﻿446 Barns and Strouhal — Viscosity of Steel. 



Method of Experimentation. 



Apparatus. — 1. For the reasons given the scope of this paper 

 has been limited to a study of the viscosity of steel, in its rela- 

 tions to temper. We endeavor therefore to determine the dif- 

 ference of viscosity which corresponds to a given known differ- 

 ence of hardness. The following differential method of meas- 

 urement, the principle of which can be made clear in a few 

 words, naturally suggests itself : 



Griven a pair of steel wires one of which is always glass-hard, 

 the other of known temper ; let them be made the threads of a 

 bifilar suspension provided with suitable means for the measure- 

 ment of the angular motion of the line denned by the lower 

 double-point of attachment, relatively to the upper line of at- 

 tachment; finally let a twist of + (p be imparted to one steel 

 wire, a twist of — (p to the other. If now the system be left to 

 itself, and if the two wires be equally viscous the bifilar system 

 {catt. par.) will remain at rest. If on the other hand the viscos- 

 ities be unequal, the lower free part of the bifilar will move 

 around the vertical let fall from the upper part. The sign of 

 this angular motion will be that of the twist of the wire of 

 greater viscosity ; its amount (ccei. par.) directly dependent on 

 the difference of viscosity of the pair of wires under experiment. 

 One of the forms of apparatus actually used is given in the 

 annexed diagram, figures 1 and 2, the former on a scale of J. 

 The couple of steel wires aivwg and ax7:g to be tested, is fastened 

 above to a massive piece of brass ABC and below to a similar 

 piece DEOF. ABC is fixed firmly to the wall at some dis- 

 tance from it, and all incidental motion or displacement is 

 registered by the mirror i/, adjustable at pleasure by aid of the 

 screw-arms fed and he. DEGFis free to move under the in- 

 fluence of the bifilar couples and motion is similarly registered 

 by the mirror iV adjustable at pleasure by aid of the screw-arms 

 mlk and hi. To keep the line of symmetry of the piece DEGF 

 vertical, to deaden incidental vibrations, and to give general 

 stability to the whole bifilar adjustment, we suspended a heavy 

 lead disk HQ (5 lbs.) from the screw r. HQ is partially im- 

 mersed in water contained in a glass bell-jar ILK also (adjust- 

 ably) attached to the wall. 



The steel wires are inserted and then fastened in the apparatus 

 as follows : To keep the two steel wires (about 33 cm long) 

 under a permanent strain of torsion, each is softened about 2 cm 

 from the ends inward. The soft parts are then bent into a hook- 

 like loop just large enough to slide over the screws of the little 

 brass bolts a and g. Having fastened the lower ends by the 

 bolt g, the upper ends are each twisted 180° against each other 

 and then also fastened by the bolt a. 



