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 VIII. Intelligence and Miscellaneous Articles. 



MICROSCOPIC OBSERVATIONS ON THE ELECTRIC SPARK. 

 BV R. FABBRI. 



'I'^O study the causes which determine the figure of the electric 

 •*- spark, I undertook to examine it with the microscope. When a 

 spark is evolved between two conductors, two facts are usually- 

 observed with regard to its form : — 1 . It is composed of different 

 rays which unite at their extremities at more or less acute angles, 

 so as to form in the whole a broken line ; and 2, these rays them- 

 selves present various curves. 



These two phaenomena appear to arise from different causes, 

 some acting iu a non-continuous manner at the points of inflection, 

 the others having a continuous action, at least in the space where 

 the spark exhibits no inflection. 



For the examination of these phaenomena, I thought it advisable 

 to employ the microscope, with which I observed a small spark pass- 

 ing between the extremities of two very fine metallic wires, fixed 

 with shell-lac to a plate of crystal. I adopted this arrangement for 

 the following reasons : — As the medium in which the spark is propa- 

 gated must necessarily have some influence on its form, there will 

 be a much greater chance of homogeneity in this medium, if we 

 operate upon a small stratum, and, consequently, with small sparks, 

 for the examination of which the microscope must be employed ; 

 moreover this instrument, which causes the disappearance or great 

 diminution of irradiation, renders the limits of the spark more di- 

 stinct, and allows small differences of form to be distinguished more 

 readily. I have never made use of high powers, which would have 

 been superfluous and even inconvenient. 



The first thing that I observed on examining the spark under the 

 microscope, was its small breadth, which is certainly much less than 

 it appears to the naked eye; it is sufficient to say that, although the 

 irradiation was not completely destroyed, I have seen sparks of 

 several millimetres in length, the apparent breadth of which under 

 the microscope wa? certainly less than -j^th of a millimetre. The 

 sparkof a small Leyden jar appeared much broader and more luminous. 



When the extremities are very close together, no point of inflec- 

 tion is ever observed in the spark, which then appears invariably 

 curved ; and it appeared to me that, when the spark started from the 

 same two points, it presented the same curve. But if the density of 

 the interposed medium be increased by placing between the two 

 extremities of the conductor a drop of a non-conducting fluid, such 

 as olive oil, and compelling the spark to traverse it, the spark is 

 seen to take the form of a broken line, even when the explosive 

 distance is very small. 



It appears that we may conclude from these experiments that the 

 zigzag of the spark is probably due to the medium in which it is 

 produced, whilst its curvature arises from the position of its extre- 

 mities in relation to the other parts of the conductors between which 

 it springs, and perhaps also to the surrounding bodies. 



In the air the spark appears of a violet colour under the micro- 

 scope, as with the naked eye ; but when the source of electricity is 



