104 KENNELLY AND VELANDER— POTENTIOMETER 



the vector change of current thereby produced in a sharply tuned 

 resonant circuit of low resistance, when the condenser is inserted or 

 removed. Other applications of the same principle will suggest 

 themselves. 



Simple RLC Circuit with Variations Made First in R Alone, 



■ AND THEN IN C AlONE. 



An example of a simple RLC circuit, with successive variations 

 in resistance and capacitance, is presented in Fig. 5. Here the total 

 resistance in the circuit could be given different assigned values be- 

 tween 52 and 802 ohms. The capacitance could also be varied be- 

 tween infinity and 0.20 microfarad. The p.d.'s were measured 

 across taps pp', Figs. 2 or 4, and the current strengths determined 

 therefrom. The constant inductance in the circuit was o.i henry. 

 The frequency was 1006'—' throughout. This produces resonance in 

 the circuit at C = o.2^ microfarad. 



If we plot the impedance of a RLC circuit, like that of Fig. 4, at 

 any constant frequency, we obtain an impedance diagram of the 

 type represented in Fig. 6. If we maintain constant resistance in 

 the circuit, such as r4 = 402 ohms, and vary only the capacitance, 

 the impedance locus will lie along the straight line AB. Such a 

 variation of impedance, substituted in equation (6), will, as we have 

 already seen, give rise to a circular current locus, the diameter of 

 the circle coinciding with the vector E of impressed e.m.f. If, 

 however, we keep the reactance constant, at some value such as 

 A\ = j 216.2 ohms, Fig. 6, and vary the resistance, the vector im- 

 pedance will move over the straight-line locus DE. This vector 

 impedance, inserted in (6), will also give rise to a circular locus; 

 but the diameter of this circle will be in quadrature with the vector 

 of impressed e.m.f. In the particular case of resonance, where the 

 reactance is kept at zero, and the resistance is varied, the current 

 locus will be a circle of infinite radius ; i.e., a straight line, coincid- 

 ing with the vector of impressed e.m.f. These results are brought 

 out experimentally in Fig. 5. It will be observed that with a capaci- 

 tance of 0.25 microfarad, the locus coincides with the straight line 

 OR. The vector impressed e.m.f. was so adjusted, by preliminary 

 tuning of the potentiometer circuit, as to coincide with this line. 



