J. Trowbridge — Thin plates of iron used <xs armatures. 361 



imperfect evidence now existing in regard to the one under 

 consideration ; and it must be remembered that no matter how 

 inadequate this evidence may at first sight appear to be. it is 

 yet as full, minute and complete as that relating to any single 

 nebula except that of Orion; and if the drawings here consid- 

 ered are not sufficient to prove a change or the absence of a 

 change, we are reluctantly driven to the conclusion that the 

 work done by astronomers in this direction has been largely 

 wasted. I hope that the evidence here adduced may be deemed 

 of sufficient importance to warrant the great expense of time 

 and labor necessary to a detailed monographic study of this 

 uebula, which may serve for future reference. There is proba- 

 bly no nebula visible in the northern hemisphere more worthy 



. Naval Observatory, AprU, : 



Art. XLIII. — Brief Co it tributions from the Physical Lahorair/ry 

 of Harvard College. No. XVIIL— On the effect of thiu plates 

 of Iron used as armatures for Electro Magnets, and a new form 

 of Induction Coil ; by John Trowbridge, assistant Profes- 

 sor of Physics. 



In a paper presented to the American Academy of Arts and 

 Sciences, April 13, 1875, 1 showed that the application of arma- 

 tures to two straight electro-magnets, which formed the primary 

 circuit of a Euhmkorff coil more than doubled the strength of 

 the induction current produced by breaking the primary cir- 

 cuit. When, however, the circuit of the secondary coil was not 

 closed, and a spark was allowed to jump across the interval be- 

 tween its poles, the striking distance of the spark and its power 

 to charge a condenser did not seem to be notably increased by 

 the application of armatures to the electro-magnets of the pri- 

 mary circuit. My experiments at that time were made with 

 solid iron cores fand I now resume these experiments with 

 bundles of fine iron wires in place of the solid iron cores. The 

 mechanical difficulty of making the ends of the bundle of fine 

 iron wires constituting the cores plain surfaces was overcome 

 by dipping them in melted solder and then filing the ends 

 smooth. In this way I had no trouble in applying the arma- 

 tures so that they should lie upon a plain surface. The re- 

 sistance of each of the two induction coils covering the two 

 straight electro-magnets was 6000 ohms ; and that of each of 

 the straight electro-magnets, -34 of an ohm. The diameter of 

 the bundles of fine iron wires constituting the cores was 5 cm. 

 and the length of the electro-magnets 28 cm. Condensers of 



