13 8 IR E TRANSACTIONS O N INSTRUMENTATION December was pointed out earlier that readings repeated in rapid pared. There is little doubt that even more startling sequence with little or no disturbance of the equipment contrasts between real an d apparent errors would ap- cannot be expected to reveal the real errors. Fo r exam- pear when the environment itself has to be carefully ple, a gage block be used to "set" a very sensitive controlled to minimize errors. Some investigations gaging device. Th e user compares tw o items and is im- should be made in order to ascertain whether some of the pressed by the sensitivity of the device. A demand is demands made fo r better standards are really justified. then made fo r better standards. What is called for is a little ingenuity to devise means CONCLUSION that will disclose the real magnitude of the errors in the At every stage in the hierarchy of calibrating labora- measurements. Closely similar test blocks should be tories, there is a laboratory that has on a bench three periodically resubmitted fo r measurement. Near equal- objects of interest: ity of the test blocks is needed to make identification 1) A standard item from the echelon above with a difficult. T h e schedule of tests should be prepared in certificate some artful or random sequence. 2) An assembly of equipment appropriate fo r com- T he following example2 illustrates t h e that paring items, an d operators calibrating clinical thermometers could "re- collection an d peat t h i rednseteeywl.vnwe h 3) A collection of items awaiting calibration fo r the eat" their readings extremely well even when the ehlnblw repeat reading followed t he reading of 23 other ther- echelon below. mometers. T he average difference between repeat read- On e safe procedure for all calibrating laboratories would ings was 0.0102°-one twentieth of a scale division. The be to quote the uncertainty in its standard, to state the average of 24 readings should have an error of about uncertainty in its comparison process, an d to tell its 0.01/V/24 or 0.002. T h e holder and t h e 24 thermometers customers that t h e simple su m of these two components were se t aside fo r a few days an d then reread. Again the is th e only safe measure of the possible error in the superb agreement between closely repeated readings wa s value assigned to the item just calibrated. observed. Unfortunately the average of the readings It should not be overlooked that the uncertainty shifted by more than 0.02°, i.e., an order of magnitude stated on th e certificate accompaning t h e standard greater than the expected error. This phenomenon w as sometimes includes a "stability" allowance. On the demonstrated over an d over again with different op- basis of broad experience a reasonable estimate of the erators an d different sets of thermometers. Operators drift effects ca n be made. When the uncertainty as - shifted relative to one another as much as 0.04° when signed to the standard includes such an allowance, this their averages on the same thermometers were com- information should also be given. Thle Speed o f Light* A. G. McNISHt Summary-Numerous measurements of the speed of light pub- INTRODUCTION lished during the last 30 years lead to widely divergent results as compared with th e assigned experimental uncertainties. Because of r HE SPEED OF LIGHT is a classic subject fo r wide diversity in the methods employed in the measurements, all scientific papers, bu t not all papers on this subject of th e data ma y not be combined effectively in a grand average. -"are classics. Th e speed of light is important to our Sufficient data had been obtained by the geodimeter method to scientific knowledge and ur engineering practice. It group them an d derive a statistical estimate of the uncertainty in the speed of light by this method. This result, and conclusions reached intrigues the theorist an d invites the experimentalist from careful examination of several experiments, leads to the con- Naturally great interest attaches to the value assigned clusion that th e value 299,792.5 km which ha s been internationally t o this important physical constant. The interest is adopted for us e in radio propagation and geodetic work is very close particularly high now because of the use of electromag- to the best value and no t likely to be in error by as much as on e part netic radiation fo r l oc at in g t he position of bodies in in one million. outer space. * Received September 4, 1962. Presented at the 1962 Interna- Oeproeo hspprist e ot h rs n tional Conference on Precision Electromagnetic Measurements as state of ou r knowledge on this subject. The author will Paper No. 3.2. dsusterlaiiyo aiu esrmnsi c t Metrology Division, National Bureau of Standards, Wash-dsusterlaiiyo a iu esrmnsi c ington, D. C. 'cordance with his o w n judgment and conclude from this AlultICLDoJ0a1UfIX Ra
