THE ‘PINCH-EFFECT” IN UNIDIRECTIONAL ELECTRIC 
SPARKS. 1 
BY ANDREW H. PATTERSON. 
Prof. Nipher has recently 2 described some interesting experi- 
ments on momentum effects in electric discharge, and writes as 
follows concerning the undirectional sparks obtained by the inser- 
tion into the circuit of strips of cloth moistened with a saline 
solution : “ . . . . the sparks are large and brilliant at the negative 
end in both positive and negative lines, and thin out towards the 
positive end. The negative terminals are large spheres of about 
10 cm. diameter. The positive terminals are small knobs, of 
1 cm. diameter. While on the large sphere the electrons repel each 
other. But when they start into motion across the spark-gap, they 
attract each other electromagnetically . This appears to be the reason 
why the spark thins out as the electrons proceed in their motion 
across the spark-gap. The italics are mine. 
According to theory, two like charges repelling each other when 
at rest, begin to develop an electromagnetic attraction for each 
other as soon as they are put in motion in the same direction. 
But this attraction does not become equal to the electrostatic 
repulsion until the charges move with the velocity of light. This 
used to seem very puzzeling to me, for I reasoned as follows: — 
Imagine a positively charged hopper filled with steel balls, which 
continually dropped into two parallel inclined glass troughs. As 
the motion of the charged balls is constantly accelerated, the 
electromagnetic attraction which they exert on the charged balls 
1 Reprinted from Science, vol. 29, p. 731, Jan. 1, 1909. 
2 Science, vol. 28, p. 805, Dec. 4, 1908. 
1908] 
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