kasperzegel:
So in my code i'm trying to make contact between PIN13 and GND.
Pin 13 is connected to the internal LED on most Arduino boards. So perhaps not the best choice...
However...
digalWrite(13, LOW);
pinMode(13, INPUT);
// Pin 13 not connected to anything - it will "float"
pinMode(13, OUTPUT);
// Pin now connected to ground - 0v
pinMode(13, INPUT);
// Pin now back to floating
This connects pin 13 to ground, or nothing, as you requested.
There's no current or voltage needed, I just want to make a contact-trigger to start an external device.
How is the external device going to detect it is closed then? I suppose you mean NO current or voltage needed from Arduino. In that case you do not want pin 13 connected to Ground - Ground is 0V.
It's a mistake to say 0v is "no voltage". It's just 0v with reference to some other voltage.
If you put the negative probe of your meter on the positive supply, then in that case the 0v pin has potential of minus 5v. Measuring voltages is all relative to what you choose as your reference point, and 0v does not necessarily mean same potential as ground.
So if your question is "how do I connect two arbitrary pins together" (and not "how do I connect a pin to ground" - which is a totally different question) then there is no way to do this on Arduino or indeed any common-or-garden microprocessor*. You'll need some external hardware, e.g. relay or opto-issolator.
- At least that I ma aware of, there may be some niche specialist devices which implement such hardware that I am unaware of.