Light Waves and Their Uses
to what was termed its “ permanent condition,” these bars were subjected to all sorts of treatment and maltreatment. The originals were cast and recast a great many times, and afterward they were cooled—a process which took several months.
After such treatment it is believed that the alteration in length of these bars will be exceedingly small, if anything at all. But, as a matter of fact, it is practically impossible to determine such small alterations, because, while there have been a number of copies made from this fundamental standard, these copies are all made of the same metal as the original; hence, if there were any change in the original, there would probably be similar changes in all the copies simultaneously, and it would therefore be impossible to detect the change. The extreme variation, however, must be of the order of one-thousandth of a millimeter or less in the whole distance of 1,000 millimeters.
The question rightly arises then: Why require any other standard, since this is known to be so accurate? The answer is that the requirements of scientific measurement are growing more and more rigorous every year. A hundred years ago a measurement made to within oue-thousandth of an inch was considered rather phenomenal. Now it is one of the modern requirements in the most accurate machine work. At present a few measurements are relied upon to within one ten-thousandth of an inch. There are cases in which an accuracy of one-milliontli of an inch has been attained, and it is even possible to detect differences of one five-millionth of an inch. Past experience indicates that we are merely anticipating the requirements of the not too distant future in producing means for the determination of such small quantities. A«min, in order that the results of scientific work already
completed, or shortly to be completed, may be compared
Light Waves as Standards of Length
and checked with those of subsequent researches, it is essential that the units and standards employed should have the same meaning then as now, and, therefore, that such standards should be capable of being reproduced with the highest attainable order of accuracy. We may, perhaps, say that the limit of such attainable accuracy is the accuracy with which two of the standards can be compared, and this is, roughly speaking, about one-half of a micron — some say as small as three-tenths of a micron. For such work neither of the three methods described above of producing a standard is sufficiently accurate. As before stated, the results obtained by them vary among themselves by quantities of the order of one part in 50,000 to one part in 20,000. Since the whole meter is 1,000,000 microns, an order of accuracy of one-half of a micron, which can be obtained with a microscope, wTould mean one part in 2,000,-
000, which is far beyond the possibilities of any of the three methods proposed.
We now turn to the interference method. Some preliminary experiments showed that there were possibilities in this method. The fact to which we have just drawn attention— namely, that the w’avc lengths are the same to at least one part in 500,000—looks particularly promising and leads us to believe that, if wTe had the proper means of using the waves and of multiplying them up to moderately long distances, without multiplying the errors, they could be used as a standard of length which would meet the requirement. This requirement is that a sufficient number of waves shall produce a length which may be reproduced with such a degree of accuracy that the difference between the new standard and the one nowT serving as the standard cannot be detected by the microscope.
The process is, in principle, an ideally simple one, and