Abstract
The method herein described is designed to eliminate some disadvantages of the standard plethysmograph, particularly the necessity of enclosing the object to be measured in a water-tight container through a tightly-fitting rubber cuff.∗ The new procedure makes it possible to determine volume changes in an open container. This is of particular interest in the clinical determination of blood flow through extremities where speed and ease with which the measurement can be performed are very desirable. The use of an open vessel offers an easy access to the extremity during the measurement and makes it possible to remove and reimmerse it within a short time, which is of advantage in experimental work.
The apparatus to be described can easily be assembled from standard parts available in any physiological laboratory. It requires a balance and a container large enough for the immersion of the object to be measured.
The principle upon which this method is based can be considered a supplement to the principle of Archimedes. According to Archimedes, an object immersed in a vessel of water loses weight equal to the weight of the volume of water which it displaces. Now it can be shown quite easily that the converse is also true namely, exactly the same amount of weight as lost by the immersed object is gained by the vessel in which it is immersed. The reason for this gain in weight is easily explainable as follows: In Fig. 1 we have a solid cylinder, C, immersed in a beaker of water, B, to the level, L. To simplify our considerations we may visualize in the liquid an imaginary rigid U-tube, as indicated in the dotted lines, having a cross section equal to that of the cylinder, C.
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