[Rob Doe]

Unless I'm confused: The comments don't make sense to me. With open/insulated points you should get 6+ volts at all test locations of the primary ig circuit this includes the points arm. Are you keeping track of the ignition switch status? It must be closed / 'on' , points open / insulated to have the correct readings.

With ig switch 'on' points closed, the input side, - coil post should be 6+volts. But the output side, + coil post, red wire should have zero or nearly zero voltage reading if a digital meter is used. The reading at the points arm should also be 0. If these two readings are still 6+ volts then the points are not fully closed or are contaminated at the contacts or there is a ground issue at the distributor or further on.

Placing a jumper across the terminal box posts will by pass a problem that exists between the passenger post, up to the ammeter and back to the driver side post if the problem is an 'open' in that portion of the ig circuit. If the problem is an unintended ground / short, the jumper will not work.

When you tested the voltage readings at the safety fuse and got zero with the ig switch 'on' and 6+ volts on both sides with it off, is this reading repeatable? We assume you are using a VOM placing the black lead on the fuse holder and the red lead on a good ground such as a clean head nut. If you put the leads on each end of the fuse holder, you might see a 0 volts or a small fraction to the right of the decimal. Analog meters won't show the fractions.

If your car is wired to Ford specs, the ig switch being on or off should not affect the voltage readings at the fuse holder.

If you car has a hot rod mod where the ig switch is wired in between the starter switch stud and the input side of the coil rather than between the coil primary winding output side and the distributor points as Ford intended, then your readings as written might indicate an intermittent grounding of the switch. This is a common issue. The zero readings at the sides of the safety switch would not allow 6+ volt readings further up the line. It seems they are intermittent???

A law of DC auto electrics is that "all voltage is consumed by the load/s of a completed circuit". The only loads between the - negative battery post and the output side of the starter switch safety fuse should be insignificant resistances at the connections. There are no meaningful loads, such as the primary ig coil windings ahead of the safety fuse. I'm using "Modern electron flow" in my analysis which states that current / electrons flow from the - post / anode to the + post / cathode.