566 



Intelligence and Miscellaneous Articles, 



of the same homogeneity made it impossible to obtain smooth 

 curves. To this difficulty was added that of breaking the primary 

 circuit in a regular manner. 



If the results of Table III. be plotted, it will be seen that the 

 increase within small limits is very nearly proportional to the 

 number of thin plates (which were ^V °f an mca m thickness). 



Table III. 



No. of 

 plates. 



Deflections of 

 galvanometer. 



No. of 

 plates. 



Deflections of 



galvanometer. 



1 

 2 

 3 

 4 

 5 



11 

 12 

 13 

 13 

 14 



6 

 7 

 8 

 9 

 10 



15 

 155 



18 

 18 

 18 5 



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 electromagnets. Plates of -£% of an inch thickness 

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

 over those of ^ of an inch. It would seem that the thin plates 

 followed the same law as the bundle of tine 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, with 

 no gain in effect. There appeared to be a slight gain by piaciug 

 the plates edgewise on the poles of the electromagnets, 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 conclusively a very great gain in 

 quantity and electromotive 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. 



Without armatures. 



With armatures. 



15 centims. 



14 



15 



32 centims. 



30 



32 „ 



Mean 14u" 



31-3 



A curious fact came up in this connexion. The lengthening of 

 the spark was not shown when the spark leaped directly between 



