AUSTIN: HIGH FREQUENCY RESISTANCES 



95 



The principle of the method is briefly the following: Two 

 equal inductance coils are placed in identical oil calorimeters, 

 one coil is heated by a high frequency current and the other by 

 du-ect. Vfhen the calorimeters are both brought to the same tem- 

 perature in equilibrium with their surroundings, the heat im- 

 parted to each per second must be the same. This heat is pro- 

 portional to I^ R and the ratios of the resistances of the two coils 

 at the given frequency and for constant current are inversely 

 proportional to the squares of the currents.^ To compensate for 

 the slight inequalities in the coils and calorimeters, the direct and 



TABLE I 



Diameter of double silk covered copper wire 



Diameter of coils = 8.6 cm. 



Turns of wire per centimeter = 18.9 



0.04 cm. 



high frequency currents are interchanged and the mean values of 

 the ratios of the current squares taken. The high frequency 

 current is produced by a rotary spark gap in an oscillatory circuit 

 coupled loosely to the circuit containing the inductance coil to 

 be measured. The two circuits are brought to resonance at the 

 frequency desired, and the high frequency current thru the induc- 

 tance, regulated by varying the coupling. The current is read 

 on a non-shunted hot wire ammeter which has been accurately 

 calibrated for high frequencies. The direct current for the other 

 coil is supplied by a storage battery and the final regulation for 

 equilibrium is made in this circuit. Equality of temperature 

 between the two calorimeters is determined by a differential con- 

 stantan copper thermoelement. The calorimeters are heated to 



2 See J. Zenneck, Elektromag. Schwingungen, p. 415, 1905, and J. A. Fleming, 

 Principles of electric wave telegraphy, p. 124, 1912. 



