Temperatiire on Modulus of Rigidity of Solid Metals. 403 



vertical glass tube Q, the jar being connected with a pump. 

 The upper brass rod B is rigidly connected with the top- 

 pieces n or c through a central hole in m. In the contact 



Fig. B. 



surface between m and n, a ring-formed groove is cut with 

 grease in it; by this contrivance we can give a twist to the 

 wire under a high vacuum. EF is a lever, which serves 

 to damp a lateral oscillation of the suspended sj^stem if it 

 occurs. The right side of the lever is made of soft iron, 

 and the left end carries a piece of cotton. When the lever 

 is left to itself, the left end falls and the cotton is free from 

 the pan H ; but when we pull down the right end of the 

 lever from outside by means of a magnet, the cotton now 

 comes in contact with the pan, and thus the lateral vibration 

 of the system is soon damped, but torsional oscillation is not. 

 To start a torsional oscillation, the top handle is slightly 

 rotated on one side, and again back to its original position. 

 This procedure usually produced a pure torsional oscillation. 

 and therefore we hardly ever used the damping lever, except 

 in very unfavourable cases. 



2 E 2 



