172 AJSTNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



sistance in absolute measure. Tlie four inductors used in this inves- 

 tigation were of different dimensions and vrere wound on nonmagnetic 

 forms of different materials, namely, porcelain, Pyrex glass, and 

 fused quartz. The most recent form consists of a heavy-walled glass 

 cylinder 120 cm. long and 35 cm. in diameter, on the surface of which 

 a very accurate screw-thread was ground and lapped to insure uni- 

 formity in the pitch of the helix. The electrical measurements re- 

 quired the use of an intermediary capacitance, so that a resistance 

 was first measured in terms of inductance and capacitance by an alter- 

 nating-current bridge ; then the capacitance was measured in terms of 

 resistance and time by the charge-and-discharge method with a 

 Maxwell bridge. Assuming the capacitance to remain the same under 

 these conditions, it is eliminated between the two bridge equations. 

 Time signals from the United States Naval Observatory were used 

 to calibrate a piezoelectric oscillator which controlled the lOO-c./sec. 

 generator. 



The most recent work by Curtis, Moon, and Sparks gives the 

 value 



1 NBS international ohm =1.000483 absolute ohms, 



which the authors believe to differ from the true value by less than 

 20 parts in a million. 



MUTUAL-INDUCTOR METHOD USING COMMUTATED CURRENT 



This method was devised by Dr. Frank Wenner. A direct current 

 is passed through the resistor and through the primary winding of 

 the mutual inductor. The current through the primary of the mutual 

 inductor is reversed at regular intervals without changing the current 

 through the resistor. Another commutator reverses the connections 

 to the secondary of the mutual inductor in such a way that the pulses 

 of induced electromotive force are always in the same direction. This 

 rectified electromotive force is balanced against the constant potential 

 drop through the resistor. A direct-current galvanometer is used to 

 indicate the balance. 



The commutators which control the connections in the primary and 

 secondary circuits are mounted on the same rotating shaft along with 

 an inductor generator and a current-reversing generator. Conse- 

 quently these units all opei'ate strictly in synchronism. 



Without attempting to discuss the circuits in detail, it may be said 

 that if the current in the secondary of the mutual inductor is zero at 

 the time the connections are reversed, and if the average value of the 

 current through the galvanometer is zero, then the resistance in abso- 

 lute measure is four times the product of the speed and the mutual 

 inductance. The precision of the method is limited theoretically only 



