504 M. B. W. Fedderscn on the Electric Spark. 



it, the measurements so obtained could only lay claim to ap- 

 proximative accuracy, because, on the one hand, the impression 

 of light on the eye is only of momentary continuance, on the 

 other hand, the manifold disturbances and irregularities of the 

 ph?enomenon totally preclude all possibility of repeating the ex- 

 periment*. 



In order to give the electric spark as long a continuance as 

 possible, I made use of the great resistance which moist con- 

 ductors offer to the electric current. Finding that moist hemp 

 cords were useless for ray purpose on account of their property 

 of dispersing the electricity, I introduced into the circuit several 

 thin columns of distilled water enclosed in glass tubes, and 

 placed them either in immediate contact with the internal coat- 

 ing of the Leyden jar (represented in the Tables by + ), or sepa- 

 rated from it by the spark-micrometer (in these cases marked 

 — in the Tables). The columns of water had the following ab- 

 solute and reduced lengths, if for the latter we take as unit a 

 column of 1 millim. in thickness and 1 millim. in lengthf. 

 Columns of water. Absolute lengths. Reduced lengths. 



The same letter in the Tables always denotes one and the same 

 tube, in which longer or shorter copper wires are inserted. The 

 distance between the two copper wires in the tube was the ab- 

 solute length of the water column. 



I. Observations of the Spark, ^c. 



On introducing resistances between 100 and 1000 of my unit 

 of resistance, I observed two very different kinds of discharge, of 

 which sometimes the one, sometimes the other predominated. 



Continuous discharge. — The first form of discharge appeared 

 in the rotating mirror in the following manner. The discharge 

 commenced by a single spark ; two luminous parallel bands 

 spread out from the two ends of the spark, enclosing a perfectly 

 dark space when the resistance was very great (from 400 to 1000 

 units) (Plate I. fig. 15), showing perhaps a similar motion of the 



* A more detailed account of the method and apparatus employed, may 

 be found in my Inaugural Dissertation, Kiel, 1857 (published by Schwer, 

 Kiel). 



t In order to distinguish this imit from that ordinarily employed, I call 

 it the " Reduced length of water." 



