Prof, von Bezold's Researches on the Electrical Discharge. 49 



oy a Grove's element, and the distance in the spark-micrometer 

 F was made = 2 millims., as neither larger nor smaller distances 

 gave such good results. 



Under these circumstances it was sufficient, in order to produce 

 a spark, if one wire D was even only 1 decimeter longer than 

 the other. When, on the contrary, they were of the same length, 

 a spark never appeared. Yet it can be instantaneously evoked 

 if, by touching one of the wires with the knob of a Leyden 

 jar, the symmetry of the two current-paths is disturbed. 



In these experiments also the material and thickness of the 

 wire exerted not the smallest influence. Whether I used a sil- 

 vered copper wire of 0*06 millim. diameter, or an iron wire of 

 0*23, or a copper wire of 0*8 millim. diameter, the spark never 

 appeared when both wires were of the same length. 



Hence the velocity of the propagation of electricity is the same 

 for all stretched* wires. 



Yet in the form above described, the experiment is not very 

 striking, as we can only work with very small distances in the 

 accessory micrometer/. I endeavoured therefore to alter it in 

 such a manner that it would be visible to a whole audience. 



Experiments with small Geissler's tubes have led to no decisive 

 result. On the contrary, with lengths of some metres at least, 

 the retardation may be very beautifully shown in the following 



manner : — 



Fig. 8. 



If a discharge (negative) (best of all, of a RuhmkorfPs appara- 

 tus) be divided, as above, just behind the spark-micrometer into 

 two branches, and if one of them be connected with the coating of 

 the perfectly insulated test-plate, while the other is led by the 

 conductor A to the upper uncoated surface, a positive, a negative, 

 or no figure at all can be made to appear, according as the upper 

 branch is larger, smaller, or as long as the lower one. Indeed 

 the experiments must succeed one another in a definite order 

 if they are intended to support the opinion that they owe their 

 origin to differences in time. For if we remember that it is 

 immaterial whether positive electricity be imparted to the plate 



* Spirally-coiled wires will, it may be presumed, give a different result. 

 Phil. Mag. S. 4. Vol. 40. No. 264. July 1870. B 



