410 



ILLUSTRATIVE EXPERIMENTS 



force per unit area necessary to maintain this condition is proportional 

 to the difference of velocity, v, of two adjacent layers, and inversely pro- 

 portional to their difference apart x. 



V 



Briefly, Force = ?/ x , where ?; is the coefficient of viscosity, which is the 



force per unit area when v = x. If, now, the quantity of fluid and the pressure 

 be kept constant and the time observed which the fluid takes to travel a 

 certain distance, the viscosity of two liquids with densities s' and s" with 

 times of flow t' and t" will be as 



/ / // ' f I ft At? 



'i rt =st /s t . 



Neglect of the difference of density introduces an error of less than 1 per 

 cent, and materially simplifies the operation, i.e. 



Apparatus (Denning- Watson Viscosimeter). The instrument is a modi- 

 fication of the Ostwald-Poiseuille viscosimeter. It consists of a U-tube 

 with a long and a short arm (Fig. 77). The long arm 

 (6 cm. in length) is blown out at its free end into a cup- 

 shaped receiver with a thin edge. 



On the short arm (2 cm. in length) there is a small 

 elliptical bulb, the capacity of which is defined by the 

 two lines m' and m". 



Method of Use. The receiving cup at the end of the 

 long arm is filled (and kept filled) with the fluid to be 

 tested, which passes down the capillary tube. A stop- 

 watch is started when the fluid reaches the point m' 

 and stopped when it reaches m". The time taken is 

 compared with the time reading of water, which is re- 

 corded on the back of the tube used. Denning and 

 Watson urge attention to the following points. (1) The 

 tubes should be scrupulously clean and perfectly dry. 

 (2) The viscosimeter and the fluid must be at the same 

 fixed temperature. (3) The receiver of the instrument 

 must be kept filled with the fluid, for pressure-height 

 must be kept constant, i.e. compared with water. (4) 

 The movement of very viscous fluids may be initiated by 

 slight suction applied to the short arm. (5) Clean the 

 tubes immediately after use. They are best cleaned by a strong reagent like 

 cone. HN0 3 (depending on the fluid last measured), followed in quick 

 succession by distilled water, alcohol and ether. Hot air is passed through 

 the tube to dry it. 



25. Imbibition, (a) A strip of thin sheet rubber (dental or patching 

 rubber) about 12"x H" is cut almost its whole length into two fingers of 

 equal width. One of the divisions is immersed in a boiling tube filled with 

 benzol, while the other half is left hanging outside. In a few minutes the 

 immersed division imbibes benzol and swells so that it is at least half again 

 as long and as broad as the unimmersed division. 



(b) Allow a sheet of ordinary glue to lie overnight on a moist surface so 

 that the under portion of the glue alone is in contact with water. Note 

 the increase in volume of the immersed portion and also the alterations 

 in colour, opacity, elasticity, etc. 



m 



FIG. 77. Viscosimeter 

 (Hawksley). 



