335 



certain phenomena which are not, I believe, generally known, I venture 

 to call attention to them. ' 



Those I propose to notice here relate altogether to the action of the 

 secondary or iDcluction helix, composed, as Dr. Callan explained to his 

 class, of thirty miles of iron wire, of about the hundredth of an inch in 

 thickness. The wire was wound up into three fiat rolls or block wheels, 

 which were placed at equal distances on the central facies of iron 

 rods composing the core. These rods, about three feet long, were bound 

 round by a helix of thick copper wire, laid on in three strata, extending 

 from about three inches of their ends. 



The secondary helix was in connexion with a multiplying apparatus, 

 composed of several hundreds of sheets of a large quarto paper with tin 

 foil between them, which was, like the coating on the iron wire, all in- 

 sulated by means of varnish invented by the professor. 



The primary or thick copper wire helix, at the time the experi- 

 ments I here refer to were performed, was in connexion with from one 

 to six four-inch plates of Dr. Callan' s galvanic battery and the action, 

 though extraordinary in producing sparks or miniature flashes of 

 lightning, in some cases sixteen and a half inches long, between the ends 

 of the secondary helix, on breaking the contact of the ends of the pri- 

 mary helix, was inferior, it was stated, to that of a larger apparatus, 

 lately exhibited in London, the cost of which, compared with that con- 

 structed by Dr. Callan, was said to be exorbitant. 



In Dr. Callan' s apparatus, every care has been taken to produce the 

 greatest philosophical results at a minimum cost. "Wood, iron, zinc, 

 tinfoil, and paper, are the chief materials. Brass is used only in the 

 break of the primary helix, and the nice works connected with it, but 

 otherwise everything indicated the greatest economy, combined with 

 complete operativeness, equal to any elaborate instrument that could be 

 produced in the workshop of the most fastidious electrician. 



The sparks produced by the secondary helix passed, either between 

 its two terminal points, or from one point to a large slightly concave 

 circular disk, to which the other end of the helix was attached. Under 

 certain circumstances, these sparks differed from each other, and also 

 from any other electric sparks I had seen before ; their apparent difference 

 becoming less and less with the decrease of the distance of the point 

 between which the sparks passed. 



"When the sparks were over six or seven inches in length, the shape 

 of no two of them appeared to be the same. They were all contorted 

 more or less ; and when the distance was the greatest, and when the 

 spark would hardly pass, its zigzag or broken character gave it the 

 appearance of a miniature flash of lightning. In every case the spark 



* Dr. Callan has communicated the following details :— One cell gave sparks 7^ 

 inches long; two cells gave sparks 12^ inches long; and six cells gave sparks 16| 

 inches long. 



