STANDARDS OF MEASUREMENT — BRIGGS 17 J 



ence would amount to approximately 1 part in 5,000 owing to the 

 discrepancy of the units. 



For these reasons the International Committee of Weights and 

 Measures decided to replace the present international electrical units 

 with absolute electrical units, the new system to go into effect January 

 1, 1940. 



In order to carry out the wishes of the International Committee it 

 has been necessary for the National Bureau of Standards and the 

 national laboratories of other countries to determine in absolute meas- 

 ure the value of their electrical standards of resistance and electro- 

 motive force with the highest attainable precision. A brief descrip- 

 tion of the Bureau's contribution to this program, which has extended 

 over a number of years, will now be given. 



Inductance and current are the most suitable quantities for evalu- 

 ation in absolute measure because they can be determined to a high 

 degree of precision without involving the measurement of any addi- 

 tional electrical quantity. But the final objective in absolute meas- 

 urements is the value of the standard of resistance and the standard 

 of electromotive force. The procedure is as follows: (1) The induc- 

 tance of a coil is computed from its geometrical dimensions and the 

 permeability of the material on which it is wound; (2) this known 

 inductance, when measured experimentally in terms of time and a 

 standard resistor, serves to fix the value of the resistor in absolute 

 measure; (3) current is determined in absolute value from the geomet- 

 rical dimensions and positions of the coils of a current balance, 

 supplemented by the absolute measurement of the force exerted be- 

 tween the coils; (4) the potential drop across the standard resistor 

 when connetced in series with the current balance serves to fix the 

 value of a standard cell in absolute measure. 



THE ABSOLUTE MEASUREMENT OF ELECTRICAL RESISTANCE 



Two independent groups at the National Bureau of Standards have 

 been working on the absolute measurement of electrical resistance. 

 Curtis, Moon, and Sparks have used an improved self -inductor with 

 an intermediary capacitance in determining the absolute value of the 

 ohm, while Wenner, Thomas, Cooter, and Kotter have determined 

 resistance in absolute measure by a method using commutative direct 

 current in a mutual inductor. 



SELF-INDUCTOR METHOD WITH INTERMEDIATE CAPACITANCE (2) 



In this method the self-inductance of an inductor is measured in 

 terms of time and a laboratory standard of resistance. The ratio of 

 the computed to the observed inductance provides the correction fac- 

 tor which is to be applied to the resistance standard to give the re- 



