378 PROCEEDINGS OF THE AMERICAN ACADEMY 



portions, each wound upon a gutta-perclia bobbin, so that one or all can 

 be lifted from around the glass bell, Avhich in like manner can be lifted 

 from the primary. The secondary helix of his first-class apparatus 

 contains about thirty miles of wire ; that of the apparatus giving a ten- 

 inch spark contains eighteen miles. 



Another novel feature in the construction of the instrument, found 

 by Mr. Ritchie materially to augment its power, is the extension of the 

 primary helix and its core to some distance beyond the ends of the sec- 

 ondary. In the smaller instruments he adopts the proportion of two to 

 one, and in the larger, of about one and a half to one. 



Mr. Ritchie, moreover, separates the condenser into several portions, 

 each connected with a screw upon the base of the instrument, enabling 

 the operator to vary at will the amount of condensing surface, or to 

 dispense with it entirely, and by these changes giving rise to many 

 remarkable varieties in the phenomena. 



In addition to these peculiarities contributing to the superiority of 

 the instrument, it should be mentioned that Mr. Ritchie uses a much 

 finer wire in the secondary coil, and a much coarser in the primary, 

 than have heretofore been employed. It should not be omitted in the 

 comparison, that while the European ajiparatus requires for the full 

 development of its action a large intensity-battery, his coil is excited by 

 a few cells to the highest display of its power. 



Professor Rogers called attention to the peculiar characters of the 

 spark of the induction coil when transmitted through the air at dif- 

 ferent distances between the terminals. When the distance is short, 

 as, for example, one or two inches in the instrument exhibited to the 

 Academy, the spark has the peculiar twofold character first noticed by 

 Du Moncel, and since minutely studied by him and Perrot and others ; 

 that is to say, it is formed of a slender, brilliant thread, enveloped by a 

 much wider and less luminous space of a somewhat ruddy, flame-like 

 aspect. As the striking distance is increased, this surrounding glow 

 becomes less conspicuous, in comparison with the brilliant thread of 

 light, and at last, when the interval has been sufficiently extended, it 

 nearly or wholly disappears, leaving the spark to consist of a slender, 

 jagged, brilliant line. 



These two parts of the luminous discharge, which we may call the 

 flame-sparh and the thread-sparlc, present remarkable differences of 

 character, indicating very unlike states of tension in the currents or 



