay in the selector, which in turn operates the rotary magnet. Details can be found in my long video entitled "The Selector in The Step-By-Step Telephone Switching System Part 1", which you can find by searching that title or my channel name "Hicken65" on KZfaq. Thanks for watching.

I believe its 600 hertz amplitude modulated by 120 hertz. Thanks for watching.

@@Hicken65 You are 100% correct. Thanks again for your great video.

Go to the "Telephone Collectors International" website. Click on "TCI Library". Click on "BC Switching Library" and search from there for Schematic Drawings (SD's) and Circuit Descriptions (CD's) for various manufacturers. Good luck with your project and thanks for watching.

@@Hicken65 WOW! What a treasure trove of information!!! Thank you so much. I have also joined as a new member.

Well, I'm glad you enjoyed the video and thanks for watching.

The outside plant cable pairs are subject to a great deal of signal loss due to the amount of capacitance built up along their length. They are also subject to noise interference from external sources, leakage, poor insulation, etc.. Keeping the two wires balanced by inductance coils along their length and at their termination points at the central office minimizes the noise interference and signal losses. As a result, the dial pulses are stronger and cleaner than if an imbalanced line with a single coil winding on one side and direct ground on the other side was utilized. The C relay on the selector monitors the incoming pulses. It is a slow release relay and remains operated while the loop is opening and closing as the pulses are being sent. When all the pulses of a particular digit have been sent, the loop closes and remains closed until the next set of pulses begin to be sent. This interdigital interval allows sufficient time for the C relay to release, which initiates the start of rotary trunk hunting.

Always great to get a comment from a fellow switcher. I have a number of switching videos. Just search "Hicken65 Telephony Videos". Thanks for watching.

@@Hicken65 thank you.

Yes, the audio is half the delight.

Wow I am glad there is no test at the end of it

You sound nice

Crossbar systems were my favorite to work on. Good luck with your search for a switch and thanks for watching.

My demonstration unit was designed to demonstrate how a call progressed through a SXS switching system. When a call is established through it, all 7 switches are tied up for the entire duration of the call. So the demo unit can only have one established call at any given time. In a very large central office, each established intraoffice call would also tie up 7 switches, but there were many thousands of these switches. Each telephone line did not have its own dedicated switch or set of switches.To make efficient use of the switches, there was a great deal of shared access to them. For example, line finders were arranged in groups of up to 20 that had shared access by 200 lines and connectors were arranged in groups of up to 10 that shared access to the same 100 telephone numbers. Between the line finders and connectors were the selectors that shared access trunks paths between the ranks (1sts, 2nds, 3rds, 4ths and 5ths).Typically, SXS systems were provided with sufficient switches such that 7% to 10% of the telephone lines could be in use simultaneously. Each office was engineered to meet specific service criteria regulated by the government. A large office serving 50,000 lines would have around 25,000 switches. If you call your own number you get busy tone.

You're welcome and thanks for watching.

The line finders are arranged in groups of up to 20. Each group serves 200 telephone lines. The 200 telephone lines are physically connected to the bank terminals. Each bank assembly terminates all 200 lines and the terminals are multipled across all 20 bank assemblies. This means that all 20 line finders have the capability to connect to any of the 200 lines.The 20 line finders are shared by the 200 lines. A telephone line does not connect to a line finder until a subscriber makes a dial tone request by taking the receiver off-hook. At that time, the line finder group selection equipment selects and activates a line finder, which moves its wipers vertically and horizontally until they make a connection on the bank assembly to the line requesting service. This connects the line from the bank terminals, through the wipers, through relay contacts in the line finder out to the 1st selector which provides dial tone to the subscriber.

Flashing the switch hook opens and closes the loop through the telephone set in the same way as the rotary dial. The rotary dial does it in a consistently timed fashion and with a particular percent break/make of around 60/40 percent. Flashing the switch hook requires good timing in order to reach the correct number, but is readily accomplished with practice. Any device that can generate the same tones as the touch tone telephone can be used to place a call if the device is close enough to the transmitter. In the "War Games" movie, the pay phone used is a Bell System 3-slot rotary dial payphone arranged for prepay service. This means it would be configured for ground start to get dial tone. The phone is connected by two wires (tip & ring) to the central office (CO). The ring wire is connected to one side of a line relay winding at the CO. The other side is of the winding is connected to negative 48-volt battery. So a ground on the ring wire at the payphone will operate the line relay. An operated line relay initiates a series of events at the CO to supply dial tone to the line. The payphone housing is always grounded to protect users from foreign voltages. Both the receiver and transmitter contacts in the handset are connected through the payphone’s internal wiring to the ring wire. So touching the grounded housing with one end of the pull-tab and the other end to the exposed transmitter contact would apply ground to the ring wire, and dial tone would be heard in the receiver. Ground is normally supplied through a pair of contacts that are closed when the pay phone hopper trigger is deflected by depositing a coin. The deflected hopper trigger also removes a short circuit across the dial contacts. Since no coin was deposited, the short across the dial remains and dialing would be ineffective. So, in this case, Broderick would get dial tone but he wouldn’t be able to dial out using the rotary dial. He could have attempted switch hook flashing. He wasn't trying to hack into the long distance trunk network. He just wanted local dial tone, after which he rotary dialed the number for long distance information (1+ area code+555-1212). Serious phone phreaking began in the 1960's and continued well into the 1980's. This involved building devices that emulated all the analog signals used in the long distance network. The evolution away from an analog multifrequency in-band signalling took a very long time to complete, before blue boxes became obsolete. If you are interested in this subject matter, I can recommend an excellent book titled "Exploding The Phone" by Phil Lapsley.

@@Hicken65I am a musician with some Morse code experiance, so tapping out 60/40 % Duty cycle sequences at about 10Hz was easy for me with virtually no practice - I just mimicked the tempo from the listening to the rotary dial clicking. You obviously must have been amused by that telephone scene in the film when you first saw it. Wow, ok, so he could have gotten a dial tone with that phone after all. One would think the network/filter circuitry in the phone would prevent hand set shorting from controlling the tip/ring opening/closing the loop. But pay phone hand sets are very, very, difficult to break, how did he open the cover so easily? I could never find a way to open a pay phone handset - even with oversized channel lock pliers.

@@Hicken65 I will see if my library has that book. I heard that ALL telephones are actually bugging devices. Even if the phone is hung up, a very high gain amplifier can be connected between the tip and ring lines anywhere between the extension and CO. This allows the bugging of criminals (and even the public) without having to get in the building (or even get a wiretap warrant). In the Air Force, our shop was so classified that a special relay external to the phone (mounted on the wall next to the phone) totally isolated the tip and ring wires that led back the CO. When we picked up the handset, inside that box, a relay clicked, and a red light came on to let us know the phone was now connected to the line and to not have classified conversations in the background. The ringer was inside the box, and relayed only the high voltage to the phone's ringer when a call came in. I open it up once to reverse engineer it... it was a very clever device. This prevented our phone from becoming a bugging device as it is designed to be and prevent electronic signals from our equipment being coupled into the lines from the network/filters/earpiece in the phone. I was told that is why the ear piece is like a speaker coil and the voice piece is a carbon element that can be removed - but it is the earpiece that acts as the microphone in bugging mode. One time President Bush visited us on base. One of the phones made available to him was next to a secure crypto phone in our command post next door to our shop. I showed the president's technician the phone. He opened it up to make sure there were no bugging devices inside and then inserted a thin strip in the various contacts. He told me that was so it cannot be used as a bugging device. I told him about our box with the red light and he said it was similar to that. It was easier to temporarily modify the phones rather than schlep a bunch of "modified" phones with him around the world. This is one reason why telephones were leased (never sold) to people for so many years. It was the best way to insure virtually every room in the USA that had a phone was bugged. To this day, I disconnect my land line phone in my shop when I have private conversations there. Did you know this?

The 3-slot pay phone didn't have any sophisticated electronics in it.You needed to put in a coin to get dial tone. The dropping of the coin physically operated a tiltable trigger which in turn closed a contact set that applied ground on the ring wire. So any method that would put ground on the ring wire would get you dial tone. The transmitter and receiver caps were glued onto the threads of the handset. Field repair technicians were instructed to replace the entire handset if there was a problem with the receiver or transmitter. It is highly unlikely banging the handset on a hard surface would have broken the glue seal. @@paulromsky9527

@@Hicken65 They were glued on! That makes sense. Thanks.

Well, thank you very much for the kind words. It's very nice for me to see such keen enthusiasm. Thanks for watching.

@@Hicken65 So, did I understand this correctly: The Cross Bar Switches are to "find" a connection circuit. As soon as it does, it hands off that info to other equipment for the actual connection and the Cross Bar resets back to Neutral while the call is being placed.

Well, the crossbar switches themselves don’t perform any “finding” tasks. They are just dumb switches that provide an interconnecting pathway between different components. Common control devices, such as markers, determine which particular crossbar switch crosspoints are to be operated to make the connection between these components. In some applications, the crosspoints only remain operated for a few seconds during the call set up process. In other applications, the crosspoints remain operated for the entire duration of the telephone call since they provide the actual talking path through the switching sytem.@@paulromsky9527

Well, I don't know what kind of device you're using, but on my computer the voice level is normal with the sound bar about a third of the way between low and high.

You're welcome. You may be interested in my long, detailed videos on this equipment. Just search on my channel name "Hicken65". Thanks for watching.

Well, I'm glad you found it informative, and thanks for watching.

No, there is no lamp signal for this condition.

J'espère que RTT restera un bon souvenir pour vous. Merci d'avoir regardé.

Enjoy your visit and thanks for watching.

There is a lot more detail in my longer videos. Just search on my channel name. Unfortunately I do not have any Automatic Number Identification or Automatic Message Accounting equipment available to make a video. Thanks for watching.

Well, you are most welcome. Good luck with your project and thanks for watching.

Yes, this was the most common method up through the 1940's. In the 1950's and and 1960's, the prepay method was the most common (in the Bell System). In the 1970's, it reverted back to a new form of post pay called Dial Tone First (DTF), to ensure people could reach emergency services without having to fumble around for a coin. Thanks for watching.

@@Hicken65 The one time I used a post-pay phone, the way it worked was when the other party answered, a loud buzzing type sound came on that didn't allow you to hear the other person until you put 15 cents in, then you could hear the person at the other end, but they would have already said Hello and hopefully they wouldn't hang up before you put your coins in. It was awkward and pre pay was much better. Pay phones that took quarters, where calls were a quarter, allowed making an emergency call or dialing the operator without a coin. You could also dial a long distance number and it would tell you how much money to put in. You could also make a collect call without a coin by dialing 0, the area code, and number.

I hope your project works out. Thanks for watching.

Always good to hear from a fellow CO tech. Thanks for watching.

Sorry, I do not. You might want to try www.A1-Telephone.com

Well, that's a daunting task. All of this equipment that remains is in the hands of collectors or telecom museums. Sometimes bits and pieces show up on EBay. You're best bet is to join the Telephone Collectors International (www.telephonecollectors.org) group email list and ask if anyone has equipment for sale.

@@Hicken65Thanks for the reply! I've been looking for Strowger switches and stuff on Ebay and there hasn't been much to see. Thank you for a fascinating video.

It's always great to receive interesting anecdotes about this old equipment. Thanks for watching.

It was the KS16297 Flushing Unit.

Thank you I have been searching for this info and could not find it. By the way cleaning banks was my worst routine to do. Now all I have to do is find the different chemicals we used in the flushing unit . @@Hicken65

I couldn't find any documentation that identified the makeup of the fluid, other than it contained 3% oil. The part number is KS16436 L1.

Thanks!@@Hicken65

Well your father certainly had a diverse and interesting career and its good that you have these fond memories. Sorry, I can't be of any help on the license plate.

To some people, the sounds in a SXS office were just cacophonous noise. I always found it to be a pleasant rhythmic chatter. Thanks for watching.

Every Central Office is equipped with alternating current tone generators for dial tone, busy tone, no circuit available tone ,etc. They were huge motor driven devices in the early years and much more compact solid state electronic devices in the later years. Dial tone is permanently connected to each 1st selector and is cut through to its attached line finder as soon as the originating telephone line has been found and connected to the 1st selector. The tone generator is removed from the voice path after the first digit is dialed. The line finder can find 200 lines and so there are 200 sets of line contacts on the bank. The line on my demo unit is connected to the tenth terminal on the tenth level and so the line finder steps up ten levels and rotates ten steps. I could have attached my line to the first terminal on level one, but it wouldn't have provided for much of a demonstration of the electro mechanical motion of the switch.

@@Hicken65 thanks, that helps!

To answer--at least--ONE question: When you picked up the "telephone instrument" "receiver" (essentially a telephone handset), you accessed the "Line Equipment," giving you--in turn--the dialtone. BTW, the DT had no special meaning... SIMPLY lets you know that the LE is available. When you begin to dial, the DT disappears...and you access the switching equipment (first "Selectors, " then "Connectors")...which routes your call..either to another phone address in the same office...or sends it out to another office (and that telephone must, then, have an "Area Code" to let the system know what office you need to access).