50 IMPEDANCE. 



generally known as " skin effects," and if employed for alter- 

 nating currents must be looked upon as " partially inductive " 

 resistances . For specially high frequency currents any form 

 of spiral may lead to errors if looked upon as non-inductive ; 

 the same caution should be applied in cases where the wave 

 form of the current is specially " peaky." 



Some tests made upon ordinary spiral resistance frames 

 are recorded in the curves shown in Fig. 22. 



The upper curve shows readings taken with alternating 

 current at 42 cycles, and with continuous current upon a 

 resistance consisting of spirals lin. diam. of No. 22 S.W.G. 

 resistance wire, the coils being spaced at an average of 

 5-5 turns per inch. The resistance consisted of 3 sets of 

 4 coils 27 inches long, the three sets being in parallel, and the 

 4 coils of each set being in series. It will be seen that at 

 this frequency no appreciable difference between the apparent 

 resistance with alternating and direct current can be 

 traced. This result can easily be verified by calculation. 



By measurement with a secohmmeter and standard self- 

 induction the inductance of the coils was found to be '26 

 millihenries, while the ohmic resistance was 69 ohms. 



The reactance of the coils was consequently = 2 TTH L 



= 27r42x -00026 

 = -069 



Hence ratio resistance = and the reactance can 



reactance 



sa'elybe neglected. 



The lower curve in Fig. 22 applies to a wire resistance 

 formed of No. 14 S.W.G. galvanised iron wire, coils 1-liri. 

 diam., and wound 5 coils to the inch, 50 coils 26 inches long 

 being in series. 



This curve has been drawn for a double purpose. Firstly, 

 it serves to show to what extent a spiral wound with thick 

 wire of fairly low resistance may be considered non-inductive. 

 Secondly, it shows the influence of the magnetic properties 

 of iron when employed as a resistance. 



As may be seen from the curve, the apparent resistance of 

 the iron spirals for alternating and continuous currents is 

 approximately the same with very small currents. The 

 exact relation may be calculated from the following data. 



