a sample holder guided by said carriage for movement relative to said carriage in a direction perpendicular to the friction surface and said first direction, said sample holder being configured to hold a sample in frictional engagement with the friction surface during {HEAD ~ relative movement} between {TAIL ~ said carriage and friction surface} in said first direction 
a sample holder guided by said carriage for movement relative to said carriage in a direction perpendicular to the friction surface and said first direction , said sample holder being configured to hold a sample in frictional engagement with the friction surface during { HEAD ~ relative movement } between { TAIL ~ said carriage and friction surface } in said first direction
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