STANDARDS OF MEASUREMENT — BRIGGS 173 



by the precision with wliich the speed of rotation of the shaft and the 

 geometrical dimensions of the mutual inductor can be measured. 



The shaft is driven by a direct-current motor, automatically syn- 

 chronized by a current of 1,000 cycles per second ; this latter frequency 

 is controlled by the primary quartz frequency-standards in the Bu- 

 reau's radio laboratory. Over periods of an hour or more the fre- 

 quency of the control current is constant and known to 1 part in 

 10,000,000, while the speed of rotation, averaged in the way in which 

 it affects a deflection of the galvanometer, is uniform within a few 

 parts in 1,000,000. 



The absolute determination of the ohm by Wenner, Thomas, Cooter, 

 and Kotter gives the relation 



1 NBS International ohm =1.000485 absolute ohms. 



THE ABSOLXTTE DETERMINATION OF THE AMPERE 



H. L. Curtis and K. W. Curtis (3) have made a new determmation 

 of the ampere in absolute measure, employing with some modifica- 

 tions the current balance originally used by Rosa, Dorsey, and Miller 

 for this purpose in 1911. The most important modification was the 

 use of coils in which the current distribution closely approached that 

 assumed in the theoretical derivation of the force. The value in 

 absolute amperes of the current in the coils of the balance was com- 

 puted from the dimensions and positions of the coils, the permea- 

 bility of the material and the electromagnetic force between the 

 coils, the latter being measured in local gravitational units. 



In such measurements, we must know the absolute value of the 

 local acceleration of gravity, and Heyl and Cook (4) have recently 

 completed this determination at the National Bureau of Standards 

 using pendulums of fused silica. They found, at the Bureau station : 



$r=980.08 cm. sec.-% 



which indicates that the absolute value of gravity at Potsdam, here- 

 tofore generally accepted, is about 2 parts in 100,000 too large. 



The current through the balance coils was measured simultane- 

 ously (1) in absolute value by means of the balance and (2) in 

 "international" amperes as determined by the potential drop across 

 a standard resistor in series with the balance coils. 



The result of the most recent measurements with balance coils of 

 improved design is : 



1 NBS International ampere =0.999852 ampere (absolute). 



THE INTERNATIONAIj TEMPERATDEE SCALE 



The thei-modynamic scale of temperature introduced by Lord 

 Kelvin has long been accepted by physicists as the ideal temperature 



