BQ asked in Consumer ElectronicsLand Phones · 3 weeks ago

what are the contact tips of telephone exchange relays made from?

4 Answers

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  • Dale-E
    Lv 7
    2 weeks ago

    Telephone exchange relay contacts and even old Bell dial phones used Platinum Contacts. The real problem was longevity. The old land line system was 50 Volts of DC. Dialing caused considerable arcing on the pulsed contacts. They all had coils which cause a voltage surge when the supply contacts are pulsed. Please keep in mind, spark coils were regularly used in auto ignitions. The points for that were always being adjusted and replaced because of the burn when battery Voltage was pulsed into the spark coil. Car battery was 12 Volts, while Telephone Talk battery is 50 volts.

    The elements stated in previous answers are guesses. None of them could withstand the kind of heat experienced on the pulsed contacts of telephones and telephone exchanges. For example, Palladium was suggested. Wikipedia says, "Palladium, platinum, rhodium, ruthenium, iridium and osmium form a group of elements referred to as the platinum group metals (PGMs). These have similar chemical properties, but palladium has the lowest melting point and is the least dense of them all."

    We are talking, Electrical / Mechanical dial exchanges, not fine circuitry. The old exchange contacts had to be extremely tough to give reliable service. Even today, good reliable automobile spark plugs will have Platinum.

    Wikipedia also says, "Platinum is used in catalytic converters, laboratory equipment, electrical contacts and electrodes."

  • 3 weeks ago

    Relay based exchanges are pretty much gone.

    Palladium was the metal of choice.

  • 3 weeks ago

    Silver or palladium.

    Some silver types may have a gold or palladium flash plating; some palladium ones have a gold flash.

    Silver is used as it's _the_ best electrical conductor, plus the surface is still conductive if it tarnishes; many other metals form high resistance or insulating tarnish/oxide/rust.

    Palladium is somewhat harder and has a longer wear life, still with good electrical characteristics.

    Examples - this is a typical telephone exchange style relay, it appears to be a silver contact type, with the silver blackened by tarnish:

    http://www.surplussales.com/Images/Relays/OpenFram...

    And an article from the 1960s promoting a make of telephone relay that uses palladium contacts:

    https://www.technology.matthey.com/article/12/1/14...

    This one is in an ebay listing - you can clearly see the tiny blocks of a silvery metal welded to each contact surface; it's not possible to tell what it is in this photo, but it's not gold.

    https://i.ebayimg.com/images/g/44MAAOSw6YtZQAz2/s-...

  • Anonymous
    3 weeks ago

    usually gold as it doesn't oxidize or decay so needs no maintenance

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