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Producing one determination of the speed of light

The distance |RM| from the rotating mirror to the fixed mirror was measured five times, each time allowing for temperature, and the average used as the ``true distance'' between the mirrors for all determinations.

The fire for the pump was started about a half hour before measurement began. After this time, there was sufficient pressure to begin the determinations.

The fixed mirror M was adjusted as described above and the heliostat placed and adjusted so that the sun's image was directed at the slit.

The revolving mirror was adjusted on two different axes. First it was inclined to the right or left so that the direct reflection of the light from the slit fell above or below the eyepiece of the micrometer. Michelson found that he had to tilt the revolving mirror as ``Otherwise this light would overpower that which forms the image to be observed.''29 The revolving mirror was then adjusted by being moved about, and inclined forward and backward, till the light was seen reflected back from the distant mirror.''30 Some adjustment in the calculations was made for the tilting of the mirror.

The distance |SR| from the revolving mirror to the cross-hair of the eyepiece was measured using the steel tape.
The vertical cross-hair of the eyepiece of the micrometer was centred on the slit and its position recorded in terms of the position of the screw.

The electric tuning fork was started. The frequency of the fork was measured two or three times for each set of observations.

The temperature was recorded.

The revolving mirror was started. The eyepiece was set approximately to capture the displaced image. If the image did not appear in the eyepiece, the mirror was inclined forward or back until it came into sight.
The speed of rotation of the mirror was adjusted until the image of the revolving mirror came to rest.

The micrometer eyepiece was moved by turning the screw until its vertical cross-hair was centred on the return image of the slit. The number of turns of the screw was recorded. The displacement is the difference in the two positions. To express this as the distance |IS| in millimetres the measured number of turns was multiplied by the calibrated number of mm. per turn of the screw.

Steps 10 and 11 were repeated until ten measurements of the displacement |IS| were made.

The rotating mirror was stopped, the temperature noted and the frequency of the electric fork was determined again.

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Next: Statistical Method and Michelson's Up: Michelson's 1879 determinations of Previous: Measurement equipment