Symons Patent GB3293, 20th September 1875
George James Symons of Camden Square filed his patent for improvements in magnetic compasses on 20th September 1875. The patent was granted the following year.
His patent sought to address the issue that Magnetic North (which compasses point to) and True North are not the same. What was appreciated at the time was that this difference varied with location, which his patent refers to. What was not appreciated was the extent of this variation over decades and even centuries. In fact the earth’s magnetic poles have switched several times over millions of years. The last switch was about 780,000 ago and they average about once every 300,000 years.
This patent is a fascinating example of how at the time of it’s filing, the understanding of the fluctuation was limited, since we now know that the poles have switched many times.
His solution was to offset the needle to correct the compass card, he quite rightly states that this is approximate correction and to be honest for general navigation at slow speeds, it can be lived with. A Symons Patent compass is not a very common one, I have only ever seen a few examples of these which is a pity since, in my view, the design is both functional and easy to read.
In reality his compass would only have been really accurate over a period of a few years, any Symons compass was very inaccurate by the turn of the century in 1900. Clearly he appreciated the issue, it’s implications and tried to compensate for the fact that the difference between true and magnetic north varied around the world by making that adjustment possible on the compass. To do this mechanically is quite challenging.