134 ELECTRICAL MEASUREMENTS 



Effective resistance, R' = R [l + co 2 C( %L - ?f 5<?# 2 )]. 

 Effective inductance, L 1 = L 

 _,<*(L 

 Phase displacement, tan l 



R and L are the total resistance and inductance of the coil and 

 C is the capacity between the wires if they be separated at the end 

 of the bight. 



It is seen that capacity and inductance tend to neutralize each 

 other. In coils of low resistance with only one layer of bifilar 

 winding, the inductance will preponderate, but when the resist- 

 ance of the coil is increased, the effect of capacity increases more 

 rapidly than that of inductance; consequently, there will be a 

 point where the two effects will be balanced. The balance point 

 is practically independent of the frequency. 



From the formulae it is evident that the phase displacement and 

 the change of resistance cannot both be made zero, but if L = 

 %CR 2 the phase displacement is zero and the change in resist- 

 ance negligible. 



After having found a construction which gives a practically 

 non-reactive coil, higher resistances may be built up by simply 

 placing coils in series. The several sections of a high-resistance 

 coil may be wound side by side on the same spool which should 

 be of porcelain. The use of a non-conducting spool avoids 

 troublesome capacity effects between the sections. 



Curtis and Grover 7 recommend the following constructions: 



The coils are designed for oil immersion and have approxi- 

 mately 50 sq. cm. surface. With an expenditure of 1 watt per 

 coil the rise of temperature above the surrounding oil is about 1, 

 the bobbin being of poor thermal conductivity. 



Tenth-ohm Coils. Only the inductance need be considered. 

 The coil is to be made of manganin strip 34 mm thick, width 

 about 3 mm., length about 10 cm. The strip is to be folded 

 back on itself at the middle of its length and the two halves bound 

 together with insulation between them. The effective inductance 

 is about -f 0.005 microhenry. 



One-ohm Coils. Only the inductance need be considered. 

 The coil is to be of manganin strip J^o mm. thick and 3 mm. 

 wide, folded back on itself at the middle of its length, and with 



