104 



MM H. Tomlinson. The Influence of 



of the end of the pointer from the rim of the plate before and after 

 torsion. 



When the horizontal adjustment was satisfactorily completed, the 

 silk threads could be made perpendicular to the axis of the rod by- 

 shifting the pulley stands up or down in a vertical plane, until any 

 given load in each pan produced a maximum twisting effect. The 

 pullies, W, were large and light, and so delicately balanced that the 

 loss from friction was very slight. This loss from friction, small as it 

 was, could be eliminated in the following manner : — Suppose that the 

 torsional stress has twisted the plate through a certain number of 

 degrees, the plate is now twisted very carefully by hand a little further, 

 and this stress then very gradually relaxed ; let ~D 1 be the present 

 position of the pointer. Again, let the original torsional stress be 

 carefully relaxed a little, and then very gradually restored, the pointer 

 will now take up a new position, D 2 . The true position of the pointer, 

 if there were no friction, would be D x + ~D 2 /2. At least ten trials were 

 made with each of the torsional stresses employed, and the mean of 

 the different readings, which accorded very well with each other, was' 

 taken. 



In order to apply the torsional stress with sufficient gentleness, the 

 following plan was adopted: — Two smooth blocks of wood were placed 

 with a face of each in contact with two opposite sides of the scale-pan, 

 S, so that neither the pan nor the plate, P, would move when weights' 

 were put in the smaller pans, T. As soon as the latter were loaded, 

 the blocks of wood were gradually and gently removed from the sides, 

 so as to permit of the torsional stress producing its effect by slow 

 degrees.* 



Prof. G. Wiedemann has already shown in his experiments on 

 torsion by the statical method, that the torsional elasticity is inde- 

 pendent of the amount of longitudinal stress which may be acting 

 on the wire at the same time as the torsional stress,f and the author 

 has also proved that this is the case when the kinetical method is 

 adopted. J However, it was considered advisable to make a few pre- 

 liminary experiments, to ascertain whether temporary loading would 

 affect the torsional rigidity. It is unnecessary to enter into the results 

 of these preliminary trials further than to say that they fully verified 

 Prof. Gr. Wiedemann's previous observations for all the metals which 

 were examined. 



Before commencing the actual testing the wire was frequently set 



* This plan was found to answer the purpose very well. Professor Gr. Wiedemann 

 has adopted a much more elaborate arrangement for effecting the same object, but 

 the author cannot help thinking that with care such a device as that mentioned 

 abore is quite sufficient to prevent the stress from being applied too suddenly. 



f 'Wiedemann's Annalen,' vol. 6, 1879; ' Phil. Mag.,' Jan. and Feb., 1880. 



% ' Phil. T aiis.,' vol. 177 (Part II), 18S6. 



