a variable weight loading device carried by said carriage for loading the sample holder such that a selected load can be applied to the sample normal to the friction surface, said variable weight loading device including a weight platform on which {HEAD ~ removable weights} can be stacked and removed to selectively vary {TAIL ~ the load} applied to the sample 
a variable weight loading device carried by said carriage for loading the sample holder such that a selected load can be applied to the sample normal to the friction surface , said variable weight loading device including a weight platform on which { HEAD ~ removable weights } can be stacked and removed to selectively vary { TAIL ~ the load } applied to the sample
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