Bell Tap is what I have to do (sometimes, if I want to connect directly to PBX bridged ringer, not to the KSU generator) to make the US telephones ring on European lines. US has higher ringing voltage ~100-105V. European standard is 75V. Frequency is also different, but it’s a minor shift: 25Hz vs 20Hz, or Cycles as one says in the US. Normal US ringer will only vibrate on EU line, not touching the bells. To fix this, I have set the bias spring to lower catch (or release it). Bell has less mechanical resistance in this case and can ring on lower voltage (though in reality I also have to play with the coil position on the core: fine-tuning the optimal magnetic transmission with matches). As far as I understand or heard, this option was included into all Bell System ringers’ designs to allow using them on remote rural lines, where ringing voltage can go down significantly due to the length of the loop. Hovewer, this setting makes the ringer “Bell Taping†on rotary dial phones (I have one Stromberg-Carlson 554 which does exactly so), when one dials out.
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