2 Prof. J. C. PoggendorflP on the Induction Apparatus 



of the coil to the other ; the number of these layers is generally 

 even, so that the beginning and end of the wire are at the same 

 extremity of the coil. But it is clear, that in a coil, whose points 

 are all exposed to about the same degree of electrical excitation, 

 the tension must increase from the middle towards each extre- 

 mity. Hence, when the wire is coiled in the manner above 

 described, points whose electrical difference is very great are 

 brought very near one another, and consequently the springing 

 of sparks between two such points is much facilitated, as well as 

 the external action of the coil weakened. 



This imperfection appeared susceptible of partial remedy by 

 dividing the length of the coil into several parts, and by covering 

 each part with wire in the same manner as the whole coil was 

 befoi'e covered, using, however, an odd instead of an even num- 

 ber of layers in order to carry the wire from one part to another, 

 and at last to let the ends run out at opposite extremities of the 

 coil. 



Further, in order to obtain eifects of great tension, it was evi- 

 dent that the M'ire must be well insulated. As far as I know, 

 whenever a second insulator has been employed, to aid the cover- 

 ing of silk generally used and never to be dispensed with, a solu- 

 tion of shell-lac in alcohol has been chosen. But in the pi-esent 

 case, where the wire, owing to its great length, cannot well be 

 vai'nished other than lengthwise, such a method only imperfectly 

 fulfils its purpose, because under such cii'cumstances the varnish 

 dries extremely slowly, and perhaps never perfectly ; so that a 

 certain degree of conductibility, even greater than that with silk 

 alone, always exists. 



Instead of the alcoholic solution of shell-lac, an easily fusible 

 insulator (spermaceti, stearic acid, or a mixture of wax and oil *) 

 appeared preferable. In applying the same, it requires to be 

 heated above its melting-point in order to prevent its immediate 

 solidification when in contact with the cold layers of wire, and 

 to allow it to permeate and thoroughly saturate the silk covering. 



Again, in order to obtain a good exterior insulation, the wire 

 was coded around a glass cylinder, at whose extremities were 

 rims of gutta-percha in which the ends of the wire could be well 

 fastened ; besides this, the whole coil was provided with a thick 

 covering of wax ; and lastly, after this had been varnished, two 

 strong rings of gutta-percha were affixed, by means of which the 

 whole could rest on a wooden stand. 



According to these principles, I have constructed three induc- 

 tion coils, each of which is 5fj inches longt, has an internal dia- 



* Paraffine would be perhaps still better, as it may now be easily pro- 

 cured, easily melts, and is, according to Riess, a good insvilator. 

 t Throughout the memoir the inches and feet arc Parisian. 



