262 



ELECTRICAL MEASUREMENTS 



70 



CO 



?50 



B40 



.3 



20 



10 



Mechanically, the needle-point spark gap is about the simplest, 

 electrical measuring device but this simplicity of construction is 

 no guarantee of simplicity of action and the needle-point gap 

 must be used by skilled experimenters if reliable results are re- 

 quired. Its indications are open to many sources of error. 

 Like all spark gaps, it is influenced by the distortion of the 



electrostatic field in its neigh- 

 boorhood, due to surrounding 

 objects. 



For high voltages, there- 

 fore, the arrangement must 

 necessarily occupy a large 

 space. In an apparatus for 

 200 kv., if designed as indi- 

 cated above, the distance be- 

 tween the needle points will 

 be about 52 cm.; the sup- 

 ports for the needles will each 

 be about 104 cm. long, so the 

 length of the apparatus will 

 be at least 9 ft., and as no 

 object should be nearer the 

 gap than 3J ft., the space 

 occupied would be about 9 by 

 7 by 7 ft. 



In addition to this large 

 space factor, which is disad- 

 vantageous, there are irreg- 

 ularities in the action of this 

 form of gap arising from the varying sharpness of the needles. 

 A new set of needles must be inserted after each breakdown of 

 the gap. 



When the voltage is gradually raised, the points are seen to 

 be surrounded by a bluish glow or corona, more or less spherical 

 in form. This happens long before the gap breaks down and 

 means that the air about the points has become conducting. 

 Serious errors may be introduced by this preliminary breaking 

 down of the air, for irregularities due to heating are thus intro- 

 duced. Again, it is found in all cases where the corona forms 



20 40 CO 80 100 120 140 160 180 

 Spark Gap in Millimeters 



Fio. 147. Plot of A. I. E. E. table 

 for needle-point spark gap. 



