528 hersey: viscosimeters 



An experimental determination of this function F affords the 

 desired calibration equation for viscosity in terms of roll time. 

 Experimental test of the theory. Equations (4) and (5) were 

 verified in the course of a series of experiments in which the 

 behaviour of the tubes was minutely studied, and the linear 

 relation 



y = a + bx (6) 



was found to hold over the range from y = 0.0001 to y = 0.1, 



the constants having the values a = — 0.0009 and b = +0.00027, 



(subject to accidental errors of several per cent due to tempera- 



d 

 ture uncertainties); provided a = 15° and j: = 0.63, and that 



the ball and tube are of ordinary smoothness. The experiments 

 on which the above generalized equation is based were made 

 with a glass tube about 1 cm. in diameter, containing a \ inch 

 (0.635 cm.) steel ball. Since x, y, a, and b are dimensionless, 

 the numerical values given are common to all systems of normal 

 units. 



Construction for use under pressure. The foregoing details 

 were duplicated, except for small corrections, in the steel tube 

 used under pressure. This tube was fitted with electric contacts, 

 and with pressure-tight plugs of the type developed by Bridg- 

 man, in whose laboratory the work was carried on. It was 

 connected to the pump and gage by a considerable length of 

 copper tubing, and swivelled so that either end could nstantly 

 be thrown up to a prescribed angle. This steel tube was cali- 

 brated for different densities by reference to equation (6). 



Residts on two lubricating oils. Lard oil and minera machine 

 oil were selected for testing, because of the well known differ- 

 ence in their behaviour as lubricants, although, under atmos- 

 pheric conditions, they have nearly the same viscosity. Express- 

 ing the results in the form 



ix = Mo (1 + «P) (?) 



and measuring pressure in atmospheres, (kg. /cm. 2 ), the value of 

 the pressure coefficient of viscosity, a, at 20°C, was found to 

 be 0.0023 for lard, and 0.0032 for machine oil, over a range of 



