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



When the sparks were taken between two pointed wires, they 

 were only 6 inches with one, and a little more than 13^ inches 

 with three 4-inch cells. Hence, when the negative end is con- 

 nected with a plate a foot square, the sparks are 1J inch longer 

 with one cell and 1 f inch longer with three cells than when each 

 end of the coil is connected with a pointed wire. 



When the negative end was connected with a circular brass 

 plate 4 inches in diameter, and the positive with a pointed wire, 

 the sparks with one cell were 6 | inches, and with three cells they 

 were 13^ inches. Therefore when the negative end is connected 

 with a plate a foot square, the sparks are 1£ inch longer than 

 when it is connected with a 4-inch circular plate. 



Since I discovered the great increase produced in the length 

 of the spark by connecting a large plate of any metal with the 

 negative end of the coil, I made a series of experiments in order 

 to ascertain the best size of the plate, and the effects of connect- 

 ing plates in various ways with the ends of the coil. Some of 

 the results of these experiments appear to be anomalous, and not 

 in accordance with the commonly received theory regarding the 

 distribution of electricity over the surface of bodies, nor with 

 the rule for determining the height to which a lightning con- 

 ductor should project above the roof of a building to which it is 

 attached. 



First, with regard to the size of the plate necessary to produce 

 the longest spark, I have found that a circular metallic plate 7 

 inches in diameter is sufficiently large when the spark does not 

 exceed 10 inches, and that a piece of wood 9 inches square pro- 

 duces the same effect as the 7-inch plate. I commonly use a 

 circular plate of tin about 12 \ inches in diameter. With this 

 plate the coil gives a spark an eighth of an inch longer than with 

 a plate containing four square feet of surface. In order to get 

 the longest spark from a coil, the outer end of the secondary 

 coil should be positive, and the inner end negative; the plate 

 should be connected with the negative end, and the pointed wire 

 with the positive end, and the central part of the plate should 

 face the point of the wire. When the plate faces the point, and 

 its circumference is opposite the point, the sparks are a little 

 shorter than when the point is opposite the middle of the 

 plate. 



Secondly, with regard to the effects of plates connected with 

 the ends of the coil, I have found that the effects of a plate in 

 connexion w r ith the negative end differ very much and in various 

 ways from those that are produced by a plate connected with the 

 positive end. 



In most of my experiments on this subject the greatest length 

 of spark the coil was capable of giving with the three cells em- 



