Like David I did my early training with relay logic but mine was associated with the signalling control of London Underground trains. We had some amazing relays, everyone designed to do a specfic task in control logic, my favourite were the glass encased Vane relays, easily big enough to use as a gold fish tank.
This is a good link to the kit i used to work with
Oh yeah. Well I had the "corrosion" and "polarity" parts OK. Couldn't make the concept work in my head because there's always two wires in a pair, would just shift issue to the other wire, but here they're talking about all the UNPAIRED wires inside the CO, have to protect THEM, the pairs are mostly "customer premises" so if the customer loses service as a result he'll pay for a service call anyway. Gotta think like a telco!
Originally, the potentials on the wires were positive with respect to earth (ground). This is called negative ground, since the negative side of the battery is grounded to earth. Telephone companies discovered that with positive voltage on the copper wires, copper wires experienced corrosion due to electrolysis. Operating in reverse, positive ground (negative voltage on the wires), the copper is protected from corrosion, a process called cathodic protection.
@JeffL_2: We're talking AT&T or the phone company, back to the days of landlines, the CO battreies were always -48 volts and I believe the reason has to do with there's less corrosion on the ground rod (that actually goes into the soil and conducts real DC current) that way
That makes a lot of sense -- OK, so now I won't feel bad about using a negative earth in my implementation (because that's the way I think :-)
We're talking AT&T or the phone company, back to the days of landlines, the CO battreies were always -48 volts and I believe the reason has to do with there's less corrosion on the ground rod (that actually goes into the soil and conducts real DC current) that way, something about electrochemistry and avoiding corrosion of the electrode, I can't provide details but they're probably not all that important.
@All: In my antique relay book, which was written by folks at AT&T, they are using a positive ground. For example, if you look at the diagram for the coil on a simple relay, the left side of the coil is connected to one side of a switch (could be a mexhanical switch, could be another relay); the other side of the switch is connected to ground.
Meanwhile, the right-hand side of the coil is connected to th enegative terminal on a battery symbol (which they are using to represent a power supply) and the positive battery terminal is connected to ground.
So, my question is -- was there any underlying reason why they should have a positive ground (i.e., the positive battery / supply terminal connected to ground)?
@Weatherbee: Therefore you can realize an XOR gate with a single relay.
I'm going to have to think about this for a bit. One thing I've discovered in my antique relay logic book is that my simple relays wre just th ebeginning -- they also had relays with multiple coils and all sorts of other clever things...
@JeffL_2: It would make a nice demo to use relays to show combinatorial logic but I think it would be too hard to take it much beyond that.
If I can lay my hands on a bunch of them, I'd certainly start with just a bunch of regular switches on the left feeding a bunch of relays wired in a combinational manner -- plus have loads of miniature incandecent bulbs scattered around so you can see everything operating.
But I would have to move up to sequential logic. I actually think you could make a D-type latch with a clear input using just two relays (but I'll have to think about thsi some more).