The Chronograph (used with the Airy Transit Circle)

 

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Description of the Chronograph (Appendix to the 1856 volume of Greenwich Observations)

 

Airy's description of the Chronograph from the introduction to the 1854 edition of Greenwich Observations

In the ground-floor room of the North Dome, there is mounted a clock of peculia construction, whose motion is governed by the rotation of a conical pendulum. One spindle of the clock gives motion to a revolving brass barrel, about 20 inches long and 12 inches in diameter, and, as the clock with conical pendulum moves without jerks, the brass barrel revolves without jerks, and with a motion sensibly uniform. The barrel revolves in two minutes of time; its cylindrical surface, therefore, moves through 0.3 inch nearly in one second of time. The barrel is covered with woollen cloth, and upon this a sheet of paper is folded: its ends arc cemented together; when it is filled by the register about to be described, it is removed, and another sheet of paper is put in its place.

Another spindle of the clock turns two long screws both parallel to the axis of the barrel, which cause a travelling-frame to traverse the whole length of the barrel. In one revolution of the barrel, the frame moves through 0.1 inch. This travelling frame carries two levers, and each lever is armed at one end with a pricking point; the mounting of each lever being such, that when the opposite end of the lever is pulled away from the barrel, the pricking end is impressed upon the barrel, and makes a permanent puncture on the paper. The prickers are mounted in such a way that they yield laterally to the motion of the revolving barrel, and do not scratch the paper. Two gal­vanic magnets are fixed on the travelling frame, so as to attract the lever-ends opposite to the pricking points. All that is required, therefore, to cause these points to make punctures upon the paper is, to send galvanic currents through the galvanic magnets.

One of the prickers is devoted to the register of seconds of the transit-clock. For this purpose, the wires of its galvanic magnet (after passing through a galvanic battery) are led to the transit-clock, and the circuit is there completed at every second of time. At first this was done by an attachment on the clock-pendulum, which at every swing united two slender springs. This method answered, for a time, very well, but the changes in the distance of the springs from the pendulum-support (in conse­quence of a peculiarity in the mounting of this clock) ultimately deranged the clock-­rate. The following method was then adopted, and it appears to answer well. Upon the escape-wheel-arbor, a wheel of 60 teeth is mounted; and, at every second, the start of a tooth of this wheel presses together two springs. Thus a series of punctures is made upon the revolving barrel, one at every second of the transit-clock. Proper means are provided for breaking the circuit at pleasure, and at the same time stopping the movement of the travelling-frame, so as to avoid unnecessary consumption of paper.

The other pricker is used for the register of the times of observations. The wires of its galvanic magnet, after passing through a battery, are led to the Transit-Circle pier, and terminate in two large springs, which touch two large insulated brass rings upon the conical axis of the Transit-Circle. From these brass rings wires are led within the Transit-Circle-Telescope, to a contact piece near the eye end, where the observer, by a touch of the finger, can complete [the] circuit, and thus make a puncture on the revolving barrel.

Branches of the same wires are led to the Altazimuth, where there is a nearly similar apparatus for completing [the] circuit.

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