Viscosity encountered on passing from Fluid to Solid. 345 



which are firmly clamped to supports projecting from the 

 wall of the room. The steel wire, w w, to be tested is tensely 

 drawn between the cylindrical cores, a I, a!V, and clamped 

 without torsion. To accomplish this the ends of the wire are 

 bent hook-shaped, and inserted in flat fissures at the ends of 

 the rods a I, a'U, the clamp-screw (shown at a') passing 

 through the hooks of the wire. The middle part oi ww 

 carries a pair of plates, b c, held together by four screws in 

 such a way as to clamp the wire firmly between the plates. 

 A brass rod of screw-wire, d d, is soldered to the upper plate, 

 and when in adjustment is horizontal and at right angles to 

 the axis of w w. Thus the rod d d is virtually the beam of a 

 balance, and two small nuts, m m' ', symmetrically placed near 

 the ends of d d, are provided with hooks from which a scale- 

 pan, h h, may be suspended. The latter carries a light hori- 

 zontal disk, vv, to be submerged in water (not shown), with 

 the object of deadening vibrations when the apparatus is in 

 adjustment. The upper plate be also carries an adjustable 

 mirror, N, for angular measurement, as well as an index, fg, 

 moving across a stationary graduated dial (not shown) by 

 which the amount of twist holding the weight, P, in equili- 

 brium is roughly registered. 



In adjusting the apparatus, the scale-pan hh is removed 

 and the rods a I so placed that the lever dd is vertical. After 

 clamping a I (care being taken to avoid twisting wiv) the 

 scale-pan is attached at m', and additional weights P added to 

 hold the lever dd horizontal. Hence the two halves of the 

 wire ww are twisted 90° each for the whole length. Other 

 twists may be applied by rotating the torsion- circles together 

 (it would be necessary to connect them rigidly for this pur- 

 pose) and adding suitable weights P. 



To fasten the wire securely between the plates the lower 

 one is clamped horizontally in a vice. If there were rota- 

 tional sliding possible here it would be detected in examining 

 very viscous wires. Some difficulty is experienced in fixing 

 ww in place quite free from initial torsion. There is also 

 some flexure ; but for wires of the radius given these discre- 

 pancies may be disregarded. When A A and B B are rigidly 

 connected, the twist of the wire ww may be expressed in 

 terms of the load P. 



13. To complete the results obtained with this apparatus, 

 let 2<£ be the amount of angular viscous motion*, in radians, 

 of any right section of the wire relative to another right 

 section, whose distance from the first is the unit of length. 

 Suppose, furthermore, that in the plane of a right section 

 * The factor 2 is supplied conformably with rny earlier notation. 





