OF ARTS AND SCIENCES. 



205 



On increasing the number of plates, a point was reached where 

 there was no additional effect. The best result was obtained where 

 the mass of the armatures was approximately equal to that of the 

 cores of the electro-magnets. Plates of ^V of an inch in thickness 

 were also used ; but no advantage resulted in their employment, over 

 those of -^-g of an inch. It would seem that the thin plates followed 

 the same law as that of the bundle of fine iron wires which constitute 

 the cores of induction coils of the present day, and that only a 

 moderate degree of discontinuity in the mass of iron submitted to 

 magnetic influence is necessary to prevent the formation of currents 

 of induction which prolong the magnetism of the cores, and prevent 

 the quick demagnetization necessary to produce intense currents of 

 induction. The effect of insulating the thin plates with thin dielectrics, 

 like paper, was also tried witli no gain in effect. There appeared to 

 be a slight gain by placing the plates edgewise on the poles of the 

 electro-magnets, instead of allowing them to repose on their flat sides. 

 This was doubtless due to better contact of the metallic surfaces. 



Since the above results proved concUisively a very great gain in 

 quantity and electro-motive force by the application of thin plates as 

 armatures, I next measured the striking distance of the spark. 

 Table IV. gives the results which are the mean of many trials. 



TABLE IV. 



A curious fact came up in this connection. The lengthening of the 

 spark was not shown when the spark leaped directly between the poles 

 of the induction coil. The increase in quantity and electro-motive force 

 was only made manifest to the eye by the employment of condensers 

 in the secondary circuit. The results in Table IV. were obtained by 

 tlie employment of a Leyden jar of large capacity. The increase in 

 the quantity and electro-motive force was not only shown by the 

 increased length of the spark, but also by its increase in volume and 

 its loud snap. The spark consisted of a thick central bolt, -surrounded 



