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4 PSYCHOLOGICAL ASPECTS

that even the most skilful experts tend to over-estimate their capacities, as is shown by Katz in the case of bread-making. It is known that rapidity and accuracy of leaming are enormously increased if the subject of the experiment can be given an objective measure of the degree and direction of his error. If morę reliable physical measurements of rheological properties could be madę, it would be possible to determine how far the experts’ judgments varied from day to day, or are affected by health, changes in room temperaturę, and the like ; and also to measure the discrepancies between different experts’ opinions. Unfortunately the physical tęsts are, in most cases, still very inadeąuate, and it is probably for this reason that the psychological aspect has hitherto been so largely neglected. As an example, it is only recently that it has been possible to make measurements of the rheological properties of flour doughs, and therefore it was not previously possible to analyse such characteristics as | body ” and " spring ” (which are assessed by the baker), in terms of physical properties, such as viscosity and elastic moduli. For similar reasons very little is known about the capacity to distinguish and measure such rheological properties by the senses. Experiments are in progress at the National Institute for Research in Dairying to investigate the róle of the senses, mainly those of movement and touch, in assessing the rheological properties of materials. A generał investigation of this kind is necessary before much progress can be madę in a particular field, but there is perhaps no industry which covers a wider rheological field than that of dairying.

In the case of some materials, the personal factor is hardly involved on the rheological side, as for example with metals. Here it is necessary to

correlate the physical properties measured in the laboratory with the factors involved in the practical use of the materiał. It is important to separate the physical properties, and, if possible, to measure them in absolute units. There is considerable danger, when an empirical test is used, that the property measured is really a mixture of several distinct physical properties, and that the proportion of these fundamental properties in this mixture will not correspond with their proportions in the function which is needed for the industrial use of the materiał. As an example, the consistency of a pastę depends both on its viscosity and its shearing strength, as will be explained later. The relative importance of these two properties may not be the same in the conditions under which the materiał is used as in those obtaining for the empirical test to which it is subjected.

The fundamental naturę of viscosity does not much affect the practical man, and hence hardly comes within the scope of this book ; but it should be remembered that its mechanism is not fully understood.¹ This mechanism is ąuite different in gases from what it is in fłuids. With gases, the viscosity is simply due to the kinetic energy of the molecules, hence the yiscosity increases with rising temperaturę. With liąuids, a rise in temperaturę brings about a fali in viscosity. There are many theories to explain this mechanism. Andrade and others put forward the generał idea that there is a momentary combination of neighbouring molecules and that in viscous fłow these groups are broken down and the molecules change. partners. The higher the temperaturę, the less static these groups become; hence the yiscosity falls with a rise in temperaturę. Stewart finds an explanation of the

¹ Vide Berna! Taylor and Ward.