Prof. Callan on an Induction Coil of great power. 415 



ployed varied from 13-j? to 15 inches. "When I used a weaker 

 battery I mention the length of the spark. The plate was about 

 12J inches, and the point was opposite to the middle of the plate, 

 except when the contrary is expressed. 



First, when a pointed wire is connected with the positive end 

 of the coil, a plate connected with the negative end lengthens the 

 spark considerably ; but when the pointed wire is connected with 

 the negative end, a plate connected with the positive one shortens 

 the spark in a greater proportion. Sparks which were 15 inches 

 long in the first arrangement were reduced to less than 11 inches 

 by the second j they did not pass at all between the positive plate 

 and negative point until the plate was brought within 8 J inches 

 from the point. 



Secondly, sparks from a positive point to a negative plate never 

 went to the circumference of the plate, and scarcely ever struck 

 the plate at a greater distance from the centre than 3 inches. 

 But sparks between a negative point and positive plate always 

 went to the circumference until the plate was brought within 2~ 

 or 3 inches from the point : even when I used a rectangular 

 plate 20 inches broad and 28 inches long, the sparks went to the 

 edge of the plate. 



Thirdly, the sparks from a positive point to a negative plate 

 never moved in a straight line, even when the point was less than 

 an inch from the plate. But when a negative point is brought 

 within 2^ or 3 inches, or even less than an inch from a positive 

 plate, the sparks pass in a straight line between the point and 

 the nearest part of the plate. 



Fourthly, the sparks from a positive point to a negative plate 

 grow weaker and less loud as the point is made to approach the 

 plate. But when a negative point is brought within two or three 

 inches from a positive plate, the sparks become as loud as if the 

 plate were charged, or as if they were produced by a small Ley- 

 den jar whose opposite coatings were connected with the ends of 

 the coil. With a plate containing four square feet of surface, the 

 sparks were louder than with the 12-inch plate. A hollow ball 

 connected with the positive end of the coil also gives very loud 

 sparks. Hence a plate or ball connected with the positive end 

 becomes charged, but a plate connected with the negative end 

 does not. 



Fifthly, when a pointed wire nearly J of an inch thick pro- 

 jected 6± inches from the middle of the 125-inch circular plate 

 connected with the negative end of the coil, and nearly at right 

 angles to the plate, and another plate of the same size was con- 

 nected with the positive end, the sparks passed not between the 

 point and positive plate, but from the circumference of one to 

 the circumference of the other, although the distance of the point 



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