KKPORT OF CHEMICAL LABOBATOEY 233 



With it determinations over very wide ranges of viscosity may be made, but for the stronger 

 solutions the method of observation recommended by the makers must be modified or very 

 serious errors may be introduced. The instrument, which deserves to be better known, is 

 shown in Fig. 98. 



A steel wire is suspended from a firm support and fastened to a stem which passes 

 through a graduated horizontal disc, thus allowing the torsion to be exactly measured. The 

 disc is adjusted so that the index points to zero, showing that there is no torsion in the wire. 

 A cylinder, 2 inches long by li inches in diameter, having a slender stem by which to 

 suspend it, is then immersed in the oil and fastened by a thumb-screw. The oil is surrounded 

 by a bath of water or paraffin, according to the temperature at which the observation is to be 

 made. This temperature being observed while the disc is resting on its supports, the wire is 

 twisted 360° by means of the knob at the top. The disc being released, the cylinder rotates 

 in the oil by reason of the torsion of the wire. The action is analagous to that of the 

 pendulum. If there were no resistance to be overcome, the disc would revolve to zero, and 

 the momentum thus acquired would carry it again to 360°. In reality, the resistance of the 

 oil to rotation causes the revolution to fall short of 360° in proportion to the viscosity of the 

 liquid. The retardation thus produced is a delicate measure of the viscosity. 



There are a number of ways in which this retardation may be read, but the simplest is 

 the number of degrees retardation between the first and second complete arcs covered by the 

 rotating pendulum. For example, suppose the wire to be twisted 360° and the disc released 

 so that rotation begins. In order to obtain an absolute reading which shall be independent 

 of any slight error in adjustment, the start from 360° is ignored, and the first reading taken 

 at the end of the first swing. The next reading, which is on the other side of the point, is 

 also ignored, as it belongs in common to both arcs. The third reading is taken, which will 

 be at the end of the second complete arc and on the same side of the point as the first 

 reading. The diiference between these two readings will be the number of degrees 

 retardation caused by the viscosity of the oil. 



Suppose the readings are as follows : — Methods of 



First reading, right-hand ... ... ... ... 356'6° 



Second ,, left-hand — ignore — 



Third „ right-hand 338-2° 



reading 



17'4° retardation 

 In order to secure freedom from error, two tests are made — one by rotating the 

 milled head to the right, and the other to the left. If the instrument is in exact 

 adjustment, these two results will be the same ; but if it is slightly out, the mean of the two 

 will be the correct reading. 



The above method will answer for comparing solutions not too strong, say up to 15 or 

 20° retardation. Beyond this a gradually increasing error is introduced which becomes a very 

 serious one when testing solutions of gum of 30 per cent, strength, and especially when the 

 viscosity in weaker solution is compared with that in stronger. It is evident that the extent 

 of retardation will depend upon the point at which the first reading is taken. 



In the example given above, the first reading was at 355-6°, at which point the wire was 

 under almost complete torsion. In the case of a very viscous solution this first reading may 

 be only, say, 200°, obviously the wire is under much less torsion at this point, and the 

 subsequent degree of oscillation being less, the retardation observed will be very considerably 



