I am reading about electromechanical relays used in the first computers and I am told that the mechanical relay is closed and opened by an electromagnetic field generated by a coil on a control wire which is itself traversed by a current.


  1. What I don't understand is how do you control this control wire ? Don't you need a switch to also turn on and off the current in the control wire ?

  2. Since there are also control wires in triodes and transistors too, is the answer valid for those too ?

  • \$\begingroup\$ The point of those relays is that their voltage/currents can be very different from the one on the control wire. You can control 220V AC on/off with 3.3V signal, for example. And 3.3V or 5V or something else this small of power is usually not a big deal - some microcontroller or even just a pushbutton. With transistors, it's a bit more complicated, you may need more than one and a few passives around them, depending on situation. Too much voltage difference on their terminals can burn them (20V between MOSFET's gate and source terminals, and you got yourself a smelly bbq) \$\endgroup\$ – Ilya Apr 20 at 8:23
  • \$\begingroup\$ In a modern computer transistors control other transistors; in a relay computer, relays control other relays. \$\endgroup\$ – user253751 Apr 20 at 9:38

Yes you might use a switch to control a relay. In a more complex circuit you might arrange for relays to control each other; in this case you can perform the elementary logic functions: inversion (where a normally-closed contact disengages a relay), an AND operation where several relays are in series and all must be energised to switch on the ‘output’ relay, and OR where several relays may be connected in parallel. In principle the same applies to triodes and transistors, although the implementation is somewhat different. Even the most complex digital circuits are made using these elementary functions.


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