44 



ELEMENTS OF ELECTRICAL ENGINEERING. 



measured and the value of the electromotive force is taken from 

 the following table.* 



TABLE. 



SPARKING DISTANCES IN AIR BETWEEN OPPOSED SHARP NEEDLE POINTS FOR VARIOUS 

 EFFECTIVE HARMONIC VOLTAGES. 



Figure 37 shows a testing transformer of the General Electric 

 Company. On the top of the containing case is a spark gauge 

 which is adjusted by means of a micrometer screw, the reading 

 of which gives the length of the spark gap. The transformer is 

 arranged to give electromotive forces (alternating) up to 10,000 

 volts. 



The indications of a spark gauge are subject to very consider- 

 able error on account of melting of needle points by the discharge 

 (needle points must be very sharp to give best results), on account 

 of drafts of air, on account of departure of electromotive force 

 curve from a curve of sines, and on account of what appears to 

 be the inherently erratic character of a spark discharge. It is, 

 however, by far the most convenient method for measuring high 



* Based on experiments of C. P. Steinmetz on the Dielectric Strength of Air, 

 Trans. American Institute of Electrical Engineers, Vol. XV, pages 281-230, 1898. 



