Timing the transit of Mercury, 1907

“About five seconds before third contact a black ligament connected the planet’s disk with the trembling edge of the Sun.”

―Winslow Upton, “Transit of Mercury at Ladd Observatory.” Popular Astronomy, 1907.

1907 transit of Mercury
Chronograph recording of the Transit of Mercury on November 14, 1907.

Prof. Winslow Upton tapped a telegraph key as he watched the planet Mercury transit the Sun. The signal was sent to a chart recorder where an electromagnet moved a pen on a slowing turning drum of paper to record the observation. The instrument is called a chronograph. This one was made by the Warner and Swasey Company of Cleveland in 1890. Primarily it was used for calibrating the astronomical regulators (precision pendulum clocks) to set the time accurately. It was also used for timing events such as lunar occultations of stars.

The chronograph with telegraph wires connected to the electromagnet holding the pen, circa 1895.

A second set of telegraph wires connected to a break circuit in the regulator. Each swing of the pendulum triggered a regularly spaced time signal that was recorded on the chart along with the astronomical observations. The precise time of the event can be read from the chart later. The Observatory was built before distribution of electricity was common. The telegraph system was powered by batteries in the basement.

gravity cell batteries
The telegraph system was powered by “gravity cell” batteries, circa 1939.

The glass jars contain “blue vitriol” which is a solution of copper sulphate in water. The “crow’s foot” shaped electrodes are made of zinc. When the battery is connected to a telegraph circuit it produces a second liquid called sulphate of zinc. The liquids separate with the heavier copper solution sinking to the bottom. The vivid blue color in the lower half of the jar fades as the batteries discharge giving a visual indication that it needs maintenance.

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