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BELL SYSTEM TECHNICAL JOURNAL 



Another way to use the tube is to have it plot the relation between 

 two quantities irrespective of time. As a simple case we may take 

 the current vs. voltage curve of a resistance through which an alter- 

 nating current is flowing. The spot travels back and forth along a 

 straight line, the slope of which measures the reciprocal of the re- 

 sistance (Fig. 30a). If inductance is added to the resistance the spot 



(a) (b) 



Fig. 30 — Diagrams of current voltage curves. 



does not come down on the same line it went up; an ellipse results, 

 the spread of which tells us the amount of the inductive impedance 

 (Fig. 30b). This method of operation has a wide variety of applica- 

 tions. Instead of the resistance, for instance, we may have a gas dis- 

 charge device of which we want to know the properties. One of the 

 applications of this method is the production of hysteresis curves of 

 the ferromagnetic materials ^5 (Fig. 31). Fig. 32 illustrates the appli- 

 cation of the method to the study of distortion in an amplifier. 



Suppose we apply to each pair of deflector plates a voltage from 

 each of two different oscillators. If the oscillators make exactly the 

 same number of vibrations per second, then the pattern on the tube 

 remains stationary, but if the frequency of the oscillators differ ever 

 so little the pattern goes through gradual changes according to the 

 different phase relations. This is one of the most sensitive means we 

 have for comparing and calibrating accurate oscillators, and we may 

 call it the Lissajous figure method. Fig. ?)i shows the appearance of 

 some of these stationary Lissajous patterns. 



Another method of comparing the frequency of oscillators has been 

 called the gear-wheel method. In this method the voltage from the 

 low frequency source is split into two equal components 90° apart in 



25 Johnson, J. B., Bell System Technical Jl., 8, p. 286, 1929. 



