Light Waves as Standards of Length 91
liundred-foot tape corresponding to a considerably smaller distance. This smaller distance is what I have termed an “ intermediate standard.” There is in this latter case the additional operation of finding the number of light waves in the intermediate standard; so that, in reality, there are three distinct processes to be considered.
In the first operation it is evident that, if an error is committed whenever we lift the tape and place it down again, the smaller the number of times wTe lift it and place it down, the smaller the total error produced; hence, one of the essential conditions of our apparatus would be to make this small standard as long as possible. The length of the intermediate standard is, however, limited by the distance at wThich wTe can observe interference fringes. The limit, as was stated in the last lecture, is reached when this distance is of the order of several hundred thousand waves. At this distance the interference fringes are rather faint, and it seemed better for such determinations not to make use of the extreme distance, but of such a smaller distance as would insure distinct interference fringes. It was found convenient to use, as the maximum length of the intermediate standard, one decimeter. The number of light waves in the difference of path (which is twTice the actual distance, because the light is reflected back) would be something of the order of three or four hundred thousand waves. With such a difference of path we can still see interference fringes comparatively clearly, if we choose the radiating substance properly.
The investigations described in the last lecture showed that the radiations emitted by quite a number of the substances which were examined were more or less highly complex. One remarkable exception, however, was found in the red radiation of cadmium vaj>or. This particular radiation proved to be almost ideally homogeneous, i, e., to con
92
Light Waves and Their Uses
sist very nearly of a series of simple harmonic vibrations. This radiation was therefore eminently suited to the purpose, and was adopted as the standard wave length.
Most substances produce a more or less complicated spectrum involving quite a number of lines, but in the case of cadmium vapor, though there are three different radiations, these three are all so nearly homogeneous that each one can be used; and the complexity of the spectrum is in this case an advantage, as will be shown below. To produce the cadmium radiation, metallic cadmium is placed in a glass tube which contains two aluminum electrodes. The tube is then connected by glass tubing with an air pump and exhausted of air. The tube is also.heated so as to drive off all residual gas and vapor, and when the required degree of exhaustion is reached, it is hermetically sealed and in condition to use. The cadmium is not very volatile, and at ordinary temperatures we should see scarcely anything of the cadmium light when the electric discharge passes. The tube is therefore placed in a metal box, as shown in Fig. GO, which is furnished with a window of mica and has a thermometer introduced into one side. If the box be heated by a Bunsen burner to a temperature in the neighborhood of 300° C., the cadmium vapor fills the tube, and can then be rendered luminous by the passage of the electric spark.
Now, it is found most convenient not to make this first intermediate standard in the form of a bar like the standard meter, with two lines drawn upon it; for then we should introduce errors of the microscope at every reading, and these errors would be added together. Thus, since this is one-tenth of the whole meter, we might have, in adding up, ten times the error of the microscope, which we said was of the order of one-half a micron; we could thus have, in the end, an error of five microns. The interference method gives us the means of multiplying the length of the intermediate