J. Trowbridge — Thin plates of iron med as armatures. 363 



both poles of the electro-magnet, thus making a magnet of a 

 horse-shoe form. On charging the condenser, I found a very 

 great increase in quantity, which was manifested by the swing 

 of the galvanometer needle; the indicator being entirelv off 

 the scale. 



Table II shows the results obtained by the use of iron plates 

 one and one-quarter of an inch in thickness, twenty in number, 

 constituting each armature. 



Here a gain of four hundred per cent was manifested by the 

 use of thin plates. The next step was to ascertain how many 

 plates were necessary to obtain the maximum effect. The dif- 

 ficulty of obtaining plates of the same homogeneity, made it 



obtain smooth < 

 ad^ed that of breaking the primary circuit in a regular manner. 

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

 increase within small limits, is very nearly proportional to the 

 number of thin plates, which were ^V of an inch in thickness. 



On 



when the mass of the armatures was approximately equal to 

 that of the cores of the electro-magnets. Plates of g^ of an 

 inch were also used, but no advantage resulted in their employ- 

 ment over those of ^^ 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 neces- 

 sary to prevent the formation of currents of induction, which 

 prolong the magnetism of the cores, and prevent the quick de- 

 magnetization necessary to produce intense currents of induc- 

 tion. The effect of insulating the thin plates with the dielec- 

 trics was also tried with no gain in effect There appeared to 

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

 the magnets instead of allowing them to repose on their flat 

 faces. This was doubtless due to better contact of the me- 

 tallic surfaces. 



Since the above results proved conclusively a very great gain 

 in quality and electrometric force by the application of thin 

 plates as armatures, I next measured the striking distance of the 



