The ball drop viscometer has a housing, in which a gravity tube (14) is positioned so that the fall of a ball (66) in the tube is detected by perpendicularly arranged sensors (7,8). A control valve (38,39) is used to regulate the supply of the fluid to be measured. A guide piece leads the fluid away from the tube to a lead off tube (67). Adjacent to the lead off tube is a permanent magnet (69), which acts upon the ball, which is of a magnetic material. The permanent magnet is held in its storage position by a compression spring, against a displaceable sealing wall, which can be acted upon by an over pressure device to force the ball upwards.
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