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→13.2.2.2 Contact Resistance
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The measuring voltage and current are usually in the range of < 20 mV and, 10 mA (DC or 1 kHz AC). The contact force is selected as 2, 5 or 10 cN,
depending on the application of the contacts. A histogram of the individual resistance data is usually used to show the frequency distribution of the data and serves as an indicator for the cleanliness of the contact surface. As shown in <xr id="fig:Frequency distribution of the contact resistance of a clean contact surface"/> (Fig. 13.3 ) for a gold contact layer, a narrow scatter range and symmetrical distribution of the contact resistance values is typical for clean and tarnish film free contact surface. In case of the contact surface being partially or totally covered with a tarnish film, characteristic asymmetrical contact resistance distributions are evident ''(Fig. 13.4)''. While the contact resistance distribution can show the presence of tarnish films, only surface analytical methods can clarify their type and composition.<figure id="fig:Frequency distribution of the contact resistance of a clean contact surface">Fig[[File:Frequency distribution of the contact resistance of a clean contact surface. 13.3: jpg|right|thumb|Frequency distribution of the contactresistance of a clean contact surface(Ag rivet with electroplated hard gold layer;test parameters: 10 mV, 10 mA, 10 cN)]]</figure>
Fig. 13.4: Frequency distribution of the contact
resistance of a contaminated contact surface
