A pretty picture and a bit of physics

This will be the last of the images from our weekend visit to the Cape; a pretty picture of a Galilean Thermometer. I think they’re beautiful and this one doesn’t disappoint. Being the sort of person who is naturally curious about such things, I was surprised to discover that neither the volume of the spheres nor the composition of the colored fluid they contain has anything to do with the way in which this intriguing instrument works. The key to understanding its operation is to know that each of the little tags is a precisely calibrated counterweight which imparts a slightly different density to each of the spheres. So how does this seventeenth century gizmo work? See if this makes sense. As the ambient temperature changes, so does the density of the fluid in the tube (which is, by the way, a mix of alcohol and water). You will recall that there is an inverse relationship between temperature and density such that the fluid in the tube will become less dense as temperatures rise and more dense as they fall. As the density of the fluid changes so too does the relationship between the buoyant force (pushing up) and the gravitational force (pulling down). The little spheres maintain their relative order in the column at all times. As temperature increases, the fluid in the tube becomes less dense and the gravitational forces pulls more strongly against the buoyant force and the balls fall (in relative order of density). As temperature falls, the fluid in the tube becomes more dense and the buoyant force becomes stronger, pushing more forcefully against the gravitational force, and the balls rise in the column. The tag on the sphere which rests comfortably in the middle of the column indicates the ambient temperature. I really do appreciate this instrument for its beauty and its mechanical elegance.


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