Subjective Assessments of Firmness: A Comparison of Methods Using Rubber Cylinders and Steel Springs

Abstract

Scott Blair and Coppen's work on comparisons of firmness of rubber cylinders by handling has been extended to include steel springs. Small differences in firmness of rubber cylinders can be detected with considerably greater accuracy than can those for steel springs over the same range. This would seem to be at least partly due to the yielding nature of the rubber surfaces, since covering them with light rigid disks raises the threshold to the level of that of the springs. Moreover, if the ends of the springs are faced with rubber caps, though the firmness of the springs only and not of the rubber caps is varied, a greater acuity is obtained, the threshold lying about half-way between that for uncapped rubbers and that for springs. This suggests that cutaneous cues play a much larger part in such judgments than Scott Blair and Coppen had supposed. A comparison of simultaneous with successive squeezing methods shows marked differences between subjects, but, on the average, no appreciable difference to threshold. While a single squeeze is inadequate for the best judgments, three squeezings show no advantage and perhaps a slight disadvantage as compared with two squeezings. In the case of steel springs, changing the whole range of firmness within reasonable limits does not alter the position of the threshold curve but, using subjects already familiar with the previous range, there is a marked increase in scatter. Varying the heights of the cylinders produces peculiar and partly unexplained changes in acuity; but in general, taller cylinders are judged relatively firmer than shorter ones. Preliminary experiments using an absolute method suggest the importance of the temperature of the samples and the relative unimportance of even quite long intervals (∼ several weeks) between tests, unless experiments on samples having a different range of firmness intervene.