Mr Searle, Apparatus for Measuring the Extension of a Wire. 323 



The apparatus was constructed by Mr F. Lincoln, second 

 mechanical assistant at the Cavendish Laboratory ; its efficient 

 working is due to his care and skill. 



The following method of destroying three degrees of relative 

 freedom of the frames would, I think, be practically convenient, 

 and would be free from the errors arising from the use of links. 

 Any extent of relative vertical motion consistent with size of the 

 apparatus might occur without giving rise to any inaccuracy. 



To the frame CD fix two vertical bars of soft iron in approxi- 

 mately the same positions as they would occupy if their axes 

 passed through the centres of the two links. Let one bar have 

 a plane face, and in the other bar let a V-shaped groove be cut. 

 To the frame CD' pivot two brass wheels, one with a flat edge 

 to run on the flat iron bar, the other with a rounded edge to 

 run in the V groove. These wheels must be adjusted so that 

 when each bears upon its own iron bar the two wires A, A' are 

 vertical. To provide the force necessary for maintaining this 

 geometrical constraint against disturbing causes two small horse- 

 shoe magnets may be fixed by the side of the wheels, so that their 

 poles are within 1 mm. of the iron bars. 



VOL. x. pt. V. 23 



