630 ELECTRICAL MEASUREMENTS 



Nominal busbar voltage 230 volts 



Minimum busbar voltage 61 volts 



Maximum busbar voltage 586 volts 



Maximum current 6,600 amperes 



Time required to open circuit... . 0.0086 second 



Time required to attain maxi- 

 mum current . 006 second 



Average rate of decrease of 



current 4,100,000 amperes per second. 



It is obvious that in this case the previous methods are not 

 applicable. 



Blondel was the first to definitely state the conditions which 

 must be fulfilled in order that a galvanometer may follow, with 

 sufficient accuracy, the rapid variations of an alternating cur- 

 rent and be capable of recording wave forms photographically 

 with but a single traverse over the photographic plate of the spot of 

 light which is used as an indicator. He applied the term oscillo- 

 graph to such an instrument. 



The conditions are as follows: 



1. High free period of oscillation, as great as 50 times that of 

 the phenomena to be investigated. 



2. Damping small and in the neighborhood of the critical 

 aperiodic value. 



3. Self-induction as small as possible. 



4. Negligible hysteresis and Foucault current effects. 



5. Adequate sensitivity. 



In addition, the design and construction must be such that the 

 necessary adjustments and repairs may be made with ease by any 

 one accustomed to handling electrical instruments. 



The moving needle, the moving coil, the string galvanometer, 

 and the hot-wire instrument have all been modified so that they 

 may be used as oscillographs. BlondePs first instrument was of 

 the moving-needle type; in its late development the needle has 

 become a thin strip of soft iron stretched over bridge pieces, as 

 shown in Fig. 395. 



Referring to Fig. 395, the thin, soft iron strip, S, is drawn 

 taut over the bridge pieces by the spring within the tube, t. The 

 tension is controlled by the nut s. The strip is supported and 

 protected by an insulating tube, T, in the sides of which soft iron 



