| Westphal's (or Mohr's) Balance is used to measure the
specific gravity (or density) of liquids.
The arm is first balanced with the plummet totally immersed in water at 15.5 degrees C. The plummet has a built-in thermometer and has a known volume (equal to 5 gms of water at 15.5 degrees C) and mass (15 gm). A 5 gm mass is placed on the hook holding the plummet, and the screw on the foot is adjusted until the index pointer on the end of the beam lines up with the point on the frame. The plummet is then completely immersed in the unknown liquid, and the system is rebalanced, using a series of riders on the nine equally paced notches on the beam, thus giving the value of the added mass for each decimal place. This gives the buoyant force of the liquid relative to water, and hence the specific gravity, which may be obtained to four decimal places..
| This Westphal Balance is shown in its box,
along with the characteristic U-shaped balancing masses and the plummet
in the upper right-hand corner. The sliding top of the box has the decorative
molding attached to it.
This device has the maker's name on it -- and it is Westphal of Celle in Germany. It was given to the Greenslade Collection by Daniel Chaucer.