RECENT PROGRESS IN PHYSICS. 



383 



The same result was given by several other series of experiments, 

 which Masson has arranged in tables. It will be sufficient to present 

 here only one of the many series, serving to establish each of the laws 

 determined. 



\ The values of Y, as given in the tables, are always the mean of two 

 jexperiraents. After the distance Y of the spark micrometer from the 

 irotating disk at which the sectors could be no longer distinguished 

 had been once determined the miciometer was brought considerably 

 inearer the disk again, and then removed the second time, until the 

 jsector disappeared. The two values of Y, thus determined, differed in 

 the various series at most by one centimetre, a proof of the exactness 

 attainable by this method of observation. 



§ 90. Variation of the brightness of the spark at different strildng 

 distances.— On this point Masson made numerous experiments. The 

 folio win <x table contains the results of one of them : 



Here X denotes the striking distance, Y the corresponding distance 

 of the spark from the rotating disk, at which the sectors cannot be 

 distinguished, under the condition that the constant illumination of 

 the rotating disk from AC remained unchanged during the whole 

 series of experiments. 



It is evident, from the above table, that the striking distance and 

 the corresponding distance of the spark from the rotating disk must 

 vary in a constant ratio, the illumination of the disk remaining the 

 same. Or, for double and treble striking distances, he spark must 

 be removed two and three times as far from the disk, if its illumina- 

 tion by the spark is to remain the same. 



By doubling the distance, the intensity of the illumination becomes 

 four times feebler, but it remains unchanged if the striking distance 

 is doubled, consequently the brilliancy of the spark mus the four times 

 greater at double the striking distance. For the distance n the illu- 

 mination by the electrical spark is n- times feebler, but it remains 

 unchanged if the striking distance is made n times greater ; hence, 

 for a striking distance n the brilliancy of the spark must be n'^ times 

 greater, or in other words, the hi^ightness of the electrical spark increases 

 as the square of the striking distance. 



§ 91. Influence of the size and form of the surface of the condenser. — 

 The form of the condenser (that is, the glass plate with metallic coat- 

 ing on both sides, the dif^charge of which passes through the spark 



