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METRIC LACQUERED BRASS SCREW COVER-GLASS MICROMETER WITH SQUARE INDICATOR DIAL

SIGNED IN A SEMICIRCLE ABOVE THE IMAGE OF A PYRAMID: 'PYRAMID'

c. 1875

MAKER: JOHN BROWNING (?)
DESCRIPTION HISTORY

DESCRIPTION:
pyramidThis is a lacquered brass screw micrometer with a square faced indicator dial. The square face measures 70 x 70 mm. The entire instrument including the knob assembly measures about 80 mm thick when the micrometer is at the zero mark. It is calibrated from 0 to 100 in increments of 0.005 mm, and labeled every ten 0.005 increments in multiples of 0.005, i.e. a measurement of '10' would be 0.05mm and 100 would be 0.5 mm etc. It has a ratcheting system built in to the control stem so that the user can easily apply just the right tension without under- or over-tightening. It has teeth that match and are held against each other by a spring inside the stem. When sufficient tension is reached, it overcomes the force of the spring holding the teeth together, and the upper stem teeth ride up and down the lower teeth(red arrows in above illustration), avoiding overtightening the lower stem which is against the coverslip. This improves accuracy and also prevents damage to the glass being measured. An adjustment screw allows setting to zero if out of calibration. Another adjusting screw adjusts the tension on the measuring screw. On the side of the C-shaped piece on the back of the instrument is the insignia and name for the Pyramid Brand. Pyramid was a brand used by Tupper & Co, a manufacturer of metals on Berkley Street in Birmingham known to be in operation in 1883.
 




HISTORY:
An almost identical micrometer, signed by John Browning is in the Collection of the Whipple Museum in Cambridge, previously in the T.H. Court collection.

Leitz Nr 1Another almost identical cover glass micrometer, but with a silvered face, was sold as part of a German microscope outfit by Leitz, the Stativ Nr 1. in which the accessory was described in the 1872 listing in Frey as: 'Instrumentchen zum genauen Messen der Dicke der Deckglaschen' or 'Instrument for accurately measuring the thickness of the coverslips' In the 1875 catalog it was referred to by the more succinct and familiar German name 'deckglastaster'. The German example shown here in its case with the Nr 1 microscope, has a silvered face, the rest of the micrometer lacquered brass, and lacks the stamp of the 'Pyramid' Brand, so it is feasible that these were made both in England and Germany. Thanks to Allan Wissner for alerting me to the existance of this German example, and for the catalog information, as well as permitting me to share his image of the micrometer in the box for the Leitz Nr 1.

For a general history of coverglass micrometers see the history section of the coverglass micrometer page.