194 GROVER AND CURTIS: INDUCTANCE MEASUREMENTS 



It has been shown by Ruedenberg 4 that for a flat top antenna 

 the constant in the above formula is in round numbers 1600, or 



E = 1600 h 2 /\ 2 -P- (2) 



The term 1600 h 2 /\ 2 is commonly spoken of as the radiation 

 resistance, inasmuch as it is a quantity which when multiplied 

 by the square of the current gives the radiated energy. 



The accuracy of equation (1) was verified in the experiments 

 on long distance radio-telegraphy already cited. It has therefore 

 seemed worth while to calculate a table of the radiation resistance 

 for various heights and wave lengths. 



ELECTRICITY. — Methods for the measurement of the effective 

 inductance of resistance coils. F. W. Grover and H. L. 

 Curtis. Communicated by E. B. Rosa. To appear in 

 the Bulletin of the Bureau of Standards. 



In the measurement of inductances which are associated with 

 relatively high resistances, such as resistance coils, the multipliers 

 of voltmeters and wattmeters and the like, the ordinary bridge 

 methods using alternating current of suitable frequency are suffi- 

 ciently sensitive, but they fail to give an accurate determination 

 due to the fact that the inductance of each arm of the bridge is 

 of the same magnitude as the inductance to be measured. The 

 method which we have used to overcome this difficulty is to sub- 

 stitute for the unknown inductance, an inductance of approxi- 

 mately the same resistance, but of such a form that its inductance 

 can be computed from its dimensions. From the changes in the 

 bridge which it was necessary to make to restore a balance, the 

 difference in the inductance of the two resistances was computed. 



Three bridge methods were used. The first was Maxwell's 

 method of comparing an inductance with a capacity; the second 

 was Anderson's modification of the above; the third was the use 

 of a variable inductance in one of the arms. These methods 

 gave concordant results and the method to be chosen in any 

 given case will largely depend upon the value of the resistance 

 associated with the inductance which is being measured. 



4 R. Ruedenberg, Ann. d. Phys. 25: 446. 1908. 



