DETERMINATION OF WAVE FORM in:; 



which shows the potential difference applied to and the current 

 in a coil with an iron core. 



In engineering work, cases are continually arising where wave 

 form determinations are of the utmost importance on account of 

 the assistance they give in explaining the behavior of electrical 

 apparatus. 



Two cases may arise: 



A. When the phenomena are periodic; for instance, the ordi- 

 nary electromotive force and current waves, Fig. 377. 



FIG. 377. Potential difference applied to and current in a coil with 



iron core. 



B. When the phenomena are transient; such as those occurring 

 when the circuit conditions are suddenly altered. This is illus- 

 trated by Fig. 394, which shows the potential difference and 

 current curves taken during a short-circuit test of an enclosed 

 fuse. 



Contact Method for Determining Wave Forms. 1 Methods 

 for dealing with case A were first developed, the earliest being 

 the contact method, used in 1849 by Lenz in investigating the 

 wave forms of alternators. In 1880 Joubert employed it to 

 determine the wave form of a Siemens machine and since then 

 it has commonly been called Joubert's contact method. 



The fundamental idea is to connect periodically the measur- 

 ing apparatus to the circuit for a time so short that during it the 

 current or voltage remains practically unchanged. This is 

 accomplished by an apparatus which is the equivalent of a key 

 operated by a rigid connection from the dynamo shaft. The 



