AN ELECTROMAGNET MODIFICATION OF ZIMM-CROTHERS VISCOMETER

An electromagnetic modification of the Zimm-Crothers rotating inner cylinder viscometer is described. This modification replaces the motor-driven permanent magnets with the stator section of an a.c. induction motor. This means that the operating shear stress, and hence shear rate, may be varied simply by altering the magnitude of the alternating current flowing through the induction motor stator. This modified apparatus is ideal for the study of non-Newtonian behaviour in polymers at very low shear rates. At extremely low shear rates the expected thixotropic build up of structure in solutions of high molecular weight calf thymus and bacteriophage DNA can be detected, but this structure rapidly breaks down as the shear rate is increased.