Silly question

[K1WI]

Hi everyone,
  For years I have had. what may seem as an odd question, but I have been wondering for a long long time. And that is why did Western Electric use 4 wire cords on the G handsets when in the vast majority of cases only three leads were required as two were connected together at the 425 or other type network. I've asked this question many times during my 35 years at "Ma Bell" and no one ever provided me with an answer that could be backed up by BSPs or other references. It always seemed atypical to not save on the expense of three versus four conductors. Just about every explaination that has been offered seems to have contrdictions.
  I do know that other manufacturers over the years did use three conductor cords on their G handsets by extending one of the transmitter leads up to the receiver.
  I know ...I've got too much time on my hands.

    Andy F    K1WI       @mgh

[AE_Collector]

I have no idea at all other than that there were likely a few models of phones or equipment that did need all 4 leads so there would be savings in not having to stock all the colors and lengths in both 3 and 4 conductor.

AE did the same thing switching from 3 to 4 conductor handset cords not too long after they came out with the type 81 handset used on AE 80's and 90's (sililar to WECo 500 & 554's). The previous handset, the type 41 had all 3 leads terminated under the transmitter. With the initial type 81 and 3 conductor handset cord, the green lead had two spades on it at the handset end. One for the transmitter and then the green lead continued up to the receiver end of the handset.

Terry

[poplar1]

Even though one white receiver lead and the red transmitter lead go to the same terminal (R) on the network, there is no DC current on either white lead. That is the reason for separate leads.  I don't have the exact quotation but it is from a discussion (probably Bell Labs) about development of the 500.

[Phonesrfun]

I have seen that quote too.  It makes sense.

[AE_Collector]

I still don't follow that. A common lead that hit the tansmitter and then carried on to the receiver would be elctrically identical to two seperate leads both connected to one terminal at the network.

Terry

[poplar1]

Terry, I don't see how the DC current from the transmitter can interfere with the AC current to the receiver if the leads are separate.

[AE_Collector]

Well unless it is maybe related to how far they both travel through a single lead. Once they hit the screw terminal on the network they both travel through a single lead within the network to the next component within the network. ut maybe the very short length of wire within the network itseld minimizes the problem. It can't be a BIG problem as prior to the 4 wire cords both currents got along okay in the same conductor.

Terry (not electronically inclined at all)

[Phonesrfun]

The common red wire in a three wire system carries the DC voltage of the primary circuit.  Because all conductors have a resistance, there is a voltage difference on the common wire at the end of the handset cord than at the point that the wire connects inside the phone.  It stands to reason that with resistance there would be a voltage difference at any point along either of the transmitter wires than at the point where they are connected inside the phone.

The receiver is not supposed to have any DC in the circuit.  A few millivolts doesn't normally hurt anything, and that would have been the norm in a 302.   Even hundredths or tenths of a volt would not necessarily be a problem with the 302 and earlier phones.

The 500 has the varistor across the receiver to clamp voltage spikes that are as small as tenths or hundredths of a volt.  Too much resistance in the handset wire could cause a voltage potential across the receiver that could interfere with the action of the varistor and cause it to silence the receiver when it shouldn't.

That is my take on the issue.

That's my story, officer, and I ain't changing it.

[AE_Collector]

uuhhhh.....huh......okay.....

I'll  have to take your word for it Bill.

Terry

[dsk]

The common red wire in a three wire system carries the DC voltage of the primary circuit.  Because all conductors have a resistance, there is a voltage difference on the common wire at the end of the handset cord than at the point that the wire connects inside the phone.  It stands to reason that with resistance there would be a voltage difference at any point along either of the transmitter wires than at the point where they are connected inside the phone.

The receiver is not supposed to have any DC in the circuit.  A few millivolts doesn't normally hurt anything, and that would have been the norm in a 302.   Even hundredths or tenths of a volt would not necessarily be a problem with the 302 and earlier phones.

The 500 has the varistor across the receiver to clamp voltage spikes that are as small as tenths or hundredths of a volt.  Too much resistance in the handset wire could cause a voltage potential across the receiver that could interfere with the action of the varistor and cause it to silence the receiver when it shouldn't.

That is my take on the issue.

That's my story, officer, and I ain't changing it.

I just partly buy this. The voltage drop will not be considerable in a normal cord, but if you use an extremely long cord, and at the same time reduce the current carrying capacity it will act as a resistor.  Its rather more likely to be a way of thinking further on, the semiconductors was not tough at that time, a transistorised circuit would need other specs, and a 4 lead cord is universal, it may be used anyway.

dsk

[K1WI]

  I love this forum. There is no other website with this much sharing of knowledge and experience .
 Over the  years I've made a few casual observations about all the reasons that have been given to this question.

    Electrically there is very little difference in the theory of operation between a WE 302 with a 101 network and a 500 set with a 425 network . The 425 has varistors to limit voltage levels an R/C circuitry to limit audio distortion. Strange that all the 302 and variants had no problems with three conductors.

     I've heard many times that the difference in voltages between the transmit and receive sides of the phone will cause noise and other problems , seems to me that a common lead will totally eliminate the voltage difference and lessen the possibility of noise picked via induction.


     There are NO transistor or other amplification devices in a 425 network , the only solid state devices are the varistors which are VERY VERY rugged.


      I doubt that 1 in 10,000 of the WECO  500 sets electrically needed a fourth wire in the handset cord ,(probably even fewer) making it hard to justify using that much more wire over all the years of production.

      All that being said I DO believe that Western Electric and Bell Labs had a good reason for going to 4 conductors . I'm also sure that the answer is out there somewhere.
      We used to joke : "There's no reason for it ... just company policy."
   Thanks to all !             Andy F   K1WI             @mgh

PS    I am NOT an electrical engineer , but I did stay at a Holiday Inn Express.  

[Phonesrfun]

Andy, all that is true in my opinion.  I am sure that the voltage differences in all practicality made little or no difference in the end result.

One thing they did gain, however was that in the E and F handset series, there was a separate handset model I.E. the F2 that had 4 wires used in switchboard applications.  With the 4 wires, they did not need another handset model, they only needed to have a separate cord with the tips made for the switchboard plug.

To DSK

I think we are all saying the same thing about the practicality of using versus not using the 4th wire.  I was trying very hard to eloquently describe something I had read a long time ago that made sense to me at the time.  There really can be a voltage difference, especially as a handset cord gets frayed or weakened.  How much difference that makes in the long run is probably nil.

[dsk]

  As Andy, this forum teach me a lot, and it keeps my curiosity on top.

I had to do some testing, whats happening if the common wire breaks on a 302?
It still works OK no anti sidetone, maybe noise, and of course we blame the tel-co
Actually the 302 cord are a little bit to stiff, the 500 has a softer cord. So whats happening when one of the 2 common (connected to R on the network) breaks? Either the receiver, or the transmitter stops working You just cant blame the tel-co
You still have the possibility to to make an internal strap in the handset until you get a new cord

But why did they decide to use 4 cords  It may actually be as the designer preferred, he/she made the prototype, it was tested, stressed and worked well, the why modify?

The 302 with metal base was more compact, more sturdy, but the 500 had the best circuit ever made, almost as simple as the 302, reliable as 302 and had a quality of sound transferring both ways who not was outdone before the carbon transmitter was replaced by electronics

dsk

[poplar1]

Here is the quotation from TCI listserv on 3/13/2008:

{500 Design Considerations


The talk was given in 1949, the year of the introduction of the 500-series

Regarding the 4th conductor in the handset cord:

"You might say, "Why do you add a fourth conductor?" Here again we are
shooting at maintenance. When a common conductor is used for both
transmitter and receiver, one of the conductors in the receiver circuit
carries current. If some of the strands of this conductor break, a situation
develops where the resistance varies as the cord is flexed, and noise is
heard in the receiver. If the two circuits are segregated, the cord can be
used much longer before the same amount of cord noise develops. We feel that
we are going to gain sufficiently in cord life to off-set, with some margin,
the cost of the additional conductor." }

[AE_Collector]

Okay, that is starting to make some sense now. In addition, an additional conductor probably goes a fair distance toward making the cord that much stronger over all and probably reduces the amount of strain on each conductor.

Thanks for looking up and posting the quote David.

Terry