[tkms002]

Whenever you make a electrical (screw) connection to an protected conductive (painted steel,stainless steel, aluminum, titanium, etc) you need a toothed washer. When tightened, the tooth "make sure" there is a good electrical connection even is the surface is painted or oxidized. Whenever you loosen the srew connection, you have to change the toothed washer.

Most (dry) oxides are not conductive,

Here a WIKI quote:

Hmm.
If I put a continuity tester on the surface it makes a connection.

[hfjbuis]

You mean, the probe is able to break through this oxidized layer. Probes should have a very sharp point. Enen if you use a (soft) copper wire, you will probably measure conductivity, the question is if this conductivity will last during the lifetime of the machine.

In Europe this toothed washer is a safety requirement.

[dharmic]

I am using a laptop that is not always plugged in.

Which changes things because most laptops don't even have a third pin for ground (at least in Oz). So you need a way of locally grounding your arduino and, indeed, the laptop via the USB connection so everyone's talking the same reference zero voltage. Star point ground, even consider soldering a nice fat wire to the board next to the USB connector's shield to that star point as a low impedance path for normalising voltages from the laptop.

You mean, the probe is able to break through this oxidized layer. Probes should have a very sharp point. Enen if you use a (soft) copper wire, you will probably measure conductivity, the question is if this conductivity will last during the lifetime of the machine.

In Europe this toothed washer is a safety requirement.

Most places it is. Don't forget that brushing a probe on continuity sense has a very low current and thus minimal voltage drop across the connection. Crank up the current and you may find that the connection to an oxidised ali surface creates such a high voltage drop it becomes next to useless.

[tkms002]

Which changes things because most laptops don't even have a third pin for ground (at least in Oz). So you need a way of locally grounding your arduino and, indeed, the laptop via the USB connection so everyone's talking the same reference zero voltage. Star point ground, even consider soldering a nice fat wire to the board next to the USB connector's shield to that star point as a low impedance path for normalising voltages from the laptop.



Most places it is. Don't forget that brushing a probe on continuity sense has a very low current and thus minimal voltage drop across the connection. Crank up the current and you may find that the connection to an oxidised ali surface creates such a high voltage drop it becomes next to useless.

my laptop does have a third wire on the charging cord. When I created my electronics box I added a grounded 120v power outlet that I plug the laptop into.

So, I will run a wire from the Arduino GRD to my common ground(a bus bar). I will add star washers in between the ground bus and the chassis.
Thanks

[tkms002]

You mean, the probe is able to break through this oxidized layer. Probes should have a very sharp point. Enen if you use a (soft) copper wire, you will probably measure conductivity, the question is if this conductivity will last during the lifetime of the machine.

In Europe this toothed washer is a safety requirement.

thanks I will add them