Measurement equipment

Michelson made use of several pieces of measurement equipment.

Distances |SR| and |RM| were measured using a steel tape, nominally 100 feet long.

The displacement |IS| was measured by means of a calibrated micrometer as shown in Figure 7.

  

Figure: Micrometer measures the displacement |IS|.

The source of the light was a narrow vertical slit that was fixed in place on the micrometer. The micrometer had a small telescope that could be moved left to right using a dial at the right. Each turn of the screw would move the telescope some small known amount. In Figure 7, the horizontal scale shown marks the amount turned. At the focus of the telescope lens (about 2 inches), and in nearly the same plane as the slit, S, was a single vertical silk fibre that served as a vertical cross-hair for alignment purposes. By turning the screw, the telescope could be positioned so that this fibre was centred on the returning image of the slit at I. The amount the telescope had to be moved from its initial position at the slit, to the position of the image would be the displacement |IS|.

The speed of rotation n, number of revolutions per second, of the revolving mirror was set using an electric tuning fork which vibrated at about 128 cps. The valve from the pump was opened to rotate the mirror R and make its speed in revolutions per second match the frequency of the electric tuning fork in vibrations per second. The speed and frequency were matched by having a small mirror attached to one arm of the tuning fork placed so that some light reflected from the revolving mirror was in turn reflected by the tuning fork's mirror to produce an image of the disk of the revolving mirror on a piece of plane glass located near the lens of the eyepiece of the micrometer. If the tuning fork frequency and the speed of the revolving mirror were the same, then the final image appearing on the glass would be distinct. In most of Michelson's determinations, the frequency of the fork was half that of the revolving mirror, so that two distinct images were produced.⁵⁰

The frequency of the electric tuning fork, called Vt₂, was measured by counting the beats per second between it and a standard tuning fork Vt₃ with known frequency 256.070 cps at 65 degrees fahrenheit. A 60 second count period was used. The temperature was recorded to correct the frequency of the standard fork for temperature. The frequency of the electric fork is thus one half of the sum of 256.070, the number of beats per second and the correction for temperature.

The final result for the speed of the revolving mirror in revolutions per second is determined from the frequency of the electric tuning fork and the number of distinct images on the glass plate