A reed switch is made up of nickel/iron and when connected to copper (a PCB trace), you end up with a thermocouple producing a high offset voltage. Since you have this thermocouple on each end, you need to compensate for these high offset voltages, otherwise they will swamp out any small offset signals the customer is trying to switch. So the key to make a low thermal relay is to develop a compensation technique that will compensate for these high offset voltages. Carefully placed thermal chips get the job done.
