If you really want to do logic with relays, you'll be happier making mux-based logic. You can think of a SPDT relay as a mux with the coil as the control input, the common as the output and each throw as an input. From that you can build all your common logic gates. I was writing about that a few months ago: http://www.drdobbs.com/embedded-systems/making-contacts/240152060
This is much denser than just using parallel and series switching circuits.
the relay I see is a balance relay. The 20 mA current is probably correct because the telephone company uses this as a max current spec on their lines. Balance relays were used in burgalar alarms for those who could afford the telephone company leased line charge. you had a battery usually 24 or 48 volts subrtract the relay voltage of 5 volts the rest of the voltage is dropped across the telephone line and series "burden" resistors placed at each end of the telephone line at the central office and the "bank".
Any disturbance in the resistance of the loop causes a change in current and when that change becomes large enough the alarm is tripped. as long as the burden resistors at each end are large compared to the telephone line resistance normal changes due to climate conditions will not trip the circuit. Anyone seeking to circumvent the circuit must keep the current constant in the loop so cutting or shorting the wires is out of the question. The A and B contacts are adjustable to set sensitivity and the spring is adjustable to set loop current balance point.
There were two types of open wire telephone lines in the '60's and 70's.Steel wire was used for local service,but long distance required copper wires.
Most poles for long distance had 10 crossarms, with 10 wires on each, and a transposition bracket every so often(can't remember exact spacing intervals).The transpositon brackets prevented noise and induction from the earth's magnetic field.
These wires ran for hundreds of miles,from office to office across the country.
The wires were left up long after they were obsoleted, then someone realized that there were millions of dollars worth of copper on the poles,and started having them removed.If they had stored the wire until today, it would have been a better investment than just about anything else.
@David: I was thinking about how to do (say) a full adder with relays....or maybe a 4-digit counter....
I must admit that I'm starting to rein myself in -- I don;t want to spend years doing this -- I just want to get a relay "thing" that's doing something vaguely useful and clicking away while doing it -- I'll be asking for suggestions in a future blog -- one think might be generating the value of Pi to an infinite number of digits (well, a large number) -- maybe outputting the values on a Teletype?
@Max "I prefer to work with 5V relays (or 12V at a push)."
Don't we all.....I think I will chuck them eventually, no use to man or beast.
You're certainly ambitious....I was thinking about how to do (say) a full adder with relays....or maybe a 4-digit counter....but it rapidly gets bigger than Ben Hur ( almost literally....) thank god for TTL and CMOS....
@Max....I have possibly 30 or 40 relays, with DPDT (ie 2 changeovers) BUT they have 48 volt coils. They are not as old looking as yours above, standard looking relay with a transparent case. If they'd be of interest I'll root them out and give you more details. I might be able to find a 48V power supply as well (oh, hang on, that'd be for our 240V AC mains.... :-( )
@SteveDAus...most of those nice relays are probably slowl leaching heavy metals into our water table, in a landfill somewhere....
Strowger exhanges were lovely to listen to, The guys who worked in them could pick up sticky relays and malfunctioning selectors just by the sound. When I lived in a very small town in Zimbabwe about 11 years ago, we still had a 1000 line Strowger exhange for our town. I told the techs I'd trained on that stuff and they let me look around. Nostalgia par excellence.... you had to remember to put ATDP commands in your modem strings (for pulse dialling) or nothing happened...
Even more primitive than a relay computer is the educational Paperclip computer, introduced by the book "How To Build a Working Digital Computer" by Alcosser, Phillips, and Wolk. See http://archive.org/details/howtobuildaworkingdigitalcomputer_jun67/