Patent GB-720333-A (GB)
Title: Methods of and means for testing a liquid for its bearing strength
Inventor: UBBELOHDE LEO
IPC: G01N21/45
Filed: 1952-06-26
Abstract: 720,333. Interferometers. LEITZ GES., E. June 26, 1952, No. 16105/52. Class 97 (1). [Also in Group XIX] An interferometer for determining the thickness of a layer of liquid under loaded conditions comprises a pair of bodies between which an adsorption layer of the liquid may be formed, means for pressing the bodies together with the layer therebetween, and means for viewing and measuring the optical interference effects created in dependence on the thickness of the layer. First embodiment. As shown in Fig. 1 light from lamp 2a passes through a semi-transparent and semi-reflecting plate 3 to a microscope 4 in a tube 11. Secured to the microscope 4 is a plate 8 having a glass plate serving as a test body 9. A further plate 14 carries a plane-convex lens 15 serving as an auxiliary body, the position of the plate 14 relatively to the tube 11 being set by an adjusting ring 16 threaded on the tube 11 acting upwardly on studs 13a and 13b extending from the plate 14 through slots 12a and 12b in the tube 11. The whole apparatus is supported by a ring 21, the test body 9 being secured directly to the ring 21 by the columns 7 whilst the tube 11 and hence the lens 15 are supported by a spring 23 resting on the ring 21, the pressure exerted by the spring 23 being adjustable by a setting ring 24 threaded on the tube 11. At the upper end of the tube 11, a plate 18 is provided which carries a lens serving as a test body 19, the plate 18 being held in position by a lid 20 screwed on the tube 11, the weight of the lid 20 being such that the tube 11 floats for a certain initial position of the ring 24. In operation, the auxiliary body 15 is set by the adjusting ring 16 at a suitable distance from the test body 9 and a liquid layer to be tested is applied to the upper surface of the test body 9. By means of a handle 5, the microscope 4 is focused on the adsorption layer formed by the liquid between the test bodies 9 and 19 through the auxiliary body 15 to ascertain whether the said layer is free from impurities. Subsequently, the microscope 4 is focused on the surface of the test body 9 adjacent to auxiliary body 15. Weights 25 are then used to create a pressure between the test bodies 9 and 19, the pressure acting on the liquid under examination. Since the auxiliary body 15 is mechanically coupled to the test body 19 by means of the tube 11, the adjusting ring 16, the guide studs 13a and 13b and the carrier plate 14, the auxiliary body 15 moves away from the adjacent surface of the test body 9 through the same distance through which the test body 19 approaches the other surface of the test body 9. Thus when the pressure exerted on the liquid layer by the weights 25 and the setting of the ring 24 becomes effective, Newton rings depending upon the thickness of said layer are created between the auxiliary body 15 and the test body 9 and can be observed by means of the eyepiece 1a and the beam-splitting plate 3. The Newton rings that are then observable through the eyepiece 1a are a measure of the thickness of the liquid layer for the given load. A reticle (not shown) may be used to measure the diameter of the Newton rings. Instead of the weights 25, hydraulic, electromagnetic or similar means may be employed. Second embodiment. To allow for any error that may arise due to the fact that the test bodies and the auxiliary body may themselves be somewhat compressed by the pressure exerted on the adsorption layer, the apparatus may comprise a common glass plate 103 against which three similar bodies 101, 101a and 101b (of which only two are visible in the drawings) are subjected to a pressure from a common source 102. An adsorption layer 117 is formed between the plate 103 and the body 101. The arrangement thus comprises one pair of test bodies 101, 103 and two pairs of compensating bodies 101a, 103 and 101b, 103. Three similar objectives 104, 104a and 104b enable the bodies 101, 101a and 101b and the layer 117 to be initially examined with regard to impurities. A beam-splitting device 105 co-operates with two of the objectives, namely 104 and 104a. Light from a source 108 divides at the beamsplitting surface 106 into two coherent light beams which are totally reflected from the surfaces 107 and 107a to reach objectives 104, 104a respectively, through two compensating plates, one in each light beam. One compensating plate comprises two glass wedges 115 and the other a tiltable glass plate 116. The two light beams are returned by the bodies 101, 101a and are re-combined by the beam-splitting surface to reach the eyepiece 114. The deformation in the bodies 101 and 101a; in the glass plate 103 and in the objectives 104 and 104a, caused by the pressure 102, are substantially the same for both light beams so that such deformations have no appreciable influence on the interference effect. The interference effect is, however, dependent on the thickness of the layer 117, and is also influenced by the refractive index of the layer 117. This latter effect is undesirable and is avoided in the device shown in Figs. 3 and 4. Third embodiment. As stated above, for greater accuracy in measuring the thickness of the adsorption layer, the refractive index of the layer should be allowed for. This is done in Figs. 3 and 4. The bodies 101, 101a and 101b are now secured to a plate 118 having two angular reflectors 119 and 120 arranged as shown. The light from the lamp 108 is again split into two coherent beams by the surface 106, said beams passing to and being laterally displaced and reflected by the angular reflectors 119 and 120 respectively. In this embodiment the compensating plates 115 and 116 equalize the optical lengths of the coherent light beams while a tiltable glass plate 121 in the shape of a circular ring serves to create two achromatic interference fringes, each light beam passing said circular ring only once. The plate 121 causes a slight relative displacement of the two light beams in the re-combined light entering the objective 113 whereby two achromatic interference fringes are caused. Coaxially with the test and compensating bodies 101, 101a and 101b, microscope objectives 122, 122a and 122b are provided, by means of which in co-operation with a rotatable prism 123, an auxiliary objective 124, reflecting surfaces 125 and 126 and an eyepiece (not shown) the bodies 101, 101a, 101b, the glass plate 103, and the layer 117 of the liquid may be examined with regard to impurities. With this arrangement any error due to the refractive index of the layer 117 of the liquid is avoided since the interference fringes are now a measure of the vertical displacement of the reflector 119 relatively to the reflector 120 due to the presence of the liquid layer 117. Fourth embodiment. Fig. 5 illustrates a modification in which the above angular reflectors 119 and 120 are avoided. The achromatic interference fringes are created by the tiltable ring-shaped glass plate 121 and two similar microscope objectives 133 and 134 positioned in the paths of the coherent light beams. The plate 118 comprises two reflecting plane regions 135 and 136. Otherwise the arrangement is analogous to that of Figs. 3 and 4. Fifth embodiment. The displacement between the wave-lengths of the interfering light beams, and therewith the accuracy of the measurement may be increased by the modification illustrated in Fig. 6. In this, the plate 118 comprises reflecting regions opposite which additional reflecting surfaces 137 and 137a are arranged so that each of the coherent light beams is reflected several times between the reflecting regions and the additional reflecting surfaces 137 and 137a respectively. Sixth embodiment. In Fig. 7, semi-transparent and semi-reflecting lenses 138 and 138a are arranged opposite the reflecting plate 118. Newton rings are thus created which are extremely distinct and can be measured with high accuracy. A beam splitting surface 139 forms two light beams passing to the lenses 138, 138a and the plate 118 as shown. The semitransparent and semi-reflecting surface of each lens 138, 138a faces the plate 118. Adjacent the other surface of each of the lenses 138 and 138a a diaphragm 141 or 141a respectively is provided which covers diametrically opposite quadrants as shown in Fig. 8. The diaphragm in one light beam is rotated through 90 degrees relatively to the diaphragm in the other light beam. Thereby, in each pair of opposite quadrants, parts of the same Newton rings appear in the viewing field of the eyepiece as shown in Fig. 9. By measuring the diameters of the Newton rings, the distances of the reflecting plate 118 from the lenses 138 and 138a and therewith the thickness of the layer 117 of the liquid can be measured. Achromatic interference fringes. Instead of measuring of the achromatic interference fringes referred to above, with the aid of a reticle, a diaphragm may be provided in the eyepiece which is darkened complementarily to the interference fringes. Such a diaphragm may be obtained by photographing two interference fringes, the diapositive of which is used as a diaphragm in the eyepiece. In use the diaphragm is initially adjusted by micrometer means until the viewing field in the eyepiece is uniformly dark. When subsequently the interference fringes are displaced, less dark streaks appear, whereupon the micrometer arrangement is re-set until, again, uniform darkness has been obtained, the differenee in the settings of the micrometer being indicative of the displacement of the interference fringes.