Transactions of the Society 



B 



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these, or to have freedom of movement in the direction of their 

 length, it is supported on one side on two V-shaped grooves, and 

 on the otlier side by a plane surface, which is intermediate in 

 position between these grooves. The diagram (fig. 1) herewith 

 illustrates the main points. 



A A are the two metal rods, and the baseplate is seen lying on 

 these. At C C the two V-shaped grooves are placed, the plane 



surface being on the side D. In sec- 

 tion, A A are again the metal rods, and 

 the relative position of the V grooves 

 and the fiat surface are seen. The 

 position of the clamp should be as 

 nearly as possible at E, so that when it 

 is brought into use the pressure or 

 downward strain is equal on each of the 

 three points of support. The two faces 

 of the V grooves are preferably at right 

 angles to one another, so that when 

 clamped down there is no tendency to 

 jamb on to the rods, and it also ensures 

 that tlie movement is perfectly free 

 when the clamp is released. 



The principle of the geometric clamp 

 is a variation on this, and in the appa- 

 ratus here shown we have as a matter 

 of expediency introduced it in a modi- 

 fied form. It is, however, so useful 

 when setting up any small piece of 

 apparatus, that it may be required to 

 move and replace in the same position, 

 that a short description of it may not 

 be out of place. 



If any piece of apparatus is sup- 

 ported on three points, and these three 

 points themselves rest on a plane sur- 

 face, there is only freedom to move or 

 rotate on a vertical axis ; and when this 

 apparatus is clamped down it has no 

 freedom to move perpendicularly to 

 the plane on which it rests. To satisfy the requirements of the 

 geometric clamp, the three points of support should be rounded ; 

 one of these points should be dropped into a shallow conical or 

 trihedral hole ; the apparatus then has only freedom to move 

 about this point as a centre. The second point should be placed 

 in a V-shaped groove, which is set in such a position that the 

 groove runs in a direction away from the centre of the triangle 

 formed by the three feet of the apparatus. The third point of 



E 



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Fig. 1. 



