Disclaimer: all content here is solely my own views, and in no way represents the views of my employer, or anyone else. Also, I'm documenting things as I learn about them, so expect frequent errors. Corrections will occur without notice and without a changelog at this point.
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Image Credit: Electro-recording Apparatus
Earliest source: William Cranch Bond. Annals of the Astronomical Observatory of Harvard College / History and Description of the Astronomical Observatory of Harvard College vol. 1. Metcalf and Company, 1856.annals1
As described in the annals, this device uses "electro-magnetism" and "galvanic current". It is electrically wired to the observatory clock, and to switches placed at all of the major instruemnts. It records clock ticks in spirals on a paper cylinder, which is pulley driven. It also records signals from any of the instruments. So for example, when a transit is being measured, the precise instant a star transits the meridian, the observer will press a switch which will record an event on the cylinder.
Here's my understanding at this time. The precise timing of star transits on the meridian circle is used to determine the star's exact position at the meridan, and hence in the sky. The prime vertical (transit circle) instrument can then be used to measure the exact time a star crosses the prime vertical circle (the vertical circle which is perpendicular to the meridian circle). Based on the star's known position, and this time, the observatory was able to make precise measurements of its latitude, which by 1856 was 42° 22' 48.1", which at the time was the most accurately measured latitude on Earth.
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