I'm not an expert on either 1800s or tachs, but....
They usually read electrical pulses and count and integrate them.
Normally they are connected somewhere between the distributor
and the coil negative (the wire that runs from the coil to the
distributor). Many also require a 12 volt connection for power.
Some have a third connection that should go to the panel lights
if they have a light inside them.

I understand some are coupled inductively to the ignition system
and just have a wire that coils around one of the connections to
the ignition. Sorry I can't furnish more specifics.
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George Downs, Bartlesville, Oklahoma, Central US