DETERMINATION OF WAVE FORM 617 



voltage between a and b is then equal to the steady voltage 

 between and /S, and the latter is read from the direct-current 

 voltmeter. 



The methods given are applied to current waves by placing a 

 non-reactive resistance (shown at a'6') directly in the circuit 

 and determining the instantaneous potential differences between 

 its terminals. 



The time consumed in mapping a wave form by the foregoing 

 methods is considerable; in itself this is disadvantageous, and 

 it also necessitates holding the circuit conditions practically con- 

 stant for a considerable time, for if the wave is non-sinusoidal, 

 many points near together must be taken. Frequently in 

 industrial testing the conditions cannot be maintained constant. 

 In addition, much time and labor must be expended in computing 

 and plotting the results. Obviously, simultaneous records of 

 two or more waves cannot be obtained with a single instrument. 



In many cases the necessity for directly connecting the con- 

 tact disc to the dynamo shaft practically prohibits the use of the 

 contact method in the forms previously given, for it is frequently 

 necessary to determine the wave forms at a place which may be 

 at a considerable distance from the generating station. 



From the potentiometer method, the Rosa curve tracer, 

 shown in Fig. 382, has been developed. The object of this 

 machine is to reduce the time necessary for making the 

 observations and plotting the results. In Fig. 381, if the direct- 

 current voltmeter is omitted, and the potentiometer wire carries a 

 definite current, the displacement, OS, of the slider from the zero 

 position will, at balance, be proportional to the instantaneous 

 voltage. The idea is to plot these displacements, as ordinates, 

 on a sheet of paper carried by a drum, the abscissae being pro- 

 portional to the displacements of the contact brush along the 

 arc AC. This is done in a semi-automatic manner, as will be 

 seen from the following. 



Referring to Fig. 382, the potentiometer wire, DE, in Fig. 

 381, is wound in a screw thread on an ebonite cylinder. When 

 the cylinder is turned, another thread of the same pitch cut in 

 th<- ebonite, serves to move the carriage to which are attached 

 the contact point, ,S, and the stile for registering the results. 



A short-period, dead-beat, moving-coil galvanometer is used 



