AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
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extremity of eacli to the same support, the other extremities being fastened in the 
one case to a scale-pan, and in the other to the centre of a bar of wood or metal 
carrying constant equal weights at each end ; the latter wire is provided with a scale, 
and the former with an index of some sort which, being level with and close to the 
scale, serves to measure any alteration of length produced by weights placed in the 
pan. By this simple and ingenious arrangement, any errors which might otherwise 
result from a slight yielding of the support, or from changes of temperature, are 
avoided. 
In my own particular experiments, the length of each wire between the support, 
which consisted of a vice firmly screwed into a stout wooden plank, and the scale and 
index was about 30 feet. To the extremities of the metal bar (fig. 1, A B), which was 
supported by one of the pair of wires, were attached two weights, each equal, in most 
cases, to one-fourth of the breaking-weight of the wire. To this wire was clamped by 
two screws a silvered metal scale (fig. 2, S), divided into half-millimetres. To the other 
wire was clamped in the same manner a vernier, V, reading to -^th of a millimetre. 
This vernier was forked (fig. 3), so that, though capable of free up-and-down motion, it 
could not readily be dislodged sideways. By means of a compound microscope, an 
alteration of length of the wire equal ju oth of a millimetre could readily be estimated. 
Fig. 1. Figs. 2 and 3. 
One-tenth, size. 
Scale and Vernier, full size. 
Mode of experimenting. 
In the case of the harder wires the one to be tested was subjected for several days 
to the stress produced by a weight three-fourths of the breaking-weight; the other wire, 
which may be called the comparison-wire, in the meantime sustaining a load equal one- 
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