C. Barus — Viscosity of Steel and 



Hence the total angular motion at the index due to a length of 

 the wire between x=k {J£>b) and x=k+a will be found by in- 

 tegration to be 



i-b<pl -f~ (2) . 



Equation (2) implies that viscous detorsion takes place only 

 along the length a of the wire, or that the remainder of the 

 wire is practically rigid with respect to this part. If, as in the 

 1# present experiments, the temperature of 



a exceeds that of the rest of the wire by 

 100° and more, these conditions are 

 nearly enough fulfilled. Equation (2) 

 therefore enables us to calculate <p at 

 once. Knowing <p or the amount of 

 viscous motion uniformly around an axis, 

 it is not difficult to find the correspond- 

 ing datum for rectilinear viscous motion, 

 where the strain impressed is a simple 

 shear. I omit this reduction here. 



Apparatus. — In the actual work it is 

 expedient to compare two identical wires, 

 one of which is kept hot, the other cold. 

 These wires are shown in the annexed 

 diagram at ab and cd. The ends, having 

 been bent loop-shaped, are screwed near 

 the middle of the apparatus to a cross- 

 piece of iron carrying the adjustable mir- 

 ror M. This is the index referred to in 

 the preceding paragraph, and angular de- 

 viations are read off by Gauss' method. 

 To deaden lateral vibrations the mirror 

 is also provided with a cross-vane, mm, 

 dipping appropriately into a fixed dish 

 of water. The upper end of the system 

 of wires is screwed to a round steel rod, 

 af, which fits snugly in the perforation of 

 the massive fixed piece of brass gh. A 

 steel pin, p, passing through both brass 

 and steel rod, secures the latter to the 

 former. The lower end of the wires is 

 similarly fastened to the steel rod. dh and 

 the fixed piece of brass hi. To keep the 

 wires tense they are stretched by a 

 weight, EQ ; and a part of the lower 

 fixed brass hi has therefore been cut 

 away in such a manner that the lower 

 pin a may slide along a plane. The sys- 



A^-"^d. 



4\***3$, 



% 



— <m)i 



Scale one-fourth. 



tern of wires is therefore free to expand. 



