Intelligence and Miscellaneous Articles, 79 



cleverly constructed by M. Duboscq, recalls the apparatus devised 

 and used by M. E. Becquerel for his important investigations on the 

 phosphorescence of bodies. 



Instead of the handle, a wooden pulley of several grooves is sub- 

 stituted, over which a catgut cord passes, which also passes round 

 another pulley of a much larger diameter fixed on the fly-wheel of a 

 gas-engine. This machine, which was very obligingly lent by its 

 inventor, M. Hugon, is of half a horse-power. It works with great 

 regularity ; it may be started and stopped almost instantaneously ; 

 and unlike steam-engines, the pressure need not be maintained 

 during the times of stoppage. Thus it is of excellent service. 



To charge the Ley den battery in which the electricity is con- 

 densed, one of Holtz's machines is used, the plate of which is put in 

 motion by the gas-engine. The sparks pass between two metal 

 knobs 11 millims. in diameter. At exactly half the distance between 

 these two knobs is formed the principal focus of the lens of a colli- 

 mator, so that the luminous rays fall perpendicularly on the vernier. 

 The aperture of the chronoscope is viewed through a magnifying- 

 glass. 



Suppose that the electric spark occurs periodically under precisely 

 the same conditions, whilst the mica disk turns almost uniformly. 

 An observer looks through the eyeglass of the telescope and calls 

 out the number of marks which he observes simultaneously with 

 each spark. Another observer registers these numbers, and counts 

 the number of turns which the handle of the chronoscope makes per 

 minute. 



Let N be the number of sparks observed, S the total number 

 of marks read, n the number of turns of the handle. 



The duration y of the spark, in millionths of a second, is given by 

 the formula 



10000/S 



'J- 



12k 





in which jj. is a constant parameter, equal to 0*70 for our apparatus. 

 If e denote the angular breadth of the marks on the mica disk, o> 

 the angle between the axes of two consecutive marks, and e' the 

 angular breadth of the marks of the vernier, then 



„-&9 oo 



Formula (1) assumes N to be a large number; thus, we usually 

 observe series of a hundred sparks. 



Other things being equal, the duration of the electric spark is a 

 function of the surface of the Leyden battery, or, in other words, of 

 the number of jars which compose it. 



By varying this number x by units from 1 to 9, we found that the 

 duration y may be expressed by the formula 



y = *(l-0 (3) 



