268 



Mr. P. E. Shaw : 



Two points in setting up may be mentioned : — 



(1) Torsional stiffness must be avoided in the attachment 

 gfe (fig. 1) . Even very thin copper wire would give trouble 

 in this respect; but if a strip of phosphor-bronze is made into 

 a permanent helix (by pulling it under the edge of a paper- 

 knife), it will be found to satisfy the conditions required. 



(2) To insulate the apparatus from vibrations it was placed 

 on a heavy platform, which was hung by four rubber door- 

 springs from a firm support above. 



(3) The connexions and contact wires were so compact 

 that they could all be enclosed, and protected from draughts 

 and dust, when the galvanometer-cover was put in place. 



To give a simple illustration of the working of the apho- 

 meter : suppose contact made when the resistance in the 

 galvanometer circuit is 4000 &> and broken when resistance is 

 3850 6); then, corresponding to these 150 &>, some sticking must 

 have occurred. But in the circuit all the elements of E.M.F. 

 and resistance are known ; hence a known current-strength 

 has been used to sunder the contact. If in an independent 

 experiment (as was actually done) current is balanced against 

 a known force (e. c/., a small weight suspended over a pulley 

 and attached to ab by a silk fibre), then at once we can 

 obtain a measure of the force o£ adherence in the case 



given. 



3. Dynamical Conditions of the Contacts. 



Before giving a statement of the experimental work, it is- 

 interesting to consider the conditions at the place of contact. 

 Consider two equal cylinders, i. e. the contact wires, crossing 

 at right angles andj^ touching one another. 



Now let the area of the cylinders be relatively displaced 

 towards one another by a distance a, let the pressure between 

 the cylinders be p, and the radius of the circular area of 

 contact be a. Then it can be shown * that we have the following 

 relations : — 



~V "1 



$(Pl+P2) 



(I.) 



3 pfti+3 2 ) 

 16 a 



. (ID 



* Miscellaneous Papers of Hertz, Paper 6 : Jones & Schott (Mac- 

 raillan). I am indebted to Mr. G. A. Schott, B.A., B.Sc, University 

 College, Aberystwyth, for showing- me the above useful modification of 

 the theory of Hertz. 



