The differential voltage across the circuit pair is the desired signal, whereas the common voltage signal is the unwanted signal which may have been coupled into the transmission pair. Contact Bounce - The intermittent and usually undesired opening of mechanical relay contacts during closure, or closing of contacts during opening. Contact bounce period depends upon the type of relay and varies from .5mS for small reed relays to 10-20mS for larger solenoid types. The maximum number of expected closures before failure. Life is dependent on the switched voltage, current, and power.
Failure is usually when the contact resistance exceeds an end of life value.
Typical failure mode is non-closure of the contact as opposed to a contact sticking closed. Also known as contact offset voltage, thermal EMF, and thermal offset. This is a major consideration when measuring voltages in the microvolt range. There are special low thermal relay contacts available to address this need. Contact Rating - The voltage, current, and power capacities of relay contacts under specified environmental conditions.