22100 str (118)

44

ROTA TION I 7S( 'OMETERs

This has a leverage otr about the axis ₀f so that the couple tending to retanl the J3fH the imaginary cylinder is    ^ot*Qn ₀j

„ l n rftł)

— 2rrnnf⁹ . -t~,

ar

The sign is negative because the eflect is n « . tion.

lntegrating this between the Umits

ROTATING CYLINDER

a

(l)

<■

SHAFT

Fig. io. (After Barr.)

rotation viscometek Gomscmm h

llml for (i, the couple rerpiiied to umiliłam a Umgth (A) p the cyUiuh-r in lolatiou

It Ilu' miler cylinder 1§ altaclunl, sny to u spring, Q orni ho nieasured ftom lho tlofommtion of lim spring, and 1 caleulnted. Moro frorptonlly tlio innor cylinder is ultaohod to a torsion wiro to moasuio the torque, and the outer cylinder is rotated.

This treatment is deduced for cylindcrs of inliuito length, and in practice, where the length is finite, it is very difficult to make adeąuate corrections to allow for end-effeets. It is clearly best to tnako aa apparatus which eliminates these, and of this type, the guard-ring device is perhaps the best (Fig. ro).

Morę complex devices than this are freąuently used, this being the simplest of them. A good variable stress version is that of Searle, in which the length of the cylinder in the liąuid can be varied (Fig. ii).

Ungar uses an apparatus not unlilce this for clays, fats, syrups and flour doughs.^{1 2}

A temporary digression is necessary here in order to consider the apphcation of these methods to anomalous systems. It has been shown that in the Bingham treatment there is in theory no flow,

1

This treatment is taken from Barr’s " Monograph on Viscometry,” p. 222.

2

I Campbell describes a somewhat similar apparatus for imrestigating jellies.