Charles Augustus Schmalcalder registered Patent N° 3545 on March 5, 1812, under "Philosophical Instruments", and was granted an "especial licence" by His Majesty King George III, which had a duration of fourteen years.  The Patent expired on March 4, 1826.

Charles Schmalcalder was born on March 29, 1781 in Germany, as Karl August Schmalkalder.  he Anglicised his name when he moved to London.  He worked from 1806 until 1840 when he was recorded as ill with disability and living in The Strand Union Workhouse, Castle Street, St. Pancras, London, where he died on December 25, 1843 in London, aged 62.  He is recorded as having been buried in The Strand Union Workhouse cemetery.  It would seem that Charles Schmalcalder fell on hard times towards the end of his life, probably caused by illness, which prevented him from earning a living.

What is the "Schmalcalder Patent"?  Schmalcalder was not known as a compass maker…. so what is it?  What is a Schmalcalder compass?
The Schmalcalder Patent refers ONLY to the folding prismatic sighting mechanism with sighting vane that appears on compasses.  It does not apply to the compass itself.  The compass had already been "invented" when he filed his patent.  The Schmalcalder Patent was purely an "improvement" to the current state of the art.

In 1811, Henry Kater devised a viewfinder and sighting vane assembly for viewing the target and the bearing at the same time, using a small mirror set at 45° above the outer edge of the card, opposite the sighting vane, and level with the eye when sighting.  This was a major step forward.  henry Kater went to Thomas Jones in The Strand and had him make two prototypes, both extant, one in a museum and the other in private ownership.  Thomas Jones, who also "made" for Schmalcalder, must have let Schmalcalder get a sight of this new design, and Schmalcalder came up with the idea of using a prism instead of a mirror, and had Thomas Jones make up his prototypes, which he moved fast to patent, beating Henry Kater in the patent race.

The Schmalcalder Patent was undoubtedly the most important development in the history of compass making, with the obvious exception of the original invention of the compass.  The design, patented in 1812, is still in use today, unchanged, on all of the world's finest prismatic compasses used by armies in nearly every country around the world.

How do you identify a Schmalcalder compass?  Any prismatic compasses signed "C. A. Schmalcalder" are very rare, and probably slightly pre-date the Patent.

Compasses bearing the mark "Schmalcalder's Patent - 82 The Strand or 399 The Strand, London" were compasses manufactured under license, or to order by Schmalcalder by third parties during the lifetime of Schmalcalder's Patent.  Schmalcalder was not known to have the ability to produce compasses in commercial quantities, so he would have done what all businesses did, and still do… have them made by a "maker" on his behalf.  The address on the card refers to the Patent more than to the maker, but it is certain that compasses bearing the wording C. A. Schmalcalder or Schmalcalder's Patent were produced to order for Schmalcalder, and were certainly sold almost exclusively from his shop, either at 82 The Strand or subsequently from 399 The Strand.  The hand-written signatures on the dial would certainly have been made in Schmalcalder's own workshops.

It is interesting to note that all compasses signed "C. A. Schmalcalder" or "Schmalcalder's Patent" bear a sequential serial number.  All the compasses bearing the address 82 The Strand, the earlier address, have the letter "B" as a suffix to the serial number (e.g. 291 B) and all compasses bearing the later address, 399 The Strand, have the letter "C" as a suffix to the serial number (e.g. 2150 C).  I believe that compasses signed "C. A. Schmalcalder may have carried the suffix "A" and may have been produced at his earliest known trading address in Soho, but I have not been able to confirm this yet.  Serial numbers were always hand-engraved on the base and inside the lid, and should match.

After 1826 a number of manufacturers produced prismatic compasses in great quantities, signing their own names, as the Patent had expired, and namely Troughton, Simms, Barker and many others.
Charles Schmalcalder did not appear to use brass lids for the compasses he signed "C. A. Schmalcalder?.  The lid, however, appeared on all compasses signed "Schmalcalder's Patent".  The lid is clearly mentioned in Schmalcalder's Patent Application.

Charles Schmalcalder worked from his home at 6, Little Newport Street, St. Ann's, Soho, London from 1806 for some years.  By 1812, when he filed his Patent Application, he was working from The Strand, London.  Charles Schmalcalder worked at 82 The Strand from 1810 until 1826, and from 399 The Strand from 1827 until his retirement in 1840.

Charles Augustus Schmalcalder was succeeded by his son, John Thomas Schmalcalder (born 1811), who traded initially from his home in 6, Little Newport Street, Leicester Square, London, and then from 1841 until 1845 from 2 Fairfax's Court, 400, The Strand, London.