4 Prof. J. C. Poggeudorff on the Induction Apparatus 



nor does an increased length of wire add anything to the bril- 

 liancy of the phaenomena as soon as the resistance of the partial 

 vacuum is small in comparison to that of the induction wire. 

 Without doubt the induction coils for obtaining sparks and 

 other phenomena of tension would be more effective and durable 

 if the thickness of the insulating medium were increased. But 

 even if the length of wire might then be reduced to some extent, 

 the coil would have to be large in size in order to produce the 

 desired effects, the inducing coil would have to be increased to a 

 corresponding extent, and a stronger voltaic battery would be re- 

 quisite ; in short, the apparatus would lose much of its present 

 convenient dimensions for practical purposes. 



I believe the above-mentioned imperfections might to a great 

 extent be remedied, — 1, by not using too thin a wire (for most 

 purposes wire of | millim. diameter would, perhaps, be best) ; 

 2, by giving the wire a very thick covering of silk ; and 3, by 

 using a liquid instead of a solid insulating medium, such as a fat 

 or essential-oil, e. g. rectified oil of turpentine. By this means, 

 at least, the gradual deterioration of the coil would be prevented, 

 for the openings broken through the silk covei'ing of the wii'e 

 by the sparks would be always stopped up again by the liquid. 



The number of parts on the coil might be greater than that I 

 have chosen ; the whole coil might be composed of loose and, 

 of course, well insulated rings of wire. In one respect this 

 would be advantageous ; the rings when defective could be re- 

 placed by new ones, but the apparatus would be thereby rendered 

 very complicated on account of the number of connecting clamps 

 that would be required. Neither can we expect any particular 

 advantage from different combinations of these rings of wire, for 

 they are elements of unequal value in consequence of their dif- 

 ferent positions with respect to the inducing coil. If it be 

 required to change the length and thickness of the wire in order 

 to study their influence upon the phaenomena, it will be found 

 more convenient to prepare complete apparatus of smaller dimen- 

 sions (say four similar to that of my third induction coil), and 

 then to combine them as it may be found desirable. 



Lastly, I am inclined to believe that the usual cylindrical form, 

 which I have also adopted hitherto, is by no means the most 

 advantageous for the induction coil. A solenoid or spindle shape 

 would be better, because the inducing coil and the soft iron core 

 are most effective in the middle. I have not yet had time, how- 

 ever, to try all these tedious changes in the apparatus. 



Primary Coil. 



I have generally used three inducing coils, all of which were 

 exactly alike. In construction they differ little from the common 



