CURTIS AND GROVER: ALTERNATING CURRENT RESISTANCES 219 



ELECTRICITY. — Resistance coils for alternating current work. 

 H. L. Curtis and F. W. Groveu. Communicated by 

 E. B. Rosa. To appear in the Bullet in of the Bureau of 



Standards. 



In an ideal resistance coil for alternating current work two 

 requirements need to be fulfilled in addition to those which are 

 of importance in direct current work; viz, (1) the resistance must 

 be independent of the frequency and (2) the phase angle must be 

 zero. Many of the resistance coils in use at the present time do 

 not even approximately fulfill these conditions. This paper 

 attempts to set forth the principles which must be followed in 

 the construction of satisfactory resistance coils for alternating 

 current work, and to show how to apply them to the design and 

 construction of several coils of different denominations. 



The most important thing in the design of a coil whose phase 

 angle is to be small, is to wind it in such a way that its inductance 

 will be a minimum. However there are three other effects which 

 need to be considered in the discussion of the phase angle; viz, 

 (1) "skin effect," (2) capacity between the wires and (3) absorp- 

 tion in the dielectric between the wires. Each of these three 

 causes also gives rise to a change in the effective resistance of 

 the coil when used with different frequencies of alternating cur- 

 rent. 



Formulas have been developed for computing the value of the 

 "skin effect." By means of these it can be shown that, with the 

 size of wire usually used with coils having a resistance of one ohm 

 and larger, the "skin effect" is in all cases negligible. 



The capacity between the wires is in parallel with their resist- 

 ance and inductance, and is distributed along their length. If 

 R' is the effective resistance of a bifilar coil and 6 its phase angle, 

 then to a first approximation 



3 " 15 



p ( L — 7T CR j 



tan 6 = 



R 



