Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

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Analytical Balance Weights, 5g & 50g

Class M, NBS Certified

William Ainsworth

provenance: Humboldt College; 19620


Class M weights are intended as primary laboratory standards and for the very highest quality work. M Class weights are of single piece construction so that their mass tends to be more stable than othe weights which are made in two pieces to allow for adjustment. Single piece construction makes it more difficult to adjust the weights, raising their cost. M Class weights are also held to higher precision for most denominations. This set includes rodium plated bronze 1–100 g weights, aluminum 1–20 and tantalum 50–500 mg fractional weights, and forceps. These weights were used as primary standards to calibrate working weights at HSU by the analytical chemistry faculty. Because of their value (NBS [now NIST] certification adds significantly to the cost of weights) and the care required in thier use as primary standards these weights were kept in their original cardboard boxes in the analytical chemistry vault, along with platinum electrodes etc.

Unfortunately even unused weights can change mass, so these would need recalibration to continue use as standards.The use of these weights diminished with the use of the Mettler balances. Due to their internal weight sets, the weights can not be calibrated by intercomparison as was traditional with two-pan balances and standard weight sets.

Descriptions of Weights and their use are provided in the links below:


The two Class M weights are of one-piece stainless steel construction, 5 and 50 grams respectively. Class M weights must be of single piece construction rather than the two piece construction of other weights, as can be seen in the comparison photograph of a 5g M Class weight and a 10g S Class weight. (A second image includes a rule for comparison.) Note that in the S Class weight the knob is a separate piece, which can be unscrewed to reveal a small compartment for adjusting the mass of the weight. The S Class weights are from the Ainsworth NBS calibrated weight set.

A variety of documents etc. have been preserved. Scanned images are avilable via links for:

The Chemical Balance at Humboldt

 HSTC icon
HSTC (1921-34)
HSC 54-73 icon
HSC (1954-1973)
HSC 35-53 icon
HSC (1935-1953)

HSC 1956-72 Instrument Collection

© R. Paselk
Last modified 18 August 2010