1. Solder joints A metal mixture is melted to join two metal parts together. When correctly done, this is a good long-term solution. Solder is however actually a slow moving liquid and it will flow under pressure. This means a solder joint will flow and even break if it is subject to much pressure or strain over a long time. If you put solder on a multi-strand wire to keep all the strands nicely together, and then tighten a screw or a nut on the wire, the solder will flow away from the pressure, and a few weeks later, the contact will be loose. This can cause arcing and even fires! 2. Elastic pressure contact This is the normal connection type for detachable connectors like RCA’s and XLR’s. You apply some pressure to keep the metal parts solid together also when vibration tries to take them apart. The pressure causes the metal parts to bend like a spring, and this tension hold the parts together. The metal parts must be made of metal mixtures that make a good spring. Copper, silver and gold are all good conductors, and they will make a good connection initially. The main problem is that air forms an oxide on the surface of the metal connectors. On silver, the contact surface is only clean for a few minutes, copper takes longer, and gold stays cleanest for the longest time. In relays and contacts, the oxide problem can be solved by sealing the relay box and injecting oxygen-free air. In other connectors, gold or a number of metal mixtures are used. You can delay the oxidation process by using a fluid to partly seal the connector. However, detachable connectors are a major problem and they should only be used when you absolutely have to. Please note that even a mirror-smooth gold surface under a microscope looks like a mountain range, and the tops on the two parts only touch each other in tiny spots. Therefore, the contact surfaces have a resistance, which will cause a temperature rise depending on the current flow. This temperature rise will limit the maximum current the connector can safely handle. 3. Plastic pressure contacts In a permanent connection, you can increase the pressure until the metal parts changes form permanently and flow into each other. The mountain tops are crushed and this process forms larger areas, where air cannot enter. Such a connection will be long-term stable. The best is the wire wrap, where a thin wire is wrapped many times around a square pin. The sharp edges of the square pin cut a bit into the wire in perhaps 40 places. Another method is a round wire placed on a flat surface, using a screw to deform the wire flat against the flat surface. The round surface means that the total force of the screw is applied on a very small area, so the local pressure against the flat surface is high enough to deform the roundness. Most mains connectors use this method. Note that in a binding post, you are not deforming the spade lugs, as the lugs are flat already! To achieve plastic deformation, you must use a round wire, unless you have extreme forces available. This was shown early by the German scientist Ohm. (Not the Ohm of Ohms law) Crimp connectors are another example of plastic deformation. Two wires inside a tube can be crimped together to form a fine airtight connection.