STANDARDS OF MEASUREMENT BRIGGS 169 



STANDARD OF CAPACITY 



The standard of capacity, the liter, is established by weighing. It 

 is deJSned as the volume occupied by a kilogram of water at its 

 maximum density. This volume is unfortunately not exactly 1,000 

 cubic centimeters as the founders of the metric system had intended. 

 One milliliter= 1.000027 cc. The discrepancy is within the error 

 of measurement of most volumetric determinations, but in precise 

 density measurements the unit of volimie (cubic centimeter or milli- 

 liter) must be specified. 



THE STANDARD OF FREQUENCY 



The national standard of frequency is maintained by means of 

 seven quartz oscillators, with natural frequencies of 100 or 200 

 kc./sec. They are carefully protected from external vibration and 

 the temperature and pressure are closely controlled. These oscillators 

 are intercompared constantly and they are also compared daily 

 with time signals from the Naval Observatory. For this purpose, 

 one of the oscillators, with the aid of a submuitiple generator, drives 

 a synchronous motor clock which indicates mean solar time. 



This group of oscillators serves to control the precision of the 

 standard frequencies of 5,000 kc./sec, 10,000 kc./sec, and 15,000 

 kc./sec. which are broadcast several days each week from the 

 Bureau's station WWV at Beltsville, Md. These frequencies do 

 not deviate more than 1 part in 5 million from the assigned value. 



By means of this service, broadcasting stations throughout the 

 United States are enabled to adhere closely to their assigned fre- 

 quencies. In addition, the emissions are modulated to give certain 

 standard frequencies in the audible range, which have been found 

 very useful by physicists and engineers. These modulations include 

 a frequency of 1,000 c/sec. as well as sharp 1-second pulses, accurate 

 to 0.00001 second. The broacasting of the standard of musical pitch, 

 440 c/sec, representing A above middle C, has also met with wide 

 favor by musicians and laboratory workers alike. 



ELECTRICAL STANDARDS 

 THE STANDARD OF RESISTANCE 



Since 1908 the national standard of electrical resistance has been 

 represented by a group of 10 1-ohm manganin coils, the average 

 value of which has been assumed to remain constant. The value 

 originally assigned to each coil was based upon standards certified 

 by the Physikalisch-Technische Keichsanstalt in 1908. When any 

 member of the basic group of 10 coils showed a pronounced tendency 

 to drift in relation to the group, it was replaced by a coil from the 

 reserve group. The NBS "international miit" of resistance is de- 



