364 J. Trowbridge— Thin plates of iron used a 



spark. Table IV gives the results which 

 many trials. 



Table IV. 



14 30 



15 32 

 Mean, 14-5 31-3 



A curious fact came to light in this connection ; the lengthen- 

 ing of the spark was not shown when the spark leaped directly 

 between the poles of the induction coil ; ' ' 

 tity and electromotive force, was only made r 

 by the employment of condensers in the 

 The results in "Table IV were obtained by the e 

 leyden jar of large capacity. The increase i 

 and electromotive force was not only shown by the increased 

 length of the spark, bat also by its increase in volume, and its 

 louder snap. The spark consisted of a thick central bolt sur- 

 rounded by curious thin, detached sparks. An attempt was 

 made to measure the increase of light in the Geissler tubes by 

 Vierodts photometric apparatus, but it was found too inexact 

 for this purpose ; if, indeed there was any increase of light, 

 which remains to be proved. I know of no results which bear 

 upon the relation of the increase of light to the increase of elec- 

 tromotive force of the induction spark. Without condensers 

 in the secondary circuit, however, the increased electromotive 

 force of the spark was shown by its greater constancy in leaping 

 over a given resistance of air. 



Unless an instrument is desired for popular scientific lectures, 

 length is not so much to be desired as quantity of electricity of 

 a spark, and in this form of induction coil the gain is princi- 

 quantity, although it is true that with the aid of 

 irs, the striking distance is increased one hundred per 

 cent. The principal points of this paper can be thus summed up : 



1. The application of thin plates of soft iron upon the poles 

 of two straight electro-magnets, with bundles of fine iron wires 

 for cores, increases the strength of the spark produced at the 

 poles of the secondary coils surrounding the electro-magnets, 

 four hundred per cent. 



2. The length of the spark is increased one hundred per cent. 

 This gain in length is only manifested by the employment of 

 leyden jars of large capacity, which are connected with the sec- 

 ondary circuit. 



3. instead of distributing the fine wire of a Ruhmkorff coil 

 upon a straight electro-magnet, as is done at present, this wire 

 should be distributed equally upon two straight electro-magnets, 

 whose poles should be provided with armatures of bundles of 

 thin plates of soft iron. 



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